option TPS 7.1.2.2 - Inele

Technik 

568 962 

MTS 7.1.2.1 

Fundamentals of 

ISDN Technology

MTS 7.1.2.1 

Fundamentals of 

ISDN Technology 

by 

Maria Spath 

and Reinhard Heermeyer

“The sensitive electronics of the equipment contained in the present experiment literature 

can be impaired due to the discharge of static electricity. Consequently, electrostatic 

build up should be avoided (particularly by utilizing appropriate rooms) or 

eliminated by discharging (e.g. at the panel frames or similar).” 

User's note 

The ISDN technology training system uses commercially available ISDN equipment. This requires 

that the following principles be adhered to by the student and instructor: 

• The digital exchange contained in the training system is equipped solely for training purposes. Any 

connection to public or internal ISDN networks is totally out of the question. Leybold Didactic can 

under no circumstances assume liability for disturbances arising from a violation of this limitation 

to utilization. 

• Leybold Didactic can under no circumstances guaranty operation for any foreign devices 

connected to this digital exchange. 

• Should there be any differences between the experiment descriptions and the user's instructions for 

the ISDN equipment the manufacturer's manuals take precedence. 

• The experiments described in this manual constitute a representative selection of important topics, 

which by no means exhaust the performance spectrum of terminal equipment and digital 

exchange. Some experiments not included in the descriptions are thus possible but are not dealt 

with by the Leybold Didactic training system.

MTS 7.1.2.1 Fundamentals of ISDN Technology Contents 

Theory 

Table of Contents 

Equipment overview ..................................................................................................7 

1 The basics of Euro-ISDN ...............................................................................8 

1.1 Historical development ....................................................................................................... 8 

1.2 Connection options ............................................................................................................. 9 

1.2.1 Basic access ......................................................................................................................... 9 

1.2.2 Primary rate access .............................................................................................................. 9 

1.2.3 Network termination............................................................................................................ 9 

1.3 Configuration ....................................................................................................................... 9 

1.4 An overview of ISDN interfaces ......................................................................................10 

1.5 Internal interfaces of a digital exchange .........................................................................10 

1.5.1 Internal ISDN interfaces....................................................................................................10 

1.5.2 Internal S 0 -bus....................................................................................................................11 

1.6 ISDN training system from Leybold Didactic ................................................................12 

2 Service features in Euro-ISDN ...................................................................13 

2.1 ISDN subscriber number as service feature ....................................................................14 

2.1.1 DDI – Direct Dialling In ...................................................................................................14 

2.1.2 MSN – Multiple Subscriber Number ...............................................................................14 

2.2 Calling/connected line identification presentation and restriction ................................15 

2.2.1 CLIP – Calling Line Identification Presentation .............................................................15 

2.2.2 CLIR – Calling Line Identification Restriction ...............................................................15 

2.2.3 COLP – Connected Line Identification Presentation .....................................................15 

2.2.4 COLR – Connected Line Identification Restriction .......................................................16 

2.3 The hold service feature....................................................................................................16 

2.3.1 HOLD – Call Hold.............................................................................................................16 

2.4 Remarks on the service features ......................................................................................16 

3 Installation techniques at the basic access terminal ................................17 

3.1 Interfaces ............................................................................................................................17 

3.2 Network terminal basic access NTBA .............................................................................17 

3.3 Power supply......................................................................................................................17 

3.4 Standard operation.............................................................................................................17 

3.5 Emergency operation ........................................................................................................17 

3.6 Connection of the S 0 bus ..................................................................................................18 

3.7 Installation for the basic access – point-to-point ............................................................18 

3.8 Installation for basic access – point-to-multipoint configuration ................................19 

3.8.1 Short passive bus ...............................................................................................................19 

3.8.2 Extended passive bus ........................................................................................................19 

3.9 Universal telecommunication socket..............................................................................20 

3.9.1 The universal telecommunications socket UAE 8 (8) ....................................................20 

3.10 Wiring ......................................................................................................................20 

4 The OSI-Reference Model ..........................................................................21 

4.1 Layer 1................................................................................................................................21 

4.2 Layer 2................................................................................................................................21 

4.3 Layer 3................................................................................................................................22 

3

4 


Theory 

5 S 0 interface ...................................................................................................22 

5.1 Logical structure of the S 0 interface .................................................................................22 

5.2 S 0 frame ..............................................................................................................................23 

5.3 Structure of the S 0 frame ...................................................................................................23 

5.4 S 0 interface code ................................................................................................................24 

5.5 Exceptions to the coding rules .........................................................................................24 

5.6 Control of access to the D channel ..................................................................................25 

6 Layer 1..........................................................................................................25 

6.1 Activation procedure for layer 1 ......................................................................................26 

6.2 Deactivation procedure for layer 1 ..................................................................................26 

6.3 Reactivation .......................................................................................................................26 

6.4 Measurement techniques and testing of layer 1. ............................................................26 

7 Layer 2..........................................................................................................27 

7.1 Tasks accomplished by layer 2 ........................................................................................27 

7.2 Structure of layer 2 ............................................................................................................27 

7.3 Layer-2 formats .................................................................................................................27 

7.3.1 The U format ......................................................................................................................27 

7.3.2 The S-Format......................................................................................................................28 

7.3.3 The I-Format ......................................................................................................................28 

7.4 Overview of layer 2...........................................................................................................29 

7.5 Protocol elements in layer 2 .............................................................................................29 

7.5.1 SAPI - Service Access Point Identifier.............................................................................29 

7.5.2 TEI – Terminal Endpoint Identifier ..................................................................................29 

7.6 Protocol sequence in layer 2 ............................................................................................30 

7.6.1 TEI-Management (assignment of addresses) ..................................................................30 

7.6.2 Establishing connections from TE → NT in layer 2 ......................................................31 

7.6.3 The layer-2 message SABME...........................................................................................31 

7.6.4 The layer-2 message UA ...................................................................................................31 

7.7 I frames in layer 2..............................................................................................................31 

7.8 S-frames in layer 2.............................................................................................................31 

7.8.1 Ending of a layer-2 connection from TE → NT .............................................................32 

7.8.2 The layer-2 message DISC ..............................................................................................32 

7.8.3 The layer-2 message UA ...................................................................................................32 

8 Layer 3..........................................................................................................32 

8.1 Functions of layer 3...........................................................................................................32 

8.2 Structure of layer 3 ............................................................................................................33 

8.3 Sequence of messages in layer 3 .....................................................................................34 

8.4 Layer 3 messages ..............................................................................................................34 

8.5 Establishment of connection from TE to NT in layer 3 .................................................36 

8.5.1 Purposes of layer-3 messages during establishment of a connection ..........................37 

8.6 Ending of a layer-3 connection from TE to NT..............................................................37 

8.6.1 Purposes of layer-3 messages during ending of a connection ......................................37 

8.7 Information elements (IE) on layer 3...............................................................................38 

8.7.1 Special features of an IE ...................................................................................................38 

8.7.2 Formats of IEs ....................................................................................................................38 

8.7.3 An overview of the most important information elements ............................................39 

8.8 Call numbers in information elements.............................................................................39 

8.9 Transmission of dialling information with an outgoing call ..........................................40


Theory 

8.9.1 Individual dialling .............................................................................................................40 

8.9.2 Block dialling.....................................................................................................................41 

8.10 The calling party number IE CGPN .................................................................................41 

8.10.1 Structure of the Calling party number IE CGPN ............................................................41 

8.10.2 Example of the Coding of a CGPN..................................................................................42 

8.11 The features CLIP and CLIR on the D channel ..............................................................42 

8.12 The HOLD feature on the D channel...............................................................................43 

8.12.1 Activation of Hold .............................................................................................................43 

8.12.2 Deactivation of Hold .........................................................................................................44 

9 Appendix .......................................................................................................45 

9.1 Literature ............................................................................................................................45 

9.2 Overview: Important Causes ............................................................................................46 

9.3 Abbreviations.....................................................................................................................47 

Exercises 

Equipment and accessories required ..............................................................................................50 

1 Installation techniques ................................................................................51 

1.1 Configuration of the teaching system..............................................................................51 

1.1.1 Graphic representation of the basic configuration .........................................................51 

1.1.2 Quick commissioning of the PBX system EURACOM 182 ..........................................51 

1.1.3 Short description of the ARGUS 10 ISDN tester ............................................................52 

1.1.4 735 912 ISDN panel..........................................................................................................54 

1.2 Installation errors ...............................................................................................................54 

1.2.1 Swapped terminals.............................................................................................................54 

1.2.2 Resistance and wiring measurements ..............................................................................55 

1.3 Line characteristics of a passive S 0 bus (Option) ...........................................................55 

1.3.1 Determining the Line coverage using the measuring bridge ........................................55 

1.3.2 Calculation of the characteristic impedance ...................................................................55 

1.4 Data transfer on the physical layer ..................................................................................55 

2 Services and features of ISDN ....................................................................57 

2.1 Connection testing .............................................................................................................57 

2.2 Testing of services .............................................................................................................57 

2.2.1 Automatic testing of services ...........................................................................................57 

2.2.2 Testing of individual services ...........................................................................................57 

2.3 Features...............................................................................................................................58 

2.3.1 The Hold feature ................................................................................................................58 

2.3.2 The features Calling Line Identification Presentation CLIP 

and Calling Line Identification Presentation CLIR ........................................................58 

3 D-channel protocol ......................................................................................59 

3.1 Establishment and ending of connections on the D channel ........................................59 

3.2 Testing a feature with the help of D-channel information .............................................59 

4 Frame structure on the ISDN telephone (option TPS 7.1.2.2) ..................60 

4.1 Sampling.............................................................................................................................60 

4.2 Quantisation and Coding ..................................................................................................60 

5

6 

MTS 7.1.2.1 Fundamentals of ISDN Technology Contents Theory 

5 Line coding on the S 0 interface (option TPS 7.1.2.3) .................................62 

5.1 Serial data transfer and security .......................................................................................62 

5.2 AMI code............................................................................................................................62 

Answers 

1 Installation techniques ................................................................................63 

1.2 Installation errors ...............................................................................................................63 

1.2.1 Terminal swapping ............................................................................................................63 

1.2.2 Resistance and wiring measurements ..............................................................................64 

1.3.1 Determining the line coverage by means of a measuring bridge .................................65 

1.3.2 Calculation of the characteristic impedance ...................................................................66 

1.4 Data transfer on the physical layer ..................................................................................66 

2 Services and Features in ISDN ...................................................................68 

2.1 Automatic Connection Test ..............................................................................................68 

2.2 Test of services...................................................................................................................69 

2.2.1 Automatic testing of services ...........................................................................................69 

2.2.2 testing of individual services ............................................................................................71 

2.3 Features...............................................................................................................................73 

2.3.2 The features Calling line identification presentation CLIP 

and Calling line identification presentation CLIR ..........................................................74 

3 D-channel protocol ......................................................................................76 

3.1 Establishment and ending of connections on the D channel ........................................76 

3.2 Testing a feature with the help of D-channel information .............................................77 

Index .......................................................................................................................80

MTS 7.1.2.1 Fundamentals of ISDN Technology Contents Theory 

Equipment overview 

TPS 7.1.2.1 Experiments 

1.1 Configuration of training system 

Equipment 

ISDN panel 735 912 1 1 – 1 1 1 1 1 1 – – – – 

PBX system 735 913 1 1 – 1 1 1 1 1 1 – – – – 

ISDN telephone 735 915 3 2 – 2 2 2 3 2 2 – – – – 

ISDN tester 735 916 1 1 – – 1 1 1 1 1 – – – – 

PAM modulator 736 061 – – – – – – – – – 1 1 – – 

PAM demodulator 736 071 – – – – – – – – – 1 1 – – 

PCM modulator 736 101 – – – – – – – – – – 1 – – 

PCM demodulator 736 111 – – – – – – – – – – 1 – – 

Data Source / Parity Generator 736 93 – – – – – – – – – – – 1 1 

Display / Parity Check Indicator 736 92 – – – – – – – – – – – 1 1 

AMI/HDB3 coder 736 94 – – – – – – – – – – – – 1 

AMI/HDB3 decoder 736 91 – – – – – – – – – – – – 1 

Accessories 

Digital storage oscilloscope 407 575 293 – – – 1 – – – – – 1 1 1 1 

BNC cable 501 02 – – 1 3 – – – – – – – – – 

Book: “Fundamentals of ISDN Technology” 568 961 1 1 1 1 1 1 1 1 1 – – – – 

Star quad cable 736 481 – – 1 – – – – – – – – – – 

Measuring bridge 736 451 – – 1 – – – – – – – – – – 

STE 100 Ohm resistor, 0,5W 577 01 – – 2 – – – – – – – – – – 

STE 10 nF capacitor, 100 V 578 10 – – 1 – – – – – – – – – 1 

STE 47 nF capacitor, 100 V 578 11 – – 1 – – – – – – – – – – 

Book: “Measurments on four-wire cables” 568 542 – – 1 – – – – – – – – – – 

Panel frames – – – – – – – – – 1 1 1 1 

DC Power Supply ±15 V/ 3 A 726 86 – – 1 – – – – – – 1 1 1 1 

Spectrum analyser 726 94 – – – – – – – – – 1 – – – 

Function generator 200 kHz 726 961 – – 1 – – – – – – 2 2 – – 

Frequency counter 0...10 MHz 726 99 – – – – – – – – – 1 1 – – 

Probe100 MHz 1.1/10:1 575 231 – – – – – – – – – 2 2 2 2 

Analog Multimeter C.A. 406 531 16 – – – – – – – – – 1 1 – – 

Cable pair 100 cm, black 501 461 – – – – – – – – – 2 2 – – 

10 bridging plugs, black 501 511 – – – – – – – – – 3 3 3 3 

Book: “Pulse Code Modulation“ 564 001 – – – – – – – – – 1 1 – – 

STE 4.7 kOhm resistor, 2 W 577 52 – – – – – – – – – – – – 1 

Book: “Baseband data transmission” 568 452 – – – – – – – – – – – 1 1 

Screened cable BNC/4 mm 575 24 – – 2 – – – – – – – – – – 

1.2 Installation errors 

1.3 Line parameters of a passive S 0 bus (Option) 

1.4 Data transfer on the physical layer 

2.1 Connection testing 

2.2 Testing of services 

2.3 Features 

3.1 Establishment and ending of connections on the D channel 

3.2 Testing a feature with the help of D-channel information 

4.1 Sampling 

4.2 Quantisation and Coding 

5.1 Serial data transfer and security 

5.2 AMI code 

7

8 

MTS 7.1.2.1 Fundamentals of ISDN Technology Theory 

1 The basics of Euro-ISDN 

1.1 Historical development 

Euro-ISDN as it exists today stems from the 

digitalization of the telecommunication system. 

This fundamental step was decided upon in 

1979 by the German Federal Post Office or 

Deutsche Bundespost (DBP), formerly the monopoly 

network operator in Germany. The entire 

development process of ISDN technology 

has been influenced by technical as well as political 

aspects and goes back as far as 1948. 

An outline of the most important events includes: 

1948/49 Shannon's theorem lays the foundations 

for the development of digital 

technology 

1970 Digital transmission using pulse code 

modulation (PCM) begins 

1972 CCITT - consensus for ISDN as the 

ideal system of the future 

CCITT = Comité Consultatif International 

Télégraphique et Téléphonique, 

later renamed ITU = International Telecommunications 

Union 

CCITT was an international body 

drafting and submitting technical recommendations, 

not only for ISDN. 

1978 CCITT's standardization work on 

ISDN commences 

1979 Fundamental decision by the German 

Bundespost (German Federal Post Office) 

to carry out complete digitalization 

of the telecommunication system 

1982 Pilot run of long-distance digital exchanges 

(DBP) 

1983 Pilot run of local digital exchanges 

(DBP) 

1984 Recommendation by the Council of 

the European Union (EG) to launch 

ISDN on the European market 

1986 EC commission agrees to introduce 

standardized norms in the area of telecommunications 

1986 The first ISDN pilot projects are set up 

in Germany 

1987 Start of ISDN pilot projects in Mannheim 

and Stuttgart 

1988 Networking of these ISDN pilot 

projects 

1988 Commissioning of the first ISDN sub- 

scriber lines by the Federal Post Office 

(DBP) 

1988 The “European Telecommunications 

Standards Institute” ETSI is founded; 

Standardization work commences for 

the Euro-ISDN protocol DSS1 (Digital 

Subscriber Signalling System No. One) 

1989 Official opening of the ISDN network 

in the Federal Republic of Germany 

(CeBIT 1989) with the national ISDN 

protocol 1TR6 

1989 “Memorandum of Understanding on 

the Implementation of a European 

ISDN” (MoU): 

26 network operators from 20 European 

nations agree to introduce Euro- 

ISDN by the end of 1993. One of the 

signatories was the Deutsche Bundespost 

Telekom 

1989 ISDN pilot project between Germany 

and the Netherlands 

1990 The German Federal Post Office renounces 

its monopoly over the installation 

of terminal equipment for communications 

technology 

1991 Interconnection of the ISDN networks 

of the German Post Office's Deutsche 

Telekom and France Télécom 

1992 Deutsche Telekom stops introducing 

national ISDN service attributes 

1993 Eurie 93: 14.-17.12.1993 

Official launch of Euro-ISDN in 17 

European nations, including Germany 

1994 Euro-ISDN becomes the standard 

package of the Deutsche Telekom and 

as of mid-year is universally available. 

1TR6 is kept temporarily running 

alongside Euro-ISDN until the end of 

the millennium 

1994 Availability of new service features in 

analog networks 

1995 Deutsche Telekom AG promotes the 

purchase of ISDN by offering discounts 

to the customers who apply for 

a Euro-ISDN line 

1997 Digitalization of the exchanges in 

Germany is almost complete. Deutsche 

Telekom makes preparations to 

confront competition from new network 

operators.


The Federal Post Office became acquainted 

with the testing of technical systems very early 

on as a pioneer in the introduction of ISDN 

technology. At that point in time there existed 

merely the recommendations of the CCITT regarding 

possible configurations, physical interfaces 

and basic protocol contents. The German 

Federal Post Office consequently developed its 

own layer 3 protocol 1R6 on the basis of the 

CCITT protocol recommendation Q.931. This 

was transformed into the 1TR6 specification already 

during the pilot project phase and ended 

up forming the essential element of the ISDN 

systems. These were marketed within the 

framework of the national ISDN network. 

Only as of 1988 did the European Telecommunications 

Standards Institute ETSI commence 

work developing a common European protocol 

meant to guarantee the compatibility of ISDN 

products and systems in all networks operating 

in accordance with this standard. Four years after 

the introduction of national ISDN, the 

EURO-ISDN protocol drafted by ETSI became 

the standard package offered by the network 

operators of the Deutsche Bundespost Telekom. 

As the two protocols are not compatible, 

1TR6 will soon disappear altogether. 

The difference between national ISDN and 

Euro-ISDN primarily affects the D-channel protocol 

(layer 3) and the service features being 

offered, but not the physical interfaces. 

1.2 Connection options 

Service integration is an essential feature of this 

technology, this is because various services can 

be used using only one connection or line. For 

that reason in the long term the variety of independent 

networks will disappear. 

If a subscriber decides to order an ISDN line 

from a network operator, he can basically decide 

between two terminals: the basic access 

and the primary rate access. Only precise 

knowledge of the respective service criteria is 

an aid in selecting the right form of access. 

1.2.1 Basic access 

Basic access provides the subscriber with two 

basic access channels (so-called B channels) 

with a transmission rate of 64 kbit/s. Both Bchannels 

can be used simultaneously, regardless 

of the service and the direction of the connection 

(outgoing or incoming). If you apply 

this to the terminology of the analog telephone 

network, the basic access makes two “exchange 

lines” available for the subscriber. 

In addition to the B channels there is also the D 

channel on the basic access for signalling with a 

transmission rate of 16 kbit/s, not to mention an 

additional 48 kbit/s allocated for synchronization 

and other functions. For that reason when 

speaking about basic access people refer to a 

net bit rate of 144 kbit/s (2xB, 1xD) and a gross 

bit rate of 192 kbit/s. 

1.2.2 Primary rate access 

The primary rate or primary multiplex access 

provides the subscriber with thirty base channels 

(so-called B channels) with a transmission 

rate of 64 kbit/s. Here too it is designed so that 

all thirty channels can be used simultaneously, 

regardless of the service and the direction of the 

connection. 

In addition to this there also exists the D channel 

(signalling channel) and the time slot 0 

(synchronisation), each having 64 kbit/s, so that 

a total transmission rate of 2.048 Mbit/s is available 

on the primary rate access terminal 

(PMXA). The PMXA basically corresponds to a 

PCM-30 system. 

1.2.3 Network termination 

In both types of access there is an interfacing 

point between the subscriber line of the network 

operator and the terminal devices of the subscriber, 

the so-called network termination (NT). 

This network terminal normally remains the 

property of the network operator. It is installed 

either by a technician sent by the network operator 

or it can be connected to the basic access 

terminal by the subscriber himself. There is a 

unique network termination for both basic access 

as well as primary rate access due to the 

fact that the two types of connection are physically 

different: 

NTBA: Network termination for basic access 

NTPM: Network termination for primary multiplex 

access 

1.3 Configuration 

In general there is a distinction made in ISDN 

between two types of configuration: point-topoint 

(P-P) and point-to-multipoint (P-MP). 

Point-to-point configuration means that only one 

“terminal device” can be connected to this terminal. 

This means that communication is restricted 

to the exchange and one “terminal device”. 

9

10 


Primary rate access is only offered in this pointto-point 

configuration. The terminal device 

which is connected to primary rate access terminal 

(NTPM) is normally a PBX system, which 

administers and allocates the thirty B channels 

for its internal subscribers. 

The basic access terminal can also be used in 

point-to-point configuration where it too is used 

in conjunction with a PBX system. Here, the 

two B channels are made available to the internal 

subscribers by the PBX system. 

Point-to-multipoint configuration means that 

several terminal devices can be operated from 

one access terminal, permitting communication 

between the PBX system (single point) and several 

devices (multipoint). 

This point-to-multipoint configuration exists 

exclusively for basic access. It is predetermined 

that up to eight terminal devices with various 

services may be connected to one basic access, 

whereby two can use one B channel simultaneously. 

A small PBX system can also be operated 

on this basic access terminal provided that it 

is configured as a point-to-multipoint device, 

meaning that it responds like a single ISDN terminal 

device. 

The terms point-to-point (P-P) and point-tomultipoint 

(P-MP) stem from the ISDN specification 

and are primarily of importance to the 

technician who installs and checks ISDN terminals. 

This is because with the measuring instruments 

being used it is frequently a necessity to 

select and make the settings for the correct configuration. 

For ISDN subscribers, two other terms are commonly 

used, which were employed heavily by 

the former monopoly network provider, Deutsche 

Telekom. These are “Multiple device connection” 

and “Equipment connection”. The 

former is a basic access terminal in point-tomultipoint 

configuration whereas the latter is 

either a primary access terminal or a basic access 

terminal in point-to-point mode. 

1.4 An overview of ISDN interfaces 

The subscriber lines between the exchange and 

the network termination are 

Basic access 

2-wire 

Primary multiplex access 4-wire 

The interfaces over which the NT is connected 

to the subscriber lines, are called 

Basic access 

Primary multiplex access 

Primary multiplex access 

where 

U 

CCITT key 

k Copper 

0 Basic access 

2M 2,048 Mbit/s 

G Glass fibre 

The following interfaces are present on the subscriber 

side (i.e. between the network termination 

and the terminal equipment): 

Basic access 

Primary multiplex 

access 

where 

S 

CCITT key 

0 Basic access 

2M 2.048 Mbit/s 

S 0 interface 

S 2M interface 

U k0 interface 

U 2M interface 

U G2 interface 

4-wire 

4-wire 

The subscriber interfaces are identical for all 

signatories of the “Memorandum of Understanding” 

and have been standardized in the 

corresponding ETSI stipulations. However, the 

interface designations can vary in the individual 

countries. Accordingly, one refers in Austria, 

for example, to an S/T interface or in England to 

the S-interface and simultaneously mean both 

the S 0 as well as the S 2M interface. 

1.5 Internal interfaces of a PBX system 

1.5.1 Internal ISDN interfaces 

If you consider the connection possibilities of 

terminal equipment to ISDN PBX systems, you


Fig. 1.4-1: Types of subscriber access terminals 

will very frequently find manufacturer-specific 

interfaces. This often involves two-wire U-type 

interfaces which permit larger distances to be 

spanned. This is, of course, unavoidable on 

company premises that cover a larger area. Naturally 

they also have a disadvantage because 

they only permit the connection of manufacturer-specific 

system terminal equipment. In this 

case, the customer must do without using commercial 

ISDN products with S 0 interfaces like 

e.g. ISDN PC cards. But in the meantime the 

manufacturers also offer a solution here, namely 

a manufacturer-specific PNT (private network 

termination), which converts the U-interface 

into an S 0 interface. This fulfils two criteria: 

larger areas can be covered and a commercially-available 

S 0 devices can be connected. 

1.5.2 Internal S 0 bus 

There are however any number of ISDN digital 

exchange systems on the market which offer, in 

addition to analog and/or manufacturer-specific 

interfaces, S 0 interfaces in the point-to-multipoint 

configuration as well. If there is an S 0 interface on 

a digital exchange for the connection of terminal 

equipment, this is then referred to as an internal 

S 0 bus, on which commercially-available S 0 

equipment can be operated. Although this interface 

has the exact same designation as the interface 

of the public network and is normally iden- 

11

12 


Fig. 1.5-1: Internal interfaces of a private digital exchange 

tical to it in its physical construction, the user always 

has to reckon with deviations. For example, 

it might be the case that the number of terminal 

devices which can be connected is limited or 

that certain service features are not intrinsically 

available. In any case, it is helpful to obtain more 

precise information regarding the capabilities of 

the internal S 0 bus. 

It is also important to know that the manufacturer 

is not obliged to implement Euro-ISDN (i.e. 

the D channel protocol and service features) as 

completely on the internal S 0 bus as prescribed 

for the public network specification. If the service 

feature “terminal portability” were not implemented 

on an internal S 0 bus, it would not 

constitute a real fault but simply a gap in the 

manufacturer's service spectrum. 

1.6 ISDN training system from 

Leybold Didactic 

The ISDN training system from Leybold Didactic 

is independent of the availability of an ISDN 

connection. All of the practical exercises and 

measurements are performed on the internal S 0 

buses of a commercially available ISDN PBX 

system. 

Naturally, however, the exercises have been 

designed so that they can also be carried out on 

an ISDN line.


2 Service features in Euro-ISDN 

In Euro-ISDN each service feature has its own 

specification with the designation ETS 300 xxx 

(each x stands for a number). 

This ETS standard not only contains the description 

of the service feature, but also defines 

information for activation or deactivation of reserved 

layer-3 messages and provides an overview 

of the signalling procedures for the 

corresponding subscriber/network interfaces. 

The ETS standards were written in the English 

language and can be obtained in Germany in 

the original with a brief German preface as a 

DIN ETS 300 xxx guideline. 

All technicians who install and perform mainte- 

nance on ISDN networks should be familiar 

with the terms listed under supplementary services 

and their abbreviations because these terms 

are required for competency in the area of 

ISDN measurement techniques. 

Having signed the Memorandum of Understanding, 

network operators have committed 

themselves to offering a minimum range of 

service features. However, it can be assumed 

that a network operator will be endeavouring to 

implement all the available service features for 

the sake of customer sales and satisfaction. 

Here is a list of the most well known service 

features provided in Germany: 

Abbr. ETS designation ETS standard 

AOC Advice of Charge 300 182 

CFB Call Forwarding Busy 

CFNR Call Forwarding No Reply 

CFU Call Forwarding Unconditional 

300 199 

300 207 

300 201 

300 207 

300 200 

300 207 

CLIP Calling Line Identification Presentation 300 092 

CLIR Calling Line Identification Restriction 300 093 

COLP Connected Line Identification Presentation 300 097 

COLR Connected Line Identification Restriction 300 098 

CW Call Waiting 300 058 

DDI Direct Dialling In 300 064 

HOLD Call Hold 300 141 

MSN Multiple Subscriber Number 300 052 

TP Terminal Portability 300 055 

13

14 


In the case of an ISDN line it is the exchange 

which provides these service features to the 

connected terminal equipment or the PBX system. 

At the same time, we should bear in mind 

that the range of available service features depends 

on the type of line connection, more precisely 

the line configuration: some service features 

are only possible for point-to-multipoint 

configuration, while others are only suitable for 

point-to-point configurations. 

Note: The service features offered to the ISDN 

subscriber for his/her line by the public network 

operator should not be confused with those 

which a PBX system makes possible for your 

connected terminal equipment. This aspect 

should be particularly taken into consideration 

during trouble-shooting. 

2.1 ISDN subscriber number as service 

feature 

One of the special features provided by Euro- 

ISDN not only to technicians but also to subscribers 

is that the call number is defined as a 

service feature. To put it more precisely there 

are even two separate specifications: DDI Direct 

Dialing In describes the direct inward dialing 

for the digital exchange and MSN Multiple 

Subscriber Number establishes how the call 

number is to function with a basic access terminal 

in point-to-multipoint configuration. 

The significance of the two service features 

should not be underestimated. On the one side, 

handling the call numbers is important for properly 

functioning communications, on the other 

side, DDI and MSN are directly related to other 

interesting service features, e.g. Calling Line 

Identification Presentation and Calling Line 

Identification Restriction. 

2.1.1 DDI – Direct Dialing In 

In Euro-ISDN Direct Dialing In designates the 

possibility of dialing directly to a terminal device 

connected to a digital exchange. Here, the 

size of the digital exchange, i.e. the number of 

available B channels (“exchange lines”) no 

longer plays a role. However, the DDI service 

feature is limited to point-to-point configurations, 

i.e. primary rate access and basic access 

with a single device. 

If the digital exchange has an internal S 0 bus, 

the ISDN terminal equipment connected there 

can also be reached by direct dialing. 

Since the DDI and MSN features do not appear 

simultaneously on one line, the subscriber must 

decide in advance which type of call number is 

most appropriate to be used with their line. 

As far as direct inward dialing is concerned 

there are no difficulties with the conversion for 

many subscribers because this possibility was 

also offered in the analog network for PBX systems 

with more than eight exchange lines. This 

is different for the multiple subscriber number 

(MSN) which is reserved for the basic access/ 

point-to-multipoint configuration. 

2.1.2 MSN – Multiple Subscriber Number 

The multiple subscriber number (MSN) is only 

available in the point-to-multipoint configuration. 

MSN simply means that, on one line, several 

call numbers are available which can be 

deployed and used by the subscriber. 

How many free and chargeable call numbers 

are available to a subscriber depends solely on 

the discretion of the network operator. For that 

reason, Deutsche Telekom's offer of up to 10 

free-of-charge MSN's for one line does not apply 

to other network operators in Germany or 

Europe. 

The best or correct way of dealing with multiple 

subscriber numbers (MSN) is sometimes 

still a problem for the subscriber, particularly 

in the face of all the various terminal equipment 

which is possible on one line. Here are 

some fundamental processes which are of significance 

particularly for incoming connections: 

• For an ISDN terminal device to know under 

which call number it can be reached, the subscriber 

needs to set the call number there. If 

this is omitted the terminal device assumes it 

is being addressed by every (incoming) call 

number. 

Example: 

A subscriber installs three ISDN telephones, 

but does not allocate any MSN to them. 

When there is an incoming telephone call, all 

three telephones ring. This can only be 

avoided by setting each phone to its own 

MSN. 

• A terminal devιce then only answers to an 

incoming call if it detects that two conditions 

are fulfilled: the call number and the service


for the terminal device have to be contained 

within the incoming call. Due to this fact the 

subscriber device can administer different 

services under one call number. Naturally 

there are restrictions on analog services like, 

e.g. Fax Group 3! 

Example: 

The subscriber possesses one ISDN telephone 

and one group-4 fax. Both are allocated 

one and the same MSN to each device. 

Communication functions smoothly. 

• Note: 

The transmission of call numbers for outgoing 

and incoming calls is performed by layer 

3 of the D-channel protocol. 

Note: 

Naturally when testing a line it is just as important 

to enter the right MSN into the ISDN testing 

instrument! 

2.2 Calling/Connected Line Identification 

Presentation and 

Restriction 

Euro-ISDN is able to transmit call numbers or 

(alternatively) restrict them in either direction. 

There are a total of four different scenarios: 

1. The call number of the calling subscriber is 

transmitted to the called subscriber and displayed 

there (CLIP) 

2. The caller activates a call number identification 

restriction so that the call number cannot 

be displayed at the called subscriber (CLIR) 

3. The calling subscriber requests from the 

network operator that the call number of the 

subscriber who actually takes the call be 

transmitted back (COLP) 

4. A subscriber requests from the network operator 

that the call number not be transmitted 

to a caller whose call has been accepted 

(COLR) 

2.2.1 CLIP – Calling Line Identification 

Presentation 

CLIP is the term to describe that the call number 

of the calling subscriber is to be transmitted and 

displayed to the called subscriber. The procedure 

for this is as follows: 

As soon as the calling subscriber (subscriber A) 

places the call – either by picking up the telephone 

receiver or by pressing the enter key on a 

PC – the call number assigned to the terminal 

device is transmitted to the exchange within the 

D-channel protocol. A's exchange establishes 

the connection to the exchange of the called 

subscriber (subscriber B) via the network. The 

B's exchange subsequently informs the B's 

equipment of the incoming call in the D-channel 

and at the same time transmits the call 

number of the subscriber A. 

2.2.2 CLIR – Calling Line Identification 

Restriction 

Subscriber A prefers not to have the call 

number seen by the called subscriber and for 

that reason activates calling line identification 

restriction before initiating the connection. The 

subsequent process is as follows: 

As soon as the subscriber A places the call the 

call number set in the terminal device is transmitted 

to A's exchange (D-channel protocol, 

layer 3!). A's exchange routes the call to the 

exchange local to the called subscriber B. B's 

exchange recognizes that A has activated the 

calling line identification restriction option and 

consequently withholds this call number when 

it routes the call to B. Although the call is signalled 

on B's equipment, the call number of the 

caller is not displayed. 

2.2.3 COLP – Connected Line Identification 

Presentation 

The calling subscriber issues an order to the 

network operator requesting the call number of 

the subscriber actually accepting the call to be 

transmitted back. 

Two aspects have to be taken into account here: 

First of all it might be the case that the calling 

subscriber wants to be sure that the subscriber 

who actually accepts the call is the one they 

have dialed; a situation of particular importance 

if, for example, confidential data is being transmitted. 

Secondly, it can certainly be the case 

that the call might arrive at a different subscriber 

than the one originally intended, e.g. if subscriber 

B has set up call forwarding to a subscriber 

C. 

As soon as subscriber B accepts the call, a protocol 

message is transmitted to the calling subscriber. 

This signals that the call has been connected, 

and at the same time indicates the actual 

call number to which connection has been 

made. 

15

16 


2.2.4 COLR – Connected Line Identification 

Restriction 

Subscribers can issue an instruction to the network 

operator that their own call number is not 

to be transmitted back to another caller. 

Even in this case there are several individual 

aspects to be taken into consideration. For example, 

it may be the case that a doctor or technician 

is working from home or is on call at home. 

For that reason he/she has activated call forwarding 

in the office or practice but wants to 

avoid having the caller receive his/her private 

number. 

The service feature COLR thus prevents callers 

from receiving the number of the subscriber 

with whom they are connected. 

2.3 The Hold service feature 

2.3.1 HOLD – Call Hold 

The service feature called HOLD is only offered 

for basic access in the point-to-multipoint configuration 

and is used with the telephony service. 

Note: 

This is not to be confused with the service feature 

consultation hold/toggle broker's calls, 

which is offered to connected terminal devices 

by the private digital exchange! 

Using the HOLD service feature an existing 

connection in the exchange can be “held”. 

In practice the following case can occur: A subscriber 

has made a phone call in the course of 

which questions arise and remain open. Consequently 

it would be useful to confer with another 

subscriber without terminating the present 

connection. This can be achieved using the 

service feature - Hold: by means of the particular 

operating procedure for the telephone the 

currently active connection is transferred to the 

exchange. The subscriber can then make anoth- 

er call and retrieve the “held” call from the exchange 

when the second call is finished. The 

subscriber has a total of 3 minutes to retrieve the 

original call. The charge metering continues for 

the call on “hold”. The other subscriber is informed 

that the call has been put on hold by a 

message from the network operator coming by 

way of the D-channel. 

2.4 Remarks on the service features 

All service features are strictly defined regarding 

their precise technical procedure. The corresponding 

specifications for the subscriber/network 

interfaces (S 0 and S 2 M) are binding. However, 

this does not apply for the internal 

interfaces to PBX systems. 

With the aid of suitable ISDN measurement 

technology the technician can at any time establish 

the functionality and/or availability of service 

features. For the technician it is indispensable 

to know the abbreviations, attributes and 

protocol procedures of the service features. 

A user is in a somewhat different situation. This 

is because there are no guidelines regarding 

how the user operation of a terminal device is 

supposed to be accomplished, what text should 

appear on the telephone display or PC monitor 

or, for that matter, the price for which a network 

operator should offer service features. One simple 

example from real life is as follows: A person 

uses a telephone made by a particular manufacturer 

to phone another subscriber whose 

line happens to be busy. The caller not only 

hears the busy signal but sees four highly explanatory 

letters on the display of his telephone: 

CCBS. Now he can take the necessary action – 

if only he knew what CCBS meant! 

More detailed information on Euro-ISDN service 

features are contained in the manual “Euro- 

ISDN for installation and service technicians”.


3 Installation techniques at the basic access terminal 

3.1 Interfaces 

Interface between NTBA 

and terminal device 

Subscriber's connection 4-wire 

Interface between NTBA 

and exchange 

Exchange connection, 

network side 

S 0 interface 

U k0 interface 

2-wire 

For the basic access connection in the point-tomultipoint 

configuration the following applies: 

• The NTBA is connected to the 230 V mains. 

• The NTBA can supply four terminal devices 

with voltage. 

• A maximum of 12 ISDN sockets can be installed. 

• A maximum of 8 standard ISDN terminal 

devices may be connected. 

• Operation of an additional 4 terminal devices 

with the service feature X.31 (Datex-P on 

the D channel) is possible. 

• There are two Β-channels available for the 

terminal devices. 

• The installation cables used correspond to 

DIN 0815. 

• A short passive bus is guaranteed to function 

up to maximum line length of 150 m. 

• For an extended passive bus the line length 

can be max. 500 m. Here the terminal devices 

must be arranged on a stretch from 30 to 

50 m at the end of the bus. 

For the basic access connection in point-topoint 

configuration the following holds true: 

• The NTBA is not connected to the 230 V 

mains. 

• The PBX system can be connected directly 

to the NTBA or to an ISDN socket, which 

may be installed additionally. 

• The number of terminal devices depends on 

the size of the PBX system. 

• The installatiοn cable used corresponds to 

DIN 0815. 

• The line length between the NTBA and the 

PBX system amounts to max. 1000 m. 

• The PBX system has two B-channels available 

from one basic access terminal. 

• There are PBX system which can be operated 

with several basic access connections 

(NTBAs) to the network. 

3.2 Network terminal basic access 

NTBA 

The NTBA constitutes a point of interface between 

the network operator and the subscriber. 

The NTBA is connected to the switching centre 

or exchange via the 2-wire U k 0 interface. There 

is a 4-wire S 0 interface available on the subscriber 

side for the connection of terminal 

equipment or PBX systems. The network terminal 

device performs an interface conversion 

from 2-wire to 4-wire, which includes adjustment 

of code, frames and voltage. 

3.3 Power supply 

In the case of the point-to-multipoint configuration, 

the NTBA is operated using the 230 V 

mains. The power supply is needed for its own 

diverse tasks and for feeding up to four terminal 

devices which do not have their own power 

supply (Note: these are normally telephones). 

The power feed to the terminal equipment is 

performed using a phantom circuit like those 

frequently used in a similar fashion in analog 

networks, e.g. when phantom circuits are used 

for multiple line utilization. 

3.4 Standard operation 

It is determined that the positive feed voltage of 

40 V is supplied to the terminal devices via the 

cable pair a2/b2 and the negative voltage via 

a1/b1. According to standard specification 

power consumption is limited to a maximum 1 

W per device in standard operation. The NTBA 

supplies power feed totalling a maximum of 4.5 

W. If you also take the losses on the installation 

line into consideration, a total of four telephones 

having no power supply of their own 

can be operated. 

3.5 Emergency operation 

If the power supply at the subscriber's end fails, 

an emergency power switchover is performed 

17

18 


Fig. 3.6-1: General circuitry of the S 0 bus 

in the NTBA. In this case, the central exchange 

takes over the function of supplying power for 

one telephone, which the subscriber has set up 

with emergency power authorization. Only 

those phones authorized for emergency power 

recognize the polarity reversal on the S 0 bus and 

can continue to perform the basic telephoning 

function. 

3.6 Connection of the S 0 bus 

The S 0 bus consists of the two cable pairs DA1 

and DA2. Seen from the terminal device the 

DA2 wire is used for the transmitting direction 

(Tx) and the DA1 wire is for the receiving direction 

(Rx). The feed to the ISDN telephone is 

carried out from the network terminal by the 

phantom circuit. Each pair of wires is to be ter- 

Fig. 3.7-1: Circuitry of a private digital exchange 

= 

= 

minated in the last socket with a 100 Ohm resistor. 

The termination resistors in the NTBA are 

installed by the manufacturer. 

3.7 Installation for the basic access – 

point-to-point 

Depending on the type of cable used the entire 

length of the cable installation may not exceed 

600 m to 1000 m between the NTBA and the 

ISDN socket. The lines are to be terminated 

with a resistor of 100 Ohm. In the point-to-point 

configuration only one terminal device can be 

operated at a time (e.g. a digital exchange or a 

multiple terminal). The connection cable between 

the terminal device and the ISDN socket 

may not exceed 10 m. The NTBA is not connected 

to the 230 V mains. 

a 

b


Fig. 3.8-1: short passive bus, variant a) 

3.8 Installation for basic access – 

point-to-multipoint configuration 

3.8.1 Short passive bus 

If all ISDN sockets starting from the NTBA are 

installed in one direction, the entire length of 

the installation cable between the NTBA and 

the last socket may not exceed 150 m. A total of 

12 sockets can be installed at any location on 

the entire section, whereby the lines are all terminated 

with 100 Ohm resistors. In addition to 

the 8 standard ISDN terminal devices permitted 

you can also connect 4 terminal devices with 

the service feature X.31 (Datex-P on the Dchannel). 

The entire length of cord between the 

terminal device and the ISDN socket may not 

Fig. 3.8-2: Short passive bus, variant b) 

exceed a length of 10 m. The NTBA is connected 

to the 230 V network. 

If the NTBA is installed within the S 0 bus, the 

total length of the installation cable may also not 

exceed 150 m. These lines must be terminated 

at both ends with 100 Ohm. The 100 Ohm resistors 

in the NTBA remain unaffected. Otherwise 

the rules pertaining to variant a) described previously 

apply. 

3.8.2 Extended passive bus 

When the passive bus is extended, the entire 

length of the installation cable – depending on 

the cable type being used – between the NTBA 

and the last ISDN socket is between 300 m and 

NTBA 

19

20 


Fig. 3.8-3: Extended Passive Bus 

. . . . . . 

500 m. The lines are to be terminated with 100 

Ohm resistors. The maximum permissible 8 

ISDN sockets may be installed at any given location 

on the last 30 m to 50 m of the bus. The 

NTBA must be connected to the 230 V mains. 

3.9 Universal telecommunications socket 

UAE 

The universal telecommunications socket (UAE 

in German) has established itself as the most 

popular ISDN connector. 

The universal telecommunications socket can 

be used as a 6-pin socket for the connection of 

analog terminal devices and as an 8-pin socket 

for the connection of ISDN terminal equipment. 

Should the telecommunication socket be used 

for ISDN terminal equipment, the adapter elements 

originally placed in the sockets must be 

removed. 

3.9.1 The universal telecommunications 

socket UAE 8 (8) 

8 ð Socket with 8-pin connector terminal 

(8) ð 8 assignable terminal contacts 

ð one ISDN terminal device 

NTBA 

Fig. 3.9-1: Circuitry of the UAE 8 (8) connector 

3.10 Wiring errors 

In practice there is a possible wiring error that not 

only leads to disturbances on the bus but which 

simultaneously can drive the technician crazy. 

Here we are referring to accidentally reversing 

the wires 2a and 2b (3 and 6 = transmission direction 

TE → NT) at the basic access point in the 

point-to-multipoint configuration. This wiring 

error is only noticeable when more than one terminal 

device is connected, whereby at least one 

terminal device is in operation in front of and one 

behind the place where the wires have been reversed. 

The wire reversal 2a and 2b leads to the 

S 0 bus becoming inoperable, as no further transmission 

is physically possible.


4 The OSI reference model 

The OSI reference model (OSI = Open System 

Interconnection) was developed by the ISO (International 

Standardisation Organisation), a 

body which develops and publishes worldwide 

standards for all disciplines including telecommunications 

technology. 

The OSI reference model is a conceptual model 

which is derived from computer engineering and 

aims to guarantee compatibility between systems 

that conform to its specifications. It divides the 

process of communication by means of telecommunications 

technology into seven layers, each 

of which is responsible for a certain task within 

the communication process. 

The OSI model is arranged hierarchically and is 

represented in one of two ways: 

a) Layers 5 to 7 are related to the application 

and are more fully specified in the application 

protocol. Layers 1 to 4 are transport-oriented 

and are concerned with the actual data 

transfer. The relevant processes are described 

in transport protocols. 

b) Layers 4 to 7 relate to the terminal device, 

i.e. the tasks these layers represent are performed 

by the terminal device. Layers 1 to 3 

are network-oriented, i.e. their part of the 

process is accomplished by the network itself. 

The entire process of communication between 

ISDN equipment and an exchange is described 

by layers 1 to 3. All the necessary information 

for the connection and disconnection of an 

ISDN link is mediated at this level. This is described 

more fully in the following sections. 

4.1 Layer 1 

Layer 1 (physical layer) is responsible for the 

physical data transfer between the terminal device 

and the network termination/exchange. It 

is used by both B and D channels. The specification 

for layer 1 is in CCITT recommendation 

I.430. 

An example of an interface that corresponds to 

layer 1 is the S 0 interface between that terminal 

device and the network terminal for basic access 

or the U k0 interface between the network 

termination and the exchange. 

The most important tasks performed in layer 1 

include: 

• Activation of a physical connection between 

TE and NT/exchange 

• Deactivation of the physical connection between 

TE and NT/exchange 

• Transparent data transfer (of D and B channel 

data) 

In the ISDN training system, the directions are 

confined to connections between a terminal device 

(TE) and the network termination (NT), i.e. 

the S 0 interface. 

4.2 Layer 2 

Layer 2 (Data link layer) belongs to the D channel 

and is responsible for ensuring security of 

D-channel information transferred between the 

terminal device and the exchange. This security 

function does not only involve control of the 

frame and the number of transmitted bits but 

also the monitoring of layer-3 messages as they 

are sent or received. 


include: 

• synchronization of frames 

• error checking 

• transmission of acknowledged and unacknowledged 

messages 

• procedures for the assignment of addresses 

• establishment and ending of layer-2 connections 

21

22 


4.3 Layer 3 

Layer 3 (Network layer) is responsible for the 

actual exchange of information between the terminal 

device and the exchange and thus takes 

on most of the work in establishing and ending 

connections and activation/deactivation of features. 

It also administers the B-channel and 

routes connections as appropriate. Layer 3 messages 

are then transported and made secure in 

layer 2. 


include: 

• Establishment and ending of connections 

between the terminal device and the exchange 

• Activation and deactivation of features 

• Transmission of information from the terminal 

device and subscribers Fig. 4.3-1: OSI reference model 

L7 Layer 7 Application Layer 

L6 Layer 6 Presentation Layer 

L5 Layer 5 Session Layer 

L4 Layer 4 Transport Layer 

L3 Layer 3 Network Layer 

L2 Layer 2 Data Link Layer 

L1 Layer 1 Physical Layer 

5 S 0 interface 

5.1 Logical structure of the S 0 interface 

In terms of the OSI reference model, the S 0 interface 

corresponds to the physical layer, layer 1. 

Apart from making the physical communications 

medium (copper wires) available, layer 1 

also manages important tasks related to secure 

data transfer, e.g. transmission of data on the B 

and D channels, frame synchronisation, control 

of access to the D-channel and the activation of 

the transmission frame. In a primary multiplexing 

terminal, the S 2M interface fulfils a similar 

role. With a basic access terminal this job is 

handled by the S 0 interface 

The S 0 interface's physical structure consists of 

a four-wire bus connected via transformers at 

both the receiving and transmitting ends. Two 

wires are available for transmitting in either direction 

and data is transferred one bit at a time. 

This kind of communication is called serial data


transfer. 

The logical structure allows each end terminal 

three-fold access to the S 0 interface: 

Fig. 5.1-1: Logical structure of the S 0 bus 

5.2 S 0 frame 

Frame pulses NT ð TE 

F Frame bit (synchronizes the start of a frame) 

L DC-bias compensation bit (parity bit) for frame 

bit F 

B1, B1 Base channels 

E Echo of the D channel information from the TE 

D D-channel information from NT ð TE 

A Indicates the synchronous state of the NT 

FA Additional frame bit (hitherto set to “0” ) 

S1, S2 Reserved bits for future use (hitherto set to “0”) 

Fig. 5.2-1: Structure of S 0 frames 

5.3 Structure of the S 0 frame 

The S 0 frame consists of a total of 48 bits, whose 

order and purpose are precisely defined. The 

structure differs depending on the direction of 

transmission (TE → NT or NT → TE cf. S 0 

frame structure). The transmission time for each 

Every terminal has access to the two user channels 

B1 and B2 which support a transfer rate of 

64 kbits/s and to the D-channel which operates 

at 16 kbits/s. The B-channels are responsible for 

subscriber information (e.g. voice or data) 

while the D-channel is primarily concerned 

with controlling the establishment of a connection 

and the activation or deactivation of features. 

With the basic access terminal, the S 0 interface 

makes available a synchronous, bit-transparent 

and precisely defined layer-1 data transfer 

structure for all traffic on the B and D-channels. 

This is the S 0 frame. 

Frame pulses TE ð NT 

F Frame bit (synchronizes the start of a frame) 

L DC-bias compensation bit (parity bit) 

– 1 bit for frame bit F 

– 1 bit for each group of 4 

Information octets (B1/B2) 

– 1 bit for each of the four 4 D-channel bits 

– 1 bit for the additional frame bit FA 

B1, B1 Base channels 

D D-channel information from TE ð NT 

A Indicates the synchronous state of the NT 

FA Additional frame bit (hitherto set to “0”) 

frame is defined to be 250 µs, corresponding to 

about 2.51 µs per bit. The voltage values representing 

the logical states for a bit are based on a 

modified version of AMI code. 

An overview of some of the most important bits 

in the S 0 frame: 

23

24 


F bit 

The F bit is the one that marks the start of every 

S 0 frame. For this reason it only appears once in 

the frame and is important for synchronization 

of the frame since every terminal device has to 

correctly detect the beginning of the frame in 

order to engage in communication. 

L bit 

The L bit is the so-called DC-compensation bit. 

Its job is to ensure that its portion of a frame and 

thus the S 0 frame as a whole is free of any DC 

bias, since the line transformer cannot handle 

DC levels. 

B1, B2 bit 

These bits form the B1 and B2 channels and are 

responsible for the transmission of subscriber 

data. The S 0 frame transmits 16 B1 and 16 B2 

bits, each divided into two blocks of 8 bits. 

D bit 

The D bit transports the D channel information 

from TE → NT or from NT → TE. There are four 

D-channel bits in each S 0 frame. The D-channel 

protocol requires octets (an octet is 8 bits) but, as 

the S 0 frame contains only 4 D bits, only ½ an 

octet can be transmitted in each one of them. 

Note: 

One speaks of octets in ISDN because it is a bitoriented 

protocol. Bit-oriented means that every 

bit has a distinct pre-defined meaning, usually 

specified in a table. A byte-oriented protocol 

is one where separate meanings are assigned to 

whole bytes. 

E bit 

The echo bit exists so that D bits received at the 

network terminal are taken up and echoed back 

to the bus. Terminal devices can, for example, 

read the E bits to determine if the D channel is 

free. A device can also use the E bits to determine 

whether it really is connected with the D 

channel. The echo channel is thus an aid to access 

control for the D channel. 

5.4 S 0 interface code 

The selection of an appropriate basis code was 

made with a view to ensuring, as far as possible, 

that the most important requirements were cov- 

ered, such as timing recovery, absence of DC 

bias, synchronization and error limiting. The 

result of this selection process was the choice of 

modified version of AMI coding (AMI = Alternate 

Mark Inversion) as the template for the S 0 

interface. 

This modified AMI code specifies the following 

voltage values for the logical states: 

logical “1” is transmitted as 0 Volts 

(zero 

voltage) 

logical “0” is transmitted as a positive or 

negative 

voltage 

In addition, the following coding rules apply to 

transmission via the S 0 interface: 

• The first logical “0” of each portion of a 

frame is always a logical zero of negative 

voltage (note each frame portion ends with 

an L bit after which the following portion of 

the frame begins). 

• If a logical “0” of negative voltage is transmitted, 

the next zero must be transmitted 

with positive voltage and vice-versa (this 

applies to the whole S 0 frame) 

• Every portion of the frame has to be free of 

DC bias 

5.5 Violations of the coding rules 

The very first bit of the frame, the F bit, breaks 

the coding rules. The F and L bits form a partial 

frame. The coding rules state that the first logical 

zero of a partial frame should always be of 

negative voltage, yet the F bit is defined as a 

logical “0” of positive voltage. The L bit which 

serves to ensure that a partial frame is free of 

DC bias is specified to be a logical “0” of negative 

voltage as expected. 

The initial coding violation involving the F bit 

always leads to a second. This occurs within the 

first 14 bits of the S 0 frame, at the latest by the 

time the FA bit is reached. This is specified to be 

a logical “0” of negative voltage. Even if all the 

B1, D and L bits in the second and third parts of 

the frame are both logical “1” at zero voltage, 

the FA bit always necessarily causes the second 

violation. 

These violations of the coding rules allow a terminal 

device to determine the start of an S 0


frame with certainty so that they can make sure 

they are synchronized. 

5.6 Control of access to the D channel 

In the case of a point-to-point connection, both 

layer 1 and layer 2 are always active. The PBX 

system is the only equipment accessing the D 

channel via the S 0 bus so that it does not have to 

compete with other transmitters. 

However, when a basic access terminal is in 

point-to-multipoint configuration (multiple-device 

connection), up to eight devices may be 

connected to the bus and all these have to share 

the D channel. This also applies to additional 

devices running the service “Datex-P on the D 

channel” (X.31). 

The S 0 bus operates on the principle of equality 

for all. No terminal device may interfere with 

the D-channel data of another. The D-channel 

may only be used by one device at a time. All 

devices must have equal priority to access the D 

channel. 

6 Layer 1 

When the basic access terminal is in point-topoint 

configuration (single device connection) 

this activation takes place when the PBX system 

is switched on. Thereafter both layer 1 and 

layer 2 remain active, even if neither of the B 

channels nor the D channel is in use. Disconnection 

of layer 1 usually only occurs in the 

case of a fault or if the equipment is intentionally 

switched off. 

If the basic access terminal is in point-tomultipoint 

configuration (multiple device connection), 

it is placed into an idle state when 

there is no activity on the D or B channels. This 

idle state is equivalent to the logical “1” state (= 

zero voltage). In this case, there is no S 0 frame 

in existence so that no data transfer between TE 

and NT can take place. Before a subscriber can 

establish a connection or the exchange relay a 

call when the equipment is in this state, layer 1 

must first be activated. 

For this purpose, D-channel access control was 

introduced. 

All devices connected to the S 0 bus read all the 

data transmitted from NT → TE on the echo 

channel (E bits). Devices can determine when 

and whether the D channel is free by counting 

logical “1” (0V) on the echo channel. The following 

number of bits must be counted by various 

devices before they can count on the Dchannel 

being free: 

Terminal devices 

with high priority 8 x logical “1”s, 

Terminal devices 

with low priority 10 x logical “1”s, 

Standard ISDN devices are usually high priority 

whilst X.31 devices are given low priority so 

that they do not block the D-channel. 

When a terminal device has counted 8 logical 

“1”s, it knows that the D-channel is free and 

that it may transmit. 

In order for layer 1 (Physical layer) to transfer Band 

D-channel data from TE → NT or vice versa, 

there has to exist an S 0 frame of the correct structure, 

i.e. layer 1 has to be activated. 

Fig. 6.1-1: Layer 1 activation procedure 

25

26 


6.1 Activation procedure for layer 1 

If no data is being sent on either the D channel 

or the B channels, the S 0 interface remains in its 

idle state. This is characterized by a constant 

logical “1” (zero voltage) which the layer-1 

specification defined as an INFO S 0 message. 

If a subscriber wishes to establish communication, 

their terminal device initiates the activation 

procedure by sending the INFO S1 message. 

INFO S1 consists of a repetition of the binary 

sequence, 00111111 (positive, negative, 

zero voltage) at a nominal bit rate of 192 kbit/s. 

This acts as a kind of wake-up call to the exchange. 

When the exchange receives INFO S1 it replies 

with an INFO S2 message. INFO S2 is a complete 

S 0 frame with all bits in the B, D and E 

channels set to logical “1”. Thus the network 

terminal sends a signal which constantly changes 

from positive to negative voltage at a rate of 

192 kbit/s. INFO S2 is so designed that the terminal 

device receives a large number of pulses 

to which it can synchronize. 

The INFO S3 message allows the terminal device 

to convey its synchronous state to the exchange 

via the network terminal. INFO S3 is a 

complete S 0 frame which prepares the B and D 

channels to accept data. 

The exchange confirms the synchronisation 

and the correct frame structure by transmitting 

INFO S4. INFO S4 is also a complete 48-bit 

frame. 

Note: 

The network termination (NT) has the task of 

implementing layer-1 activation of the S 0 interface 

on the U k0 interface and vice versa (code 

conversion). 

After layer 1 has been connected, this is conveyed 

to layer to 2 where the necessary steps 

are taken (cf. layer 2). Only after communication 

has been established on layer 2 can layer 3 

initiate the connection requested by the subscriber, 

e.g. by transmitting the dialed number 

to the exchange. 

6.2 Deactivation procedure for layer 1 

In the point-to-multipoint case, layer 1 is deactivated 

again after the last device has finished its 

Fig. 6.2-1: Layer 1 deactivation procedure 

activity on the bus. This deactivation can only 

be managed by the exchange since only there is 

it known whether other terminal devices on the 

bus are still active. 

Deactivation of layer 1 is achieved by the transmission 

of an INFO S 0 signal (constant “1“ – 

zero voltage), i.e. there are no further S 0 frames 

in existence. The terminal device also ceases 

transmission of its S 0 frame and confirms deactivation 

by sending INFO S 0 . Thus the S 0 interface 

is returned to its idle state. 

6.3 Reactivation 

After the S 0 bus has returned to its idle state, outgoing 

and incoming connections can only be 

established if layer 1 is activated again (see activation 

procedure). If the exchange needs to relay 

an incoming call to a subscriber, then it has 

to initiate the activation of layer 1. As the direction 

of the INFO messages is fixed, the activation 

in this direction begins with the transmission 

of the INFO S2 frame. 

6.4 Measurement techniques and testing 

of layer 1. 

Test devices used for the installation and troubleshooting 

of ISDN equipment use two different 

ways of representing the activation of layer 

1. The activation is displayed either by means 

of LEDs or the INFO messages themselves are 

displayed on a device display or on a PC 

equipped with appropriate decoding software. 

This enables any possible loss of synchronisation 

on the line to be easily identified.

7 Layer 2 


7.1 Functions of layer 2 

Layer 2 (Security layer) is managed by the D 

channel and is primarily responsible for the secure 

transmission of D-channel data between 

terminal devices and the exchange. 


include: 

• establishment and ending of layer-2 connections. 

The assignment and administration of 

layer-2 addresses 

• transport of layer-3 messages in layer-2 

frames 

• monitoring the integrity of each individual 

frame 

• controlling the flow of messages 

• error checking by means of a check sum 

• error handling in the case of aberrations from 

the above 

7.2 Structure of layer 2 

The format of layer 2 corresponds essentially to 

the fundamentals of the HDLC protocol (High 

Level Data Link Control), where data is conveyed 

to the application level in packets. ISDN 

includes some specialized adaptations of the 

original HDLC protocol. For this reason, ISDN's 

layer-2 protocol is designated LAP-D (Link 

Access Protocol for D-Channel). It is described 

in the CCITT recommendations I.440 and I.441 

and has been incorporated into the ETS standard. 

These rules stipulate that all D-channel data 

should be packaged into frames. Every frame 

begins and ends with a specified bit pattern, the 

begin and end flags of layer 2. The D-channel 

data to be transmitted is located between the 

beginning of the frame (the begin flag) and its 

end (the end flag). The data is gathered into oc- 

tets (1 octet = 8 bits). The sequence of these 

octets is firmly specified (cf. structure of layer 2 

and structure of layer 3). The meaning and 

structure of each octet is also defined in specific 

detail. 

All information belonging to layer 2 is transmitted 

as “1/0” sequences - as it is for layer 3. The 

meaning of each bit or bit group is precisely 

defined in tables. These tables contain not only 

the logical role played by each bit but also a 

description of that role or the meaning of an individual 

bit. 

7.3 Layer 2 formats 

Every D-channel message consists of frequently 

repeated protocol elements: the start flag that 

specifically marks the start of a protocol message, 

the layer-2 address consisting of two octets, 

the layer-2 contents field that can cover 

one octet or two, the two-octet check sum and 

the end flag that marks when the protocol message 

is over. 

Fundamentally, layer 2 is aware of three formats, 

the U format, the S format and the I format. 

7.3.1 The U format 

U frames carry layer-2 or layer-3 messages as 

unnumbered frames. If a frame is lost during 

transmission, neither the exchange nor the terminal 

device can detect it. 

There are three main types of frame in the U 

format (U = unnumbered): 

1. The U format is used in establishing and ending 

connections in layer 2 and for error messages. 

The frames then take the above format 

Octet 1 Octets 2 & 3 Octet 4 Octets 5 & 6 Octet 7 

Start flag 

Layer 2 

Address 

Layer 2 

Contents 

Check sum End flag 

In this case U frames only contain layer 2 information. 

27

28 


2. The U format is also used for the assignment 

of addresses (TEI management): 

Octett 1 Octets 2 & 3 Octet 4 Octets 5 to 9 Octets 10 & 11 Octet 12 

Start flag 

Layer 2 

Address 

Layer 2 

Contents 

In addition to the prescribed protocol elements 

(start and end flag, contents and check sum) this 

U frame transmits information pertaining to socalled 

TEI management (cf. address designa- 

3. The U format can contain layer-3 information 

in some cases 

In the case of a communication process in 

which a device has not been recognized as 

capable of supporting secure data transfer, 

then layer-3 messages will be sent without security 

information. Example: the exchange 

7.3.2 The S format 

The S format (S = supervisor) is used when the 

receipt of a layer-3 message is to be confirmed 

or when necessary to communicate that a certain 

layer-3 message needs to be retransmitted. 

S frames are also used for the continuous con- 

TEI Management Check sum End flag 

tion procedures) in octets 5 to 9, e.g. querying 

for a layer 2 address or the assignment or refusal 

of such an address. 

Octet 1 Octets 2 & 3 Octet 4 Octets 1 - 260 Octets 5 und 6 Octet 7 

Start flag 

Layer 2 

Address 

Layer 2 

Contents 

7.3.3 The I format 

The I format (information) is used for the numbered 

transmission of layer-3 messages. The 

layer-3 message is transmitted between layer 2's 

contents field and the check sum. 

Layer 3 Check sum End flag 

wishes to signal an incoming call to a terminal 

device. As the device to receive the call is not 

yet recognized, the layer-3 information is 

transmitted to the bus in an unnumbered layer- 

2 frame. 

trol and monitoring of an existing connection. 

With single-device terminals (Basic access terminal/point-to-point 

and primary multiplex terminals) 

the permanently active layer-2 connection 

between the PBX system and the exchange 

is maintained by the exchange of S frames. 

Octet 1 Octets 2 & 3 Octets 4 & 5 Octets 6 & 7 Octet 8 

Start flag Layer 2 Address Layer 2 Contents Check sum End flag 

The S frame consists entirely of layer-2 elements. 

As opposed to unnumbered transmission of layer-3 

messages in U frames, the transmission of 

layer-3 data in an I frame is secured by the inclusion 

of transmit and receive sequence 

counters in octets 4 and 5. 

Octet 1 Octets 2 & 3 Octets 4 & 5 Octets 1 - 260 Octets 6 & 7 Octet 8 

Start flag 

Layer 2 

Address 

Layer 2 

Contents 

Layer 3 Check sum End flag


7.4 Overview of layer 2 

Fig. 7.4-1: Overview of layer 2 

This illustration shows an overview of all the 

layer-2 octets. Among the most informative 

components for technicians are the SAPI and 

TEI protocol elements. 

7.5 Protocol elements in layer 2 

7.5.1 SAPI - Service Access Point Identifier 

The SAPI element is always contained in the 

second octet of the layer-2 frame and consists 

of 6 bits. The SAPI affirms the function for 

which layer 2 is being used at any given time. 

The specification currently defines a total of 

four SAPI values, corresponding to four different 

functions. 

SAPI- 

Value Meaning 

0 

16 

32 

63 

Call Control Procedure “Signaling”: is 

used for all layer-2 connections used for 

the exchange of messages on 

layer 3 

Datex P on the D channel (X.31): 

indicates that the D channel should 

be used for packet oriented transfer 

Loop test: allows the exchange to 

test the line up to and including the 

network termination 

TEI management procedures: used 

for all layer-2 address assignment 

procedures 

all other values are reserved for future 

applications and may not be put to use 

Notes: 

1. SAPI values are decimal numbers. 

2. When an ISDN test device is used to decode 

the D-channel protocol, the octets 

containing the start flag, the end flag and the 

check sum (FCS) are not usually displayed! 

The most common values to appear as SAPI 

values when the D channel is monitored are 0 

and 63. With the help of these values it can be 

determined whether a message is concerned 

with TEI management or if a terminal device is 

already engaged in establishing a connection. 

7.5.2 TEI – Terminal Endpoint Identifier 

Every D-channel message has to include a layer-2 

address. This is called the TEI = Terminal 

Endpoint Identifier. It consists of 7 bits and is 

contained in the 3rd octet of the layer-2 frame 

(see frame structure). 

In regard to layer-2 addresses there is a vast difference 

between a point-to-point connection 

and a point-to-multipoint connection. In the 

former (basic access terminal and primary multiplex 

terminal), the TEI is given the fixed value 

0 by both the equipment and the exchange. This 

is adequate to allow perfect communication on 

the D-channel because there is only one transmitter 

or receiver on either side of the connection. 

With the multiple-device connection (basic access 

terminal – point-to-multipoint configuration), 

however, the exchange can have up to 

eight standard ISDN terminal devices at the 

subscriber end with which it might be communicating. 

At such a terminator, the assignment 

of layer-2 addresses is managed by having 

every terminal device contact the exchange for 

a TEI whenever it is plugged in or switched on 

(automatic TEI). The device is only able to 

work via the ISDN connection after the TEI 

has been assigned by the exchange. The TEI 

assigned to the device must then be included 

in every D-channel message. A device keeps 

this TEI until it is removed from the bus, until 

the power is switched off or until the exchange 

has a reason to deassign the TEI. 

For X.31 terminal devices (packet-oriented 

transmission on the D channel) layer-2 addresses 

are handled rather differently. After 

the customer has registered the device with 

their network provider, a TEI is supplied by the 

provider which the customer must manually 

set the device by hand. This TEI then remains 

fixed (non-automatic TEI). 

29

30 


The specification defines the following TEI values: 

TEI- 

Value Meaning 

0 

0 - 63 

64- 

126 

reserved for point-to-point configuration 

(single device connection); valid for basic 

rate access and primary multiplex access 

non-automatic TEI; hard-programmed in 

the terminal device 

automatic TEI; assigned by the exchange 

upon request; valid only for the multiple 

device connection (basic rate access) 

127 common TEI, broadcast or group TEI 

Notes: 

1. TEI values are given as decimal numbers. 

2. The general TEI, 127, is always used when a 

definite TEI is not yet known, when all terminal 

devices are to receive a message or 

during the procedure for assigning address. 

7.6 Protocol sequence in layer 2 

7.6.1 TEI management (procedure for 

assigning addresses) 

The first time a terminal device is connected to 

the basic access terminal (in point-to-point configuration) 

it must request a layer-2 address 

(TEI) from the exchange before it can begin the 

actual connection process. To do so it sends the 

layer-2 message “ID request” (ID = identity) to 

Fig. 7.6-1: Procedure for assigning a TEI 

the exchange and waits for an “ID assign” message 

to acknowledge the request and supply the 

assigned TEI value. 

The exchange can also refuse to assign a TEI 

address, e.g. when eight TEIs have already been 

assigned to a single connection and all eight are 

still connected to the bus. If the exchange refuses 

the assignment of an address, it sends an “ID 

denied” message. 

Fig. 7.6-2: Denial of a TEI request 

Within the scope of TEI management there are 

other messages which are used for certain purposes: 

Identity check 

request 

Identity check 

response 

The ID check request message is 

used by exchange to check for 

the existence of a specific TEI 

value or indeed all TEIs on the 

bus. 

ID check response is the 

terminal device's answer to the 

exchange's ID check request 

giving its TEI value. If no 

response is received, the 

exchange deletes this TEI. 

Identity remove When the exchange deletes a 

TEI from its index, it informs 

the terminal devices on the bus 

by means of the message ID 

remove. 

Identity verify 

The terminal device can request 

the exchange to check its TEI. 

To do this, it send the ID verify 

message to the exchange. The 

exchange then initiates the test 

procedure by responding with 

an ID check request message.


All TEI management procedures are protected 

by a time-out. Their structures are gathered together 

in a table. 

7.6.2 Establishing connection from TE → 

NT in layer 2 

Should one subscriber wish to make a call to 

another, the process begins with the raising 

of the telephone receiver. In terms of the OSI 

reference model this transmits a layer-3 

(transport layer) command for the establishment 

of the D-channel connection. This is the 

main purpose of layer 3 in this case. In order 

for the terminal device to send layer-3 messages 

to the corresponding layer at the exchange, 

the security functionality of layer 2 is 

required. In order for these functions to work 

correctly, layer 2 must be synchronized at the 

terminal and the exchange. This synchronization 

is achieved by the establishment of the 

connection on layer 2. This can occur in either 

direction, i.e. TE → NT or NT → TE, depending 

on whether it's an incoming or outgoing 

call. 

Note: 

Prerequisites for the establishment of a connection 

on layer 2 are: 

a) that layer 1 has been successfully activated 

and 

b) that the terminal device has already been assigned 

a TEI. 

When circumstances are positive, the establishment 

of a connection on layer 2 requires the 

exchange of just two messages: 

Fig. 7.6-3: Establishment of a connection on layer 2 TE → NT 

7.6.3 The layer-2 message SABME 

The SABME command (Set Asynchronous Balanced 

Mode Extended) instructs both the terminal 

device and the exchange to set their I and S 

frame counters to the predefined initial state of 

0 and to prepare for secure transfer of data. The 

SABME message itself is sent in an unnumbered 

U frame. 

7.6.4 The layer-2 message UA 

The reply to SABME is UA (Unnumbered 

Acknowledgement ) by which the exchange 

confirms that it is in a defined initial state and is 

prepared for secure transfer of data. The UA 

message itself is sent in an unnumbered U 

frame. 

Establishment of the layer-2 connection between 

terminal device and exchange only follows 

after the UA-message is received. This allows 

the numbered transmission of layer-3 messages 

to begin. 

7.7 I frames in layer 2 

After the layer-2 connection is established, 

layer-3 messages can now be sent in I frames. 

An I frame is a numbered frame which includes 

its transmit and receive counters within 

its structure. The number of layer-3 messages 

transmitted and received is counted so 

that both the terminal device and the exchange 

can determine whether all their messages 

have reached their intended destination. 

7.8 S frames in layer 2 

S frames serve to acknowledge receipt of layer- 

3 messages (I frames). The rule for the basic 

access terminal is that an acknowledgement is 

immediately sent in an S frame containing the 

appropriate receive count to confirm the receipt 

of every layer-3 message. 

There are three message altogether which can 

be sent in the S format: 

31

RR receive 

ready 

RN receive 

not 

ready 

32 


The receiver acknowledges receipt 

of a layer-3 message with a receive 

count and confirms that it is ready 

to receive again. 

The receiver acknowledges receipt 

of a layer-3 message with a receive 

count and informs that it is 

currently unable to accept any 

further D-channel data. Readiness 

to respond can only be 

demonstrated by the further 

transmission of an RR message. 

REJ reject Reject is sent by the receiver when 

a layer-3 message has an incorrect 

count. It requests the transmitter to 

send a specific frame again. 

When a connection has eventually been established 

between the two subscribers, layer 2 has 

the task of ensuring the continuity of the connection 

by means of the D channel for the entire 

period until the connection is ended. This involves 

the regular exchange of supervisor messages 

between the terminal device and the exchange. 

An RR message is sent by the terminal 

device and, when all is well, the exchange replies 

with an RR message of its own. 

7.8.1 Ending of a layer-2 connection from 

TE → NT 

When the subscriber finishes a conversation, by 

hanging up the telephone for example, a command 

is sent instructing layer 3 to end the Dchannel 

connection (cf. Layer 3). When layer 3 

has completed this task, a basic access terminal 

in point-to-multipoint configuration needs its 

layer-2 connection to be ended as well. 

As in the case of establishing a layer-2 connection, 

ending requires the exchange of two messages: 

Fig. 7.8-1: Ending of a layer-2 connection from TE to NT 

7.8.2 The layer-2 message DISC 

The DISC command (DISConnect) severs the 

connection and deactivates layer 2. DISC is an 

unnumbered message sent in U format. 

7.8.3 The layer-2 message UA 

To confirm the ending of the connection and 

the deactivation of the layer, a UA (Unnumbered 

Acknowledgement ) is sent. 

Note: 

After layer 2 and layer 3 have been disconnected, 

as long as there are no other devices connected 

to the bus, the exchange will initiate the 

deactivation of layer 1.

8 Layer 3 


8.1 Functions of layer 3 

Layer 3 is managed by the D-channel – just like 

layer 2. Its main task is the establishment and 

ending of connections and the activation or deactivation 

of features. Some of the most important 

functions of layer 3 include: 

• initiating the establishment of a call connection 

with the exchange (outgoing call) or 

with the terminal device of the subscriber 

who is being called (incoming call), including 

the transmission of telephone numbers. 

• ending of layer-3 connections when the subscribers 

have finished their communication 

or if a connection between them cannot be 

established. 

• management of the B channels, i.e. the assignment 

or enabling of a B channel or the 

transmission of the appropriate message 

when there is no longer a free B channel. 

• transmission of service identifiers required 

by the exchange or the terminal device of 

the party being called to monitor compatibility. 

• activation/deactivation of features, e.g. callforwarding, 

transfer of calls to other bus outlets 

or the display of charging information 

during a call. 

• addressing of layer-3 messages. Since several 

terminal devices can effectively be transmitting 

or receiving D-channel messages at 

the same time, it must be ensured that layer- 

3 messages can be assigned to their proper 

processes. This is achieved by means of layer-3 

addresses. 

• time-out function. For layer 3 too, there is 

a specific time-limit during which replies 

to a message should be received. If this 

period is exceeded an error procedure is 

initiated. 

• monitoring of protocol sequences within the 

layer. The sequence of layer-3 messages and 

the possible actions and responses are predetermined 

for all processes. 

8.2 Structure of layer 3 

Every layer-3 message is embedded in a layer- 

2 frame. The first layer-3 protocol element is 

preceded by the layer-2 header, containing the 

start flag, the address field (SAPI and TEI) and 

the contents field (control field). After the last 

protocol element in layer 3 comes the end of 

the layer-2 frame with the check sum (Frame 

Check Sequence = FCS) and the end flag (cf. 

layer 2). 

Layer-3 messages can be transported in two different 

layer-2 frames. 

a) as an unnumbered frame (U format) if the 

identity of the other party in the communication 

is not yet known. In this case the control 

field only contains one octet as the U frame 

doesn't include layer 2's transmit and receive 

sequence counters. 

Layer 2 Layer 2 Layer 2 Layer 3 Layer 2 Layer 2 

Start flag Address field Control field Message Check sum End flag 

b) as a numbered frame (I format) when the 

identity of the other party is known and secure 

communication is possible. An I frame 

has a two-octet control field with the transmit 

and receive sequence counters. 

Layer 2 Layer 2 Layer 2 Layer 3 Layer 2 Layer 2 

Start flag Address field 

Control field 

(counter) 

Message Check sum End flag 

33

34 


8.3 Sequence of messages in layer 3 

Fig. 8.3-1: Overview of layer 3 

The sequence of messages in layer 3 is precisely 

defined in the specification. Note the following 

basic rules: 

a) Layer 3's so-called “header” consists of four 

different protocol elements which always 

appear in the same order: 

Start of layer 3: Protocol 

discriminator (PD) 

Layer-3 address: Length of call 

Reference, 

Call Reference 

Value (CR) 

Message name: Message Type (MT) 

b) Further messages that describe the contents 

in more detail follow on from these: 

Contents of the message: Information 

elements (IE) 

The Information elements contain, for example, 

information about services, call numbers, 

features, time and date or the reasons for the 

ending or failure of a connection. The length 

of these contents varies according to their 

contents. 

c) The total length of a layer-3 message may 

not be more than 260 octets. 

d) Every layer-3 message has its own table 

which precisely defines which information 

elements are mandatory or optional. 

e) Every layer-3 message is embedded in a 

layer-2 frame. 

8.4 Layer 3 messages 

The designation of a layer-3 message is contained 

in its “header” and is called Message 

Type (MT). The message type indicates which 

layer-3 is involved and the purpose of the message. 

There are basically five types of layer-3 message: 

1. Messages for the establishment of a connection 

All these messages have a specific role in the 

establishment of a connection, from the request 

for a connection using the B channel to display 

and acceptance of the call. 

2. Messages sent during the course of the connection 

It can also be necessary to exchange layer-3 

messages over the course of a connection, e.g. 

when a device is switched to another outlet on 

the bus. 

3. Messages for ending a connection 

When a subscriber ends communication, the 

disconnection process is carried out in full on 

layer-3. There are specific messages for this 

purposes. 

4. Miscellaneous messages 

Other messages communicate important information 

such as enquiries about system states or 

the handling of errors. 

5. Messages for the control of features 

The activation and deactivation of features is 

also accomplished by means of layer-3 messages. 

As well as the service features, which work 

on an active connection (e.g. Hold), there are 

others which can be operated while the connection 

is idle (e.g. forwarding of calls). 

8 7 6 5 4 3 2 1 HEX MT Short description 

0 0 0 0 0 0 0 0 00 Escape Code


Messages for the establishment of connections 

0 0 0 0 0 0 0 1 0 1 Alerting 

Messages sent during the connection phase 

0 0 1 0 0 1 1 0 26 Resume 

0 0 1 0 1 1 1 0 2E Resume Ack 

Requests restoration of a 

connection after suspend 

Acknowledgement/ 

restoration of a connection 

after resume 

0 0 1 0 0 0 1 0 22 Resume Reject Rejection of resume 

0 0 1 0 0 1 0 1 25 Suspend Suspends a transaction 

0 0 1 0 1 1 0 1 2D Suspend Ack Acknowledges suspend 

0 0 1 0 0 0 0 1 21 Suspend Reject Rejection of suspend 

0 0 1 0 0 0 0 0 20 User Info 

Messages for ending a connection 

0 1 0 0 0 1 0 1 45 Disconnect 

User-User messages during 

a connection 

Requests the ending of a 

connection 

0 1 0 0 1 1 0 1 4D Release Ends a connection 

0 1 0 0 1 0 1 0 4A 

Release 

Complete 

0 1 0 0 0 1 1 0 46 Restart 

Confirmation of release 

Initiate layer-3 reset 

(Error handling) 

0 1 0 0 1 1 1 0 4E Restart Ack Confirm layer-3 reset 

0 1 1 0 0 0 0 0 60 Segment 

0 0 1 0 0 0 0 0 20 User Info 

Indicates that a call has 

arrived 

0 0 0 0 0 0 1 1 3 Call Proceeding Retain dialing information 

0 0 0 0 0 1 1 1 7 Connect 

0 0 0 0 1 1 1 1 0 F Connect Ack 

Accept call/connection 

made 

Confirms acceptance of 

call 

0 0 0 0 0 0 1 0 2 Progress Connection information 

0 0 0 0 0 1 0 1 0 5 Setup 

Initiates establishment of a 

connection 

0 0 0 0 1 1 0 1 0 C Setup Ack Acknowledges setup 

Segments of a layer-3 

message 

User-user messages 

during a connection 

35

Other messages 

36 


0 1 1 1 1 0 0 1 79 

Messages for service features 

0 0 1 0 0 1 0 0 24 Hold Hold a call 

0 0 1 0 1 0 0 0 28 Hold Ack Acknowledge Hold 

0 0 1 1 0 0 0 0 30 Hold Reject Reject Hold 

0 0 1 1 0 0 0 1 31 Retrieve Retrieve held call 

0 0 1 1 0 0 1 1 33 Retrieve Ack 

0 0 1 1 0 1 1 1 37 Retrieve Reject 

0 1 1 0 0 0 1 0 62 Facility 

0 1 1 0 0 1 0 0 64 Register 

Tab. 8.4-1: Message Types (MT) – Layer-3 messages 

8.5 Establishing connection from 

TE → NT in layer 3 

When a subscriber A calls another subscriber B, 

there are precisely three connection procedures: 

1. from subscriber A to the exchange (via D 

channel) 

2. from subscriber A's exchange to subscriber 

B's exchange (Central signalling channel No. 

7 = CSC #7) 

3. from subscriber B's exchange to subscriber 

B (via D channel) 

The sequence of D-channel messages between 

the subscribers have to be separately 

Congestion 

Control 

0 1 1 1 1 0 1 1 7B Information 

0 1 1 0 1 1 1 0 6E Notify 

0 1 1 1 1 1 0 1 7D Status 

Flow control for User-User 

service 

Conveys information , e.g. 

dialing information, ... 

Displays status of 

terminal portability 

Indication of state, e.g. 

error conditions 

0 1 1 1 0 1 0 1 75 Status Enquiry Requests an operating state 

Acknowledge/retrieve held 

call 

Reject restoration of held 

call 

Request or assignment of 

feature 

Request for feature - not 

based on current 

connection 

recorded for the purposes of measurement 

and analysed. In the case of a fault, the Dchannel 

data recorded at a location can only 

give a provisional idea of what might be occurring 

at the other end of the connection. An 

outcoming call is designated to be a connection 

established from TE → NT, and an incoming 

call is the establishment of a connection 

from NT → TE. 

The illustration below represents the successful 

establishment of a connection and shows the sequence 

of D-channel messages for both ISDN 

subscribers.


Fig. 8.5-1: Successfully established connection 

8.5.1 Meanings of layer-3 messages during 

establishment of a connection 

SETUP The SETUP message is sent 

when the subscriber or the exchange 

wishes to establish a connection 

using the B channel. 

SETUP ACK SETUP ACKNOWLEDGE indicates 

to the receiver, that the request 

to connect itself has been 

accepted; however, additional 

information (e.g. dialing codes) 

is necessary to complete the connection 

INFO The INFORMATION message 

contains the information required 

to complete the establishment 

of the connection. 

CALL PROC CALL PROCEEDING indicates 

that the dialing information has 

been recognized as complete 

and the connection process has 

been initiated. No further information 

will be accepted. 

ALERT ALERT signals that the dialed 

subscriber's terminal equipment 

is ready to take a call (“it starts 

ringing”). 

CONN CONNECT acknowledges that 

the subscriber has accepted the 

call (e.g. by lifting the telephone 

receiver). 

CONN ACK CONNECT ACKNOWLEDGE 

is sent to acknowledge the receipt 

of CONNECT. 

Every layer-3 message has a defined meaning 

and can only be used for the function ascribed 

to it. 

8.6 Ending of a layer-3 connection 

from TE → NT 

If the caller terminates the connection, as in the 

following example, then it is the caller's terminal 

device that initiates the ending of the layer- 

3 connection on the D channel. 

Fig. 8.6-1: Ending of an active connection. 

8.6.1 Meanings of layer-3 messages during 

ending of a connection 

Only three messages are sent during the ending 

of a connection: 

DISC DISCONNECT sends a request 

the “D-channel partner” to end 

the connection 

REL RELEASE indicates that the transmitting 

device has switched off its 

channel-B connection and released 

both the B channel and the 

Call Reference. 

The transmitter requests its “Dchannel 

partner” to do the same. 

REL COM RELEASE COMPLETE lets the 

transmitter know that the B channel 

and call reference have indeed 

been released. 

37

38 


8.7 Information elements (IE) 

on layer 3 

8.7.1 Special features of IEs 

Information elements are the actual detailed 

content of a layer-3 message. The use of information 

elements is either mandatory or optional 

depending entirely upon the type of message 

and the direction in which it is being transmitted. 

It is therefore important to know whether a 

message is being sent from the network to a 

subscriber (n ð u) or from the subscriber to the 

network (u ð n). When there are information 

elements in a message, they directly follow the 

octet Message Type (MT). 

8.7.2 Formats of IEs 

An information element normally consists of 

three components, name, length and content. In 

Example a) 

Example b) 

In variable length information elements, the extension 

bit (bit 8 of the first octet) is set to 0. 

Here the following rules apply: 

• If the extension bit is 0, then it is a variablelength 

information element. 

• The remaining seven bits of the first octet 

Example c): Information element of variable length 

some cases there are no contents when the message 

type contains in itself all the necessary information. 

The information element has two different formats: 

• Information elements the length of a single 

octet 

• Information elements of variable length 

For single-octet information elements, bit 8 is 

set to 1. This bit is the so-called “extension bit”, 

a mechanism which allows the receiver to recognize 

whether or not there are further octets in 

the present group. If the extension is set, there 

are no further octets. 

In a single-octet information element the extension 

bit signalling the length is followed by the 

name and any contents which fit into the single 

octet. 

contain the name of the information element. 

• The second octet contains the length of the 

message. 

• The length is the number of octets following 

the length-octet (not including it).


There are also further rules for variable-length 

information elements which should be noted 

• The contents of an information element itself 

can be subdivided into individual octets or 

octet groups 

• Each octet group is a separate entity. 

8.7.3 An overview of the most important information elements 

• The sequence of octets within an information 

element is predefined and may not be 

altered. 

• A table indicates whether an octet or octet 

group is mandatory or optional for an information 

element 

8 7 6 5 4 3 2 1 Hex IE Meaning 

1 0 1 0 0 0 0 0 A0 

0 0 0 0 0 1 0 0 04 

Sending 

complete 

Bearer 

capability 

Call number complete 

0 0 0 0 1 0 0 0 08 Cause Error cause 

0 0 0 1 1 0 0 0 18 

Channel 

identification 

Transmission capacity of 

network 

Channel information 

0 0 0 1 1 1 0 0 1C Facility Control of features 

0 0 1 0 1 0 0 1 29 Date / Time Datum / Zeit 

0 1 1 0 1 1 0 0 6C 

0 1 1 1 0 0 0 0 70 

0 1 1 1 1 1 0 0 7C 

0 1 1 1 1 1 0 1 7D 

0 1 0 0 1 1 0 0 4C 

8.8 Call numbers in information 

elements 

The call number assigned by the network provider 

(cf. features MSN and DDI) can be distinguished 

by two sorts of information elements 

which are independent of the direction of transmission. 

1. The number of the subscriber wishing to establish 

the connection is called the Calling 

Calling Party 

Number 

Called Party 

Number 

Low Layer 

Compatibility 

High Layer 

Compabitility 

Connected 

number 

Party Number (CGPN). 

Caller's number 

Number of person receiving 

the call 

Transmission service for 

layers 1-3 on the B channel 

Transmission service for 

layers 4-7 on the B channel 

Number of the connected 

subscriber 

2. The number of the subscriber receiving the 

call is termed the Called Party Number 

(CDPN). 

The numbers of both subscribers are thus conveyed 

via the D-channel in different information 

elements. 

Call number Information element Abbreviation 

Caller Calling party number CGPN 

Callee Called party number CDPN 

39

40 


When establishing a connection, the dialing information 

plays an important part in some senses, 

e.g.: 

• The destination number can be transmitted 

as a block or as individually dialed digits - 

depending on the way the subscriber has actually 

dialed. 

• The source and destination numbers are important 

for various features (e.g. CLIP, CLIR 

or CF), 

• The call numbers of both the caller and the 

called party are usual exchanged via the D 

channel. 

• Dialed digits can, depending on the application, 

be transmitted in individual layer-3 

messages. 

Fig. 8.9-1: Individual dialing 

8.9 Transmission of dialing information 

in an outgoing call 

A simple example from everyday practice demonstrates 

the information elements CGPN and 

CDPN: A subscriber A connected to the network 

in Bremen (dialing code 0421) wishes to 

call directory enquiries (the number: 11833) 

from a multiple device connection. Subscriber 

A has entered the MSN 9801736 into their own 

ISDN telephone. 

The transmission of the destination number depends 

on what the subscriber does. 

8.9.1 Individual dialing 

This happens when the subscriber raises the telephone 

receiver and then taps in the individual 

digits of the number they wish to reach. The 

protocol for this is as follows:


Lifting the receiver causes the subscriber's telephone 

to send the exchange a SETUP message, 

informing it of the desire to establish a connection. 

Since subscriber A has a full MSN entered in the 

telephone, the SETUP message includes this 

number as the CGPN. 

The exchange responds to the SETUP message 

with SETUP ACKNOWLEDGE, indicating to 

the terminal device that the request to establish 

a connection has been accepted but the necessary 

dialing information still needs to be sent. 

If the subscriber taps in all the digits of the destination 

number one by one, each one is transmitted 

in its own layer-3 INFORMATION message 

as the information element CDPN. 

8.9.2 Block dialing 

Block dialing implies that the entire destination 

number is sent in a single block, such as when 

the subscriber calls a number that has been 

saved in the telephone's memory and only lifts 

the receiver afterwards. 

Fig. 8.9-2: Block dialing 

Subscriber A already has a full MSN programmed 

and the number is sent as the CGPN 

in the SETUP message when using the blockdialing 

procedure too. 

As the called number is known before the raising 

of the receiver, the telephone can also transmit 

this number to the exchange in the outgoing 

SETUP message where it is contained in the 

CDPN information element. 

Notes: 

The D-channel protocol is, of course, different 

if subscriber A has not programmed an MSN or 

has entered it only partially. 

If the D-channel data is recorded at the receiving 

end of the connection, the picture also looks 

different. 

8.10 The Calling Party Number 

IE CGPN 

The calling party number identifies the caller's 

number. 

If the caller's terminal device has its phone 

number programmed into it, then this number 

is sent to the exchange as CGPN within the 

outgoing SETUP message. The exchange 

adds the local dialing code to this and switches 

the call into the network to reach the called 

number's local exchange. This exchange then 

passes the caller's number to the destination 

as the CGPN in the incoming SETUP message 

(as long as this is permitted by the CLIP feature). 

8.10.1 Structure of the Calling Party 

Number IE CGPN 

The structure and the purpose of all bits within 

information elements is defined in ETS 300 

102-1. The calling party number is an information 

element of variable length depending 

41

42 


on the length of the call number being transmitted. 

Explanations: 

1. The first octet of the information element 

contains the element's ID. Calling party 

number (CGPN) is identified by the hex code 

6C. 

2. As bit 8 of the 1st octet is a 0, the IE is of 

variable length. That means that the 2nd octet 

contains the length of the element. This 

length is the number of octets to follow. 

3. Within some octets, a sequence of bits may 

be specifically defined (e.g. Type of 

number). These are specified in separate tables. 

4. There are four bit sequences in the CGPN 

element which have their own ID: 

Type of 

number: indicates whether the number is, 

for example, national, international 

or network-specific. If the 

terminal device does not recognize 

the type of number, this section 

is marked “unknown” 

Numbering 

plan 

identification: indicates the numbering plan of 

the terminal device, e.g. ISDN / 

telephone number plan, data or 

telex number plan. If the number 

plan is not known, this section is 

marked “unknown” 

Presentation 

indicator: specifies whether the call 

number may be displayed or not 

Screening 

indicator: indicates, for example, whether 

the number has been checked for 

validity by the exchange 

5. The numbers themselves are coded according 

to International Alphabet No. 5 (IA5). 

8.10.2 Example of the coding of a CGPN 

Subscriber A from the previous example has the 

MSN 9801736 entered into the ISDN telephone 

and is agreed to having the call number displayed 

on subscriber B's telephone. The calling 

party number information element is sent in the 

outgoing SETUP message where it appears in 

the following manner: 

Binary Hex Meaning 

0110 1100 6C 

0000 0001 01 

1000 0000 80 

Name = Calling party 

number 

09 Length: 9 octets to follow 

Type of call number: 

unknown 

Call number plan: 

ISDN/Telephone plan 

Call number may be 

displayed 

0011 1001 39 Number: 9 

0011 1000 38 Number: 8 

0011 0000 30 Number: 0 

0011 0001 31 Number: 1 

0011 0111 37 Number: 7 

0011 0011 33 Number: 3 

0011 0110 36 Number: 6 

8.11 The features CLIP and CLIR on 

the D channel 

The calling party number information element 

clearly shows how easy it is to change the CLIP 

and CLIR features on the D channel. Bits 6 and 

7 of octet 3a are defined as the presentation indicator:


A table in the original specification lists the binary 

values that the presentation indicator can 

assume and the meaning of these values: 

Octet Bits binary Meaning 

3a 7 – 6 00 Display permitted 

01 Display suppressed 

10 

Called number not 

available 

11 Reserved 

If subscriber A is making use of the CLIP feature, 

the presentation indicator element in the 

transmission protocol for the transfer of the 

caller's number (calling party number) is set 

to 00 and the exchange at the destination end 

sends the caller's number to subscriber B via 

the D channel. 

If the caller activates CLIR, however, the terminal 

device sets the presentation indicator in the 

CGPN information element to 01. This is how 

subscriber B's local exchange can recognize 

that it is not permitted to pass the number on to 

the destination. 

Note: 

Whether or not subscriber A wishes the number 

to be displayed, it will still be sent along the 

network as far as the destination exchange. It is 

only this final exchange that interprets the presentation 

indicator and acts accordingly. 

8.12 The HOLD feature 

on the D channel 

In contrast to the CLIP and CLIR features, 

which can be implemented in two bits, the activation 

and deactivation of the HOLD feature 

require their own layer-3 messages. 

Bits Hex Message Meaning 

0010 0100 24 Hold 

0010 1000 28 Hold Ack 

0011 0000 30 Hold Reject 

0011 0001 31 Retrieve 

0011 0011 33 

0011 0111 37 

Retrieve 

Ack 

Retrieve 

Reject 

The terminal 

device requests 

that the exchange 

hold the call 

The exchange 

confirms that the 

call is held 

The exchange 

rejects the 

terminal device's 

request 

The terminal 

device requests 

that the exchange 

restore the held 

call 

The exchange 

restores the 

connection to the 

terminal device 

The exchange 

rejects the 

requested 

connection 

8.12.1 Activation of Hold 

When a subscriber puts a connection on hold, 

the following layer-3 messages are exchanged 

between the terminal device and the exchange: 

Fig. 8.12-1: Activation of Hold 

43

44 


The terminal device sends the HOLD message 

to request the exchange to put the connection 

on hold and to make the B channel it had been 

using available for another call. 

By sending HOLD ACKNOWLEDGE, the exchange 

confirms 

• that the connection has been put on hold, 

• that the exchange at the other end has been 

informed that the connection is on hold 

• and that the B channel that had been in use is 

now available again. 

The subscriber may now use the freed B channel 

to accept a call or to establish a connection 

with another subscriber. 

After HOLD ACK has been received, the subscriber 

has three minutes to return to the held 

call. If this is not done, the exchange disconnects 

the call. Charges continue to be calculated 

while a call is being held. 

8.12.2 Deactivation of Hold 

Fig. 8.12-2: Deactivation of Hold 

To return to a held call the subscriber performs 

the relevant procedure on their telephone. The 

terminal device then sends the layer-3 message 

RETRIEVE to the exchange to request the connection. 

If the connection is still in existence, 

the exchange will respond to the terminal device 

with RETRIEVE ACKNOWLEDGE. If the 

time has expired, however, the exchange will 

refuse the request by sending RETRIEVE RE- 

JECT.


9 Appendix 

9.1 Literature 

ISDN – Das neue Fernmeldenetz der Deutschen Bundespost TELEKOM 

Author: Peter Kahl 

Publisher: R. v. Decker’s 

3rd newly revised and expanded edition, 1990 

Euro-ISDN für Installateure und Service-Techniker 

Spath / Heermeyer 

Publisher Franzis, 1997 

Neues Terrain – ISDN erfordert fundierte Aus- und Weiterbildung 

Heermeyer / Spath 

in: Gateway, January/February 1994 

Neue Herausforderungen für die Berufsbildung am Beispiel der ISDN-Technik 

Maria Spath, in: lernen & lehren, 45, Donath-Verlag, 1997 

Methodisch-didaktische Umsetzung des Themas ISDN in der Berufsschule, 

Karl Boscher, in: lernen & lehren, 45, Donath-Verlag, 1997 

Qualifikation des technischen Personals bei der österreichischen Post, 

Hans Proksch, in: lernen & lehren, 45, Donath-Verlag, 1997 

Integrated Services Digital Network (ISDN); User-network interface layer 3; 

Specifications for basic call control, ETS 300 102-1, ETSI, 1990 

Illustrated ISDN – a painless primer to principles and protocols 

Lindgren / Jonsson, Infotrans, Sweden, 1991 

ISDN: The Integrated Services Digital Network: Concept, Methods, Systems 

P. Bocker, Springer Verlag, 1992 

The ISDN Literacy Book 

Gerald L. Hopkins, Addison-Wesley Pub Co, 1995 

ISDN For Dummies® 

David Angell, IDG Books Worldwide, 1996 

ISDN Explained: Worldwide Network and Applications Technology 

P.F. Adams / John M. Griffiths, John Wiley & Sons, 1998 

45

46 


9.2 Overview: important causes 

No. Cause 

16 Normal call clearing 

17 User busy 

18 No user responding 

19 No answer from user (user alerted) 

21 Call rejected 

22 Number changed 

27 Number changed 

28 Invalid number format 

29 Facility rejected 

31 Normal, unspecified 

34 No circuit/channel available 

38 Network out of order 

41 Temporary failure 

44 Requested circuit/channel not available 

50 Requested facility not subscribed 

57 Bearer capability not authorized 

58 Bearer capability not presently available 

65 Bearer capability not implemented 

81 Invalid call reference value 

83 A suspended call exists, but this call identity does not 

84 Call identity in use 

85 No call suspended 

88 Incompatible destination 

96 Mandatory information element is missing 

100 Invalid information element contents 

Note: The complete list of failure causes for 

Euro-ISDN is to be found in ETS 300 102-1 or 

in the book Euro-ISDN für Servicetechniker 

und Installateure, Spath/Heermeyer, page 408.


9.3 Abbreviations 

ALERT Message that indicates that the subscriber has been called (“the telephone 

rings”) 

AMI Alternate Mark Inversion 

AOC Advice of Charge 

B channel Base channel; used for speech, images, text etc. 

B-subscriber Dialed subscriber 

BC Bearer Capability 

CALL PROC CALL PROCEEDING; message sent in response to SETUP which confirms 

that the full dialing number for the destination has been included in the 

SETUP message. 

CAU Cause; information element, giving a reason for a failure 

CCBS Completion of Calls to Busy Subscriber = call back automatically if the line 

is engaged 

CCITT Comité Consultatif International Télégraphique et Téléphonique 

CDPN Called Party Number 

CGPN Calling Party Number 

CLIP Calling Line Identification Presentation = transmit the caller's number to the 

dialed subscriber 

CLIR Calling Line Identification Restriction = suppress transmission of the caller's 

number to the dialed subscriber 

COLP Connected Line Identification Presentation = transmit the number of the 

connected subscriber to the caller 

COLR Connected Line Identification Restriction = suppress transmission of the 

connected subscriber's number to the caller 

CONN CONNECT; message to confirm that the connection has been accepted 

CONN ACK CONNECT ACKNOWLEDGE; Response to CONNECT 

CR Call Reference = Layer-3 address 

D channel Signal channel 

DDI Direct Dialing In = Dialing into an ISDN switching system 

DISC DISCONNECT; message that initiates the ending of a connection (Layer 2 

or Layer 3) 

DSS1 Digital Subscriber Signalling System No. 1 

E-Bit Echo bit for a terminal device's D-channel data (S 0 frame) 

ETSI European Telecommunications Standards Institute 

ext extension 

F-Bit Frame bit, used for synchronising the beginning of a frame (S 0 frame) 

FA-Bit Additional frame bit (S 0 frame) 

FCS Frame Check Sequence = Layer-2 check sum 

HDLC High level Data Link Control 

47

48 


HOLD Call Hold 

ID assign Identity assign = TEI assignment by the exchange 

ID denied Identity denied = refusal to assign a TEI 

ID request Identity request = a terminal device's request for a TEI 

INFO S0 Idle state of the S0 interface 

INFO S1 Signal to awaken the S0 interface TE ðNT 

INFO S2 Synchronising frame NT ðTE 

INFO S3 Synchronisation confirmation TE ðNT 

INFO S4 Synchronisations confirmation NT ðTE 

ISDN Integrated Services Digital Network 

L-Bit DC-bias compensation bit (S 0 frame) 

L1 layer 1 = Physical layer 

L2 layer 2 = Security layer 

L3 layer 3 = Transport layer 

LAPD Link Access Procedure for D-channel 

MGA Multiple-device connection 

MSN Multiple Subscriber Number = Mehrfachrufnummer 

MT Message Type 

NT Network Termination 

NTBA Network termination equipment for a basic-rate connection 

NTPMxA Network termination equipment for a primary multiplex connection 

OSI Open System Interconnection 

P-MP Point-to-multipoint (configuration) 

P-P Point-to-point (configuration) 

PCM Pulse Code Modulation 

PD Protocol discriminator 

REL RELEASE; message sent during the ending of a layer-3 connection 

REL COM RELEASE COMPLETE; message confirming the ending of a layer-3 connection 

RNR Receive Not Ready (Layer 2) 

S2M Interface between TE ↔ NT on primary multiplex connection 

SABME Set Asynchronous Balanced Mode Extended; instruction to establish a layer-2 

connection 

SAPI Service Access Point Identifier; identifies the purpose of a layer-2 connection 

SETUP message for initiating the establishment of a connection on layer 3 

SETUP ACK Acknowledgement of SETUP 

S0 speech 

Interface between TE ↔ NT on a basic-rate connection


TA a/b Terminal adapter for analogue terminal devices 

TE Terminal Equipment = terminal devices (in general) 

TEI Terminal Endpoint Identifier = layer-2 Address 

TK-Anlage PBX system 

TP Terminal Portability = the ability to unplug from one socket on the bus and 

plug into another 

UA Unnumbered Acknowledgement (Layer 2) 

UAE Universal-Anschluss-Einheit (Universal telecommunications socket) 

UI Unnumbered Information 

Uk0 Interface between NT ↔ Exchange 

2-wire interface between PBX system and terminal equipment 

U P0 

VSt Vermittlungsstelle (Exchange) 

X.31 Datex-P on the ISDN-Network (B and D channel) 

CSC Central Signalling Channel 

49

50 

MTS 7.1.2.1 Fundamentals of ISDN Technology Exercises 

Equipment and accessories required 

MTS 7.1.2.1 Fundamentals of ISDN technology 

1 ISDN panel 735 912 

1 PBX system 735 913 

3 ISDN telephone 735 915 

1 ISDN tester 735 916 

Accessories 

1 Digital storage oscilloscope 407 575 293 

3 BNC cable 501 02 

1PC 

1 Book: “Fundamentals of 

ISDN Technology” 568 962 

Additional accessories for experiment 1.3: 

1 Star-quad cable 736 481 

1 Measuring bridge 736 451 

2 STE resistor 100 Ohm, 0,5 W 577 01 

1 STE capacitor 10 nF, 100 V 578 10 


1 Book: Measurements on 

Four Wire Lines” 568 542 

2 Screened cable BNC/4 mm 575 24 

TPS 7.1.2.2 Frame construction in longdistance 

ISDN devices 

1 PAM modulator 736 061 

1 PAM demodulator 736 071 

1 PCM modulator 736 101 

1 PCM demodulator 736 111 

Accessories 

1 Panel frame 

1 DC power supply ±15 V/3 A 726 86 

1 Spectrum analyzer 726 94 

2 Function generators 200 kHz 726 961 

1 Frequency counter 0...10 MHz 726 99 


2 Probes 100 MHz 1.1/10:1 575 231 

1 Analog multimeter C.A. 406 531 16 

2 Cable pairs 100 cm, black 501 461 

3 Packs of 10 bridging plugs, black 501 511 

1 Book: “Pulse Code Modulation” 564 002 

TPS 7.1.2.3 Line codes for the 

S 0 interface 

1 Data source / parity generator 736 93 

1 Display / parity check indicator 736 92 

1 AMI/HDB3 coder 736 94 

1 AMI/HDB3 decoder 736 91 

Accessories 

1 Panel frame 

1 DC power supply ±15 V/3 A 726 86 


2 Probes 100 MHz 1.1/10:1 575 231 

1 STE resistor 4,7 kOhm, 2 W 577 52 


3 Packs of 10 bridging plugs, black 501 511 

1 Book: “Baseband data 

transmission methods” 568 452


1 Installation techniques 

1.1 Configuration of the training system 

1.1.1 Graphic representation of the basic configuration 

123 ISDN Telephone (735 915) 7 PC 

4 PBX system (735 913) 8 Oscilloscope (575 293) 

5 ISDN tester (735 916) 9 ISDN panel (735 912) 

6 Option 

The experiment set-up shown here represents a configuration with the maximum amount of equipment. In the following 

experiments, the number of terminal devices may be less than shown for example. All the measuring equipment may also 

be unnecessary for some experiments. 

1.1.2 Quick commissioning of the PBX system 

EURACOM 182 

• Connect the EURACOM 182 to the mains (230 

V) 

• Connect the internal S0 bus of the PBX system 

to the ISDN panel in the following way: 

73513 PBX system 735912 ISDN panel 

(EURACOM 182) 

S02 → NT2 (S02) 

S03 → NT1 (S01) 

• When connecting an oscilloscope, the ground 

of the PBX system should be connected to the 

GND socket on the oscilloscope by means of 

the cable. This avoids problems caused by 

earth loops. 

• ISDN terminal devices (e.g. telephones) should 

be plugged into the Western sockets of the 

ISDN panel and programmed with the appro- 

priate MSNs (look up "setting the MSN" in the 

handbook for the terminal devices.) 

Notes: 

• The EURACOM 182's default settings are as 

follows: 

Port S02 Port S03 

20 Multiaddress 

call 

30 Multiaddress 

21 pre-set MSN 31 pre-set MSN 




51

52 


• To start operation quickly, you may program 

the terminal devices with the MSNs preinstalled 

in the PBX system, (21..24, 31..34). 

• If MSNs are not programmed into the individual 

terminal devices, then every device on the 

bus will ring at once. 

• When the multi-address call number is active, 

all the devices on the bus ring together even if 

they are each assigned an MSN. 

If the MSN is incorrect or missing, the dial 

tone may not appear. 

• To alter the MSN using EURACOM 182, start 

the configuration program and proceed according 

to the handbook: 

System Configuration Programme 

Configuration 

Basic Configuration 

Assign the appropriate subscriber call number 

to port 2.3 or alternatively deactivate it. 

1.1.3 Short description of the ARGUS 10 

ISDN tester 

Please note that the operating steps and the menu 

entries for the Argus 10 may change should the 

manufacturer make alterations to the software. 

Important elements for operating the ARGUS 

test equipment 

→ shows the menus that can be activated by 

pressing the ü button. 

↓ allows scrolling of menu entries. 

ü used to activate a menu, a submenu or a procedure 

Press cancel to return to the previous menu or 

menu entry. 

Display 

Example: 

ARGUS 10 

Settings 

S0 NT 

x.xx Software version 

X.XX 

OK? 

P – P 

YES NO 

The first thing to do after switching on the device 

is to select the interface over which the test will be 

carried out: 

a) S 0 interface: for all measurements involving 

the S 0 bus 

b) analog interface (a/b-interface): for the simulation 

of terminal devices and operating the monitor 

on the a/b interface 

c) open, S 0 bus with no voltage applied (line test): 

for all physical tests 

The LEDs above the display show whether layer 1 

(S 0 interface) or layers 2 and 3 (D channel) are 

active. Detailed descriptions of the individual layers 

are to be found in the theory section. 

Methods of operation for the test equipment 

If the user wishes to test the S 0 interface, they 

should select that interface from the menu which 

brings up the display, Connection mode. By scrolling 

with the arrow keys and confirming with the ü 

key, the user can now select the operating mode 

for the testing device 

a. Automatic TE mode 

By activating the Automatic TE mode entry, 

the user causes the connection to be tested automatically. 

The result of this test is shown on 

the display. It includes details of the B channels, 

the interface, the configuration, the operating 

mode and the D-channel protocol currently 

in use. 

The second step enables the user to repeat the test 

or call up the main menu so that the Argus 10 device 

keeps the configuration that it has detected. 

For the user this means that further tests can be 

started immediately. 

Important: If the Argus 10 test device is used as 

a terminal device on the S 0 bus,the 

TE mode must always be selected. 

b. NT-S0 P-P 

In this operating mode the Argus 10 test device 

emulates two components: 

– in physical terms, it takes the role of the network 

terminal (NT) with its S 0 in point-to-point 

configuration, i.e. an NTBA, to which a PBX 

system is connected, and 

– in protocol terms (i.e. on the D channel), it 

takes the role of the exchange on layers 2 

and 3. 

This so-called NT simulation serves as the connection 

of a PBX system (point-to-point configuration) 

to the testing device.


c. NT-S0 P-MP 

In this operating mode the test device acts once 

again as the exchange on layers 2 and 3 in protocol 

terms and as the network terminal on a physical 

level. However, this time the configuration is 

point-to-multipoint so that all the terminal devices 

that can operate on the multiple device connection 

can be connected. 

d. S0-Monitor 

When used as the S 0 monitor the Argus 10 is connected 

across the S 0 bus where it has high impedance. 

Its job is then to display all protocol data, 

transmitted on the D channel over this interface. It 

doesn't matter whether this D-channel data has 

been sent from the network terminal or another 

terminal device. The test device is otherwise completely 

passive. 

Overview: Use of the Argus 10 in the experiments 

Automatic TE Mode menu: 

This instructs the equipment to test the line automatically. 

The Argus test equipment is usually 

configured for the internal S 0 interface of a Euro 

ISDN PBX system in point-to-multipoint mode 

using the Euro ISDN Protocol (DSS1). 

Connection menu (Individual tests): 

This test menu allows a connection to an external 

telephone number to be established (B-channel 

user). The service and the B channel can be selected 

and a number may also be dialed individually. 

Service features menu (Individual tests): 

This menu allows the availability or functionality 

of the features, TP, HOLD, CLIP, CLIR, COLP, 

COLR, CF, CCBS to be checked. 

Services test menu (Individual tests): 

This checks whether a certain service is available 

or working for the line under test. The 

service test can be carried out by self-calling or 

by connection with an external subscriber 

WinPlus and WinAnalyse 

Here follows a short description of how to record 

D-channel data using these two software packages. 

WinPlus and WinAnalyse should be installed 

on the computer according to the supplied operating 

manual. The WinPlus software can only show 

layer-2 and layer-3 messages but WinAnalyse fea- 

tures an additional window in which individual Dchannel 

messages can be shown with various 

depths of decoding. Both these software packages 

can be operated in Trace mode or Monitor mode. 

Recording of D-channel data in Trace mode 

Next the L-Test socket is connected to the serial 

port on the PC which has been configured by the 

software during installation. It is advisable to use 

the supplied cable for this. 

The next step is to start the software on the PC. 

The main menu should appear on the screen. 

The Argus 10 is now switched to Trace mode by 

selecting the submenu Trace mode from the Communication 

menu so that the Trace LED comes on 

After starting Trace mode you can return to any 

text menu to carry out a test. For this purpose, all 

D-channel data is automatically transmitted to the 

PC and shown there by the software. 

Definition of Trace 

In measurement technology, the term “trace” is 

similar to “record”. With an ISDN tester this 

means recording D-channel messages between the 

terminal device and the unit with which it is in 

communication. In the case of the TE simulation 

(TE mode), it is the D-channel data that the Argus 

10 exchanges with the PBX system or the exchange 

when it is emulating a terminal device. 

Tracing of D-channel data when operating as 

a passive monitor 

After the software is installed on the PC, the Argus 

10 is connected to the S 0 bus and the PC (see Argus 

handbook). 

The next steps are to start the software running on 

the PC and turn on the Argus 10's S 0 monitor (see 

Argus handbook). 

As soon as D-channel data appears on the S 0 bus 

this is conveyed to the PC and displayed with the 

help for the Argus software installed there. 

Explanation of the “passive monitor” operating 

mode 

When the Argus 10 is connected to the S 0 bus as a 

high impedance passive monitor, it records all Dchannel 

data transferred between the connected 

terminal devices and the PBX system or the exchange. 

This process allows the technician to specifically 

determine what data has been sent from 

each individual ISDN device and to discover if 

53

54 


there are any deviations which might indicate operational 

errors, programming errors or protocol 

errors. 

Further information on the operation and uses of 

the Argus 10 test equipment can be derived from 

its handbook. 

1.1.4 735 912 ISDN panel 

• Noise voltage can arise between the oscilloscope 

and the PBX system. This can visibly 

affect the oscillogram. To help avoid this the 

PBX system should be connected to the oscilloscope's 

earth socket. 

• The 735 912 ISDN PANEL includes 2 independent 

S 0 buses: S 01/2 . The buses each have 

their own power supply, designated NT 1/2 . The 

PBX system's two internal S 0 buses should be 

connected to these points. 

• To simulate both normal and emergency 

modes, the power supply voltage for the S 01 bus 

can be reversed. (EMERGENCY / NOR- 

MAL). This is achieved by means of ISDN's 

usual differential transformer. 

• Installation experiments (connection tests, e.g. 

short circuit, swapped terminal etc.) should be 

performed on the S 01 bus using the supplied cable. 

In addition, the 4-mm socket for the 736 481 

star quad may be connected. The terminating 

resistance for this bus can be switched in and out 

of circuit by means of the TERMINATION 

switch. 

Pay attention to this: 

Connection tests using the ISDN tester (735 916) 

can only be performed when the S 0 bus is turned 

off (S 01 bus = OFF), otherwise the bus lines 

are short-circuited by the differential transformer. 

For the same reason, such tests cannot 

be performed on S 02 either. 

• The power supply portion of NT 2 is decoupled 

from the power supply for the Trigger and 

Noise electronics by means of the differential 

transformer. The Trigger socket allows the 

oscilloscope to be externally triggered by a 

pulse. This pulse corresponds to the start of a 

layer-1 frame. The two sockets TE → NT (3- 

6) and NT → TE (4-5) represent the bits for the 

direction of transmission and reception on 

NT2. 

• Note: 

PBX systems usually maintain constant communication 

with the terminal devices connect- 

ed to them, i.e. layer 1 is never deactivated. 

Trigger pulses are therefore always present. 

The trigger ceases, however, when NOISE is 

switched to ON and BER is set too high (using 

the potentiometer). The coding violation that 

marks the beginning of the frame is then no 

longer detected. 

Notes on the use of the ARGUS 10 with the 

735 912 ISDN panel: 

Short-circuit test: S 01 → OFF, 

else display: 3,6 / 4,5 

short circuit 

Termination: ON produces the display: 

100 Ohm 

OFF produces the display: 

no resistance 

Line test: Swapped terminals, Short 

circuit, 

Termination only possible with 

NT 1 -S 01 . Because the differential 

transformer cannot be 

switched off, the display for 

S 02 is always “Short circuit” 

3,6 / 4,5. 


1.2.1 Swapped terminals 

Extend an internal S 0 bus from the existing PBX 

system by four sockets with the help of the ISDN 

panel (NT1). Intentionally wire the connections to 

the two right-hand sockets as stated. Connect an 

ISDN telephone to each socket and check out the 

functionality of the bus with the help of these two 

phones. 

Questions 

1. What settings need to be set on the ISDN panel 

(NT 1 )? 

2. How should the two sockets of the ISDN panel 

(NT 1 ) be wired together correctly? 

3. What is the result of your test? 

4. How do the terminal devices react when you 

disconnect the terminating resistance? Give a 

reason for your result. 

5. How do the telephones behave when the power 

supply voltage is switched between normal and 

emergency operation? Give a reason for your 

result. 

6. How do the terminal devices behave when 1a 

and 1b are crossed over? 


and 2b are crossed over?


1.2.2 Resistance and wiring measurements 

When you install an ISDN bus for multiple devices 

at a customer's site, the line can be up to 150 m 

long and support up to 12 sockets In practice, to 

ensure that there are no faults with the ISDN bus, 

a test using appropriate measuring techniques is 

performed. 

Use the ISDN panel (NT 1 ) to take measurements 

on the ISDN tester which will indicate installation 

faults. Note the device-specific operating steps for 

the ISDN tester used. 

Note: 

When the wiring and resistance measurements are 

being performed, there must be no voltage supplied 

to the ISDN bus. That means that the ISDN 

panel (NT 1 ) must be completely disconnected 

from the internal S 0 interface. Furthermore, no terminal 

devices may be connected. 

When there is voltage on the bus, the red LED on 

the test adapter will come on. 

1. Use the ISDN tester to measure the resistance 

on a correctly wired S 0 bus. 

1.1 How do you connect the test equipment to the 

S 01 bus an, in order to take measurements? 

1.2 What does the ISDN tester show the results of 

the test to be... 

a) with “Termination off”? 

b) with “Termination on”? 

2. Perform the same test after creating a short 

circuit between 3 (2a) and 4 (1a). What does 

the ISDN tester show the results of the test to 

be? 

3. Use the ISDN tester along with the ISDN panel 

(NT1) to test for swapped terminals. 

3.1 How do you connect the ISDN tester to the 

bus for the swapped-terminals test when you 

want to take a measurement of the S 01 bus between 

the first and last sockets? 



a) with correct wiring? 

b) when 1a and 1b are swapped over? 

c) when 2a and 2b are swapped over? 

d) when 2a and 1b are swapped over? 

1.3 Line characteristics of a passive S 0 

bus (Option) 

The cable pairs of the passive S 0 bus can be more 

easily represented by the following equivalent cir- 

cuit diagram: 

Fig. T2. 

The characteristic impedance of the line can be 

derived from this equivalent circuit diagram: 

Z w = 

R'+ jωL' G'+ jωC' 1.3.1 Determining the line quantities per unit 

length using the measuring bridge 

1.3.2 Calculation of the characteristic impedance 

The experimental investigation of the S 0 bus as a 

data transfer medium is described in the training 

system MTS 7.2.4.2 Measurements on Four Wire 

lines. 


The data on the receive and transmit lines of NT 2 's 

ISDN bus should be recorded using the oscilloscope. 

The oscilloscope should be triggered externally. 

Experiment set-up see Chapter 1.1.1. 

1. Connect NT2 to the PBX system then plug an 

ISDN telephone into a free socket. 

a) What does the oscilloscope show? Settings: 

0.5 V/cm; 10 µs/cm. 

b) Make a sketch of the oscilloscope display. 

c) Compare your sketch of the display with the 

S 0 frame structure. Which bits can you 

identify straight away? 

2. Lift the telephone receiver. 

d) How does the oscilloscope display alter? 

e) Where is the dial tone carried? 

3. Add noise to the S 0 bus by turning the potentiometer 

(NOISE) slowly. (Switch ON.) 

f) What does this noise do? 

4. Dial on the telephone the number of a terminal 

connected to the PBX system's second internal 

S 0 bus. Establish a connection between the two 

devices and conduct a call. 

55

56 


g) In what ways does the oscilloscope display 

change when you send as constant a tone as 

possible, e.g. a whistling noise? 

h) Can you conduct an analysis of the D channel 

using the oscilloscope as it is presently 

set up? Give reasons for your answer.


2 Services and features of ISDN 

2.1 Automatic line test 

A customer calls to report a fault. The customer is 

aware that they possess an ISDN line but is unable 

to give any technical details about it. For this reason 

you conduct an initial test using the ISDN tester 

to check 

• whether the B channels are present, 

• the interface on which the test is being performed, 

• the line configuration, 

• and the protocol the connection is using. 

Questions 

1. How can you conduct a simple automatic line 

test using the ISDN tester? 

2. What results are shown on the display after 

completion of the test? 

3. How do you interpret these results? 

4. How do the test results appear when a B channel 

is used during the course of the test? How 

can a B channel be occupied using the existing 

terminal devices for the duration of a line test? 

5. How do the test results appear when two B 

channels on the S 0 bus are used? How can you 

occupy both B channels on the internal S 0 bus, 

even though only three ISDN telephones in 

total are available to you? Can you achieve the 

exercise with only two telephones? 

6. How does the testing device react when it is not 

connected to the S 0 interface during the line 

test? 

2.2 Testing of services 

2.2.1 Automatic testing of services 

You are installing an ISDN line for a customer. 

After installing the NTBA and laying the S 0 bus, 

you wish to prove the functionality of the line. In 

doing this you also check whether various terminal 

devices (e.g. telephones, group-3 faxes, group- 

4 faxes etc.) can be operated on the bus, i.e. whether 

they can transmit and receive a call. Since you 

are not in possession of all these types of device 

where you are, use the ISDN tester to carry out this 

check. 


Questions 

1. What sort of test can you use to check the various 

services most easily? 

2. Which test menu do you call up on the ISDN 

tester for this? 

3. How do you carry out this test with the ISDN 

tester? Describe the testing process step by 

step. 

4. Carry out the services test by means of selfcalling. 

What are the objectives of the test 

when calling the test device's own number? 

Interpret the results of the test. 

5. Conduct a test of services by calling an external 

number (= establishing connection to the 

S 02 bus). What are the objectives of the test 

when calling a different number to that of the 

test device? Interpret the results of the test. 

6. Why do the LEDs L1, L2 and L3 turn on or 

flash during the course of the test? 

2.2.2 Testing of an individual service 

Your customer has bought an ISDN PC card and is 

planning to use it as a group-3 fax receiver. The 

card has already been installed by the customer. 

As it turns out, faxes can neither been transmitted 

nor received, so the customer has called you to 

solve the problem. To find out if the terminal device 

service for fax group three is functioning at 

all on this line, you initially make use of the ISDN 

tester to determine whether connections using this 

service can be established. 

Experiment set-up Chapter 1.1.1. 

Note: 

1. As an alternative to a PC card the customer 

could be connecting a group-3 fax machine via 

a terminal adapter (TA a/b) or a small PBX 

system (point-to-multipoint configuration). 

Questions 

1. What sort of test can you use to check a specific 

service most easily? 

2. Which test menu on the do you call up on the 

ISDN tester for this? 


tester? Describe the testing process step by step. 

57

58 


4. What information are you given by the device's 

display during establishment of the connection 

(= dialing)? 

5. What is the result of the test when a connection 

is successfully established? How do you interpret 

this result? 

6. What is the result of the test when there is no 

terminal device to receive the call at the dialed 

number? How do you interpret this result? 

2.3 Features 

2.3.1 The Hold feature 

You are called to a customer on account of a fault. 

The customer has a multi-device basic access terminal 

with several telephones connected to it. 

According to the description of the fault, the customer 

is unable to use the Hold feature of their 

ISDN line. Because of this you are testing to see if 

the Hold function works properly with the help of 

the ISDN tester. 


Questions 

1. What is the original designation for Hold in the 

Euro-ISDN specification? 

2. Why must you know the technical abbreviations 

and the original descriptions of a feature 

in practice? 

3. Where is the Hold feature described in detail? 

4. Describe the process of holding? 

5. State how many B channels are used by the 

Hold feature. 

6. How do you test the availability of the hold 

feature using the ISDN tester? Which menu do 

you use for the individual test? Describe the 

operating method step-by step. 

7. What result is shown when Hold is working 

properly at the connection (and if it isn't)? 

8. What reasons could there be for Hold not to 

work? 

Additional exercise 

Perform a Hold with an existing telephone on the 

PBX system's internal S 0 bus. Take note of the 

instructions concerning Hold in the manufacturer's 

manuals. 

Explain the process to the customer. 

2.3.2 The features Calling Line Identification 

Presentation CLIP 

and Calling Line Identification Presentation 

CLIR 

A customer complains that when calling the 

number of subscriber B, the person at the other end 

of the line can see the call number even though 

Calling Line Identification Restriction is activated. 

It is your task to check this statement by means 

of the ISDN tester and to find out where the fault 

lies. 


Questions 

1. What are the technical abbreviations for 

Calling Line Identification Presentation and 

Calling Line Identification Restriction? 

2. Explain to the customer how Calling Line 


Identification Restriction work? 

3. How does it affect Calling Line Identification 

Presentation on the B channel when... 

a) subscriber A has entered an incorrect MSN 

into their telephone or 

b) subscriber A has not entered an MSN into 

their telephone? 

4. How can you use the ISDN tester to check 

whether Calling Line Identification Presentation 

or Calling Line Identification Restriction 

are activated on the customer's line? Describe 

the method of operation step by step. 

5. How do you interpret the results? 

6. What reasons could there be for Calling Line 

Identification Restriction not to work? 

Additional exercise: 

Test the features Calling Line Identification Presentation 

and Calling Line Identification Restriction using 

an existing telephone on the internal S 0 bus of the 

PBX system. Explain the process to the customer.


3 D channel protocol 

3.1 Establishment and ending of connections 


The customer complains to you of being unable to 

reach a certain number by telephone. You learn 

that the customer has a multiple device connection 

with more than one ISDN telephone connected. 

To find out whether the fault is really with your 

customer's system or with that of the dialed subscriber 

B, you establish a connection to B with the 

ISDN tester to check the telephone service. 

In order to glean an idea of the protocol sequence 

on the D channel at the same time, you record the 

D channel data using the ISDN tester and load 

this onto a PC. The WinPlus and WinAnalyse 

software is available to you for the analysis of 

your data. 

As you only need to see the sequence of protocol 

messages, you can begin using the WinPlus software. 

Questions 

1. How do you connect the ISDN tester to the S 0 

bus and the PC at the same time? 

2. Which function do you have to activate on the 

ISDN tester to enable communication with the 

PC? 

3. What does Trace mode mean? 

4. How do you establish a connection? 

5. What is the sequence of data on the D channel 

in layer 3 when subscriber B accepts the telephone 

call? Create a diagram containing all the 

D-channel messages with their directions of 

transmission. 

6. What are the layer-3 messages that appear and 

what are their meanings? 


when subscriber B has no telephone connected 

under the dialed number? (Draw a diagram!) 

To simulate this case in an experiment, remove 

all terminal devices from the internal S 0 bus of 

the dialed number (subscriber B). 

8. What are the layer-3 messages that are sent in 

this case and what are their meanings? 

3.2 Testing a feature with the help of 

D-channel information 

The customer has purchased top-of-the-range 

ISDN telephones for a basic access terminal. It 

isn't long before you receive a call to say there is a 

fault. The customer maintains that the telephones 

don't work because the Hold feature can't be used. 

Use the ISDN tester to check whether the Hold 

feature is available on the customer's line. 

To be safe, record the D-channel data exchanged 

during the process using the testing device and 

load them into the PC. You can decode them there 

with the help of the WinPlus software. 

Questions 



2. Describe, step by step, the actions needed to 

record the D-channel data using WinPlus. 

3. How do you test the Hold function using the 

ISDN tester? 

4. What result is shown on the ISDN tester? 

5. What layer-3 D-channel data is sent during the 

test procedure? Prepare a flow diagram. 



59

60 

MTS 7.1.2.2 

7.1.2.1 Fundamentals of ISDN Technology Exercises 

4 Frame structure on the ISDN telephone (option TPS 7.1.2.2) 

In order to be transmitted via ISDN, speech needs 

to be converted by an analogue to digital converter. 

The data so created is bundled into the B channel 

of the S0 frame. The bundling of the B channels 

in ISDN is achieved on the S 0 bus using a 2channel 

PCM transmission method. ISDN 

telephones are the most important group of terminal 

devices using the system. They perform 3 conversion 

steps before the analogue speech signal 

can be represented as digital data: 

• sampling 

• quantisation 

• coding 

To try some experiments that investigate this 

process more thoroughly, the TPS 7.2.2.1 Pulse 

Code Modulation training system is available. 

4.1 Sampling 

Sampling produces a signal that is discrete in time 

and has continuous values. This is the first step in 

the process of creating a digital signal. Sampling is 

a reversible process as long as the relevant conditions 

of the sampling theorem are fulfilled. The 

receiver is thus able to reproduce the signal in its 

entirety. 

There are two kinds of PAM (Pulse Amplitude 

Modulation) 

1. Natural PAM. The output of the electronic 

sampling circuit is a signal whole PAM pulses 

have an envelope that reflects that of the input 

signal. 

Natural PAM 

2. PAM using a sample-and-hold circuit (S&H). 

The PAM pulses in this case are rectangular. 

The amplitude corresponds to the value at the 

end of the sample. The differences between the 

two types of pulse modulation is reflected in 

their spectra. 

PAM using a sample-and-hold circuit 

PAM spectrum 

A 2-channel time multiplex system. One channel carries a 

sine-wave signal, the other a triangular one. 

4.2 Quantisation and coding 

The PAM signal produced by sampling is discrete 

in time and has continuous values. In order to convert 

this into a digital signal quantisation and coding 

must be carried out in addition. To improve the

MTS 7.1.2.2 7.1.2.1 Fundamentals of ISDN Technology Exercises 

signal-to-noise ratio for small amplitudes, a process 

known as "companding" is used where the portions 

of the signal with small amplitudes are given 

finer resolution than the larger signals. The illustration 

shows the experimentally recorded compressor 

and expander characteristics. 

In addition, an example of non-linear quantisation 

is shown 

Compressor characteristic 

Expander characteristic 

Non-linear transmission characteristic (5 bit) 

The illustration below shows the meanings of the 

individual bits and the times at which they appear 

in the transmitted signal. This is simplified in the 

training system in that, instead of a 48-bit frame, a 

2 x 12 bit is transmitted. The LSB appears on the 

oscilloscope as the 2nd bit from the left, i.e. the 

second bit to be sent. The sign bit is the ninth bit in 

the transmission, bit 10 is the CHI and is active 

low. This bit is designated the frame synchronisation 

bit. The first bit in either channel is always 

low and represents the stop bit. 

Frame structure in a 2-channel PCM system 

61

62 

MTS 7.1.2.1 7.1.2.3 Fundamentals of ISDN Technology Exercises 

5 Line coding on the S 0 interface (option TPS 7.1.2.3) 

Data transfer on the baseband implies the transmission 

of data at the original frequency, i.e. unmodulated. 

This uses line coding and is usual behaviour 

in ISDN, Datex and PCM30/32. 

The training system TPS 7.2.5 Baseband Data 

Transmission contains experiments which investigate 

this theme more thoroughly. 

5.1 Serial data transfer and security 

Important points to consider regarding serial data 

transfer include problems with 

• timing recovery 

• error correction 

• operating methods 

etc. 

5.2 AMI code 

Data transfer on the S 0 interfaces employs a modified 

AMI code. The following illustration shows 

an example.

MTS 7.1.2.1 Fundamentals of ISDN Technology Answers 

1 Installation techniques 


1.2.1 Swapped terminals 

1. What settings need to be entered on the ISDN 

panel (NT 1 )? 

These are the settings required for the ISDN panel 

(NT1): 

S 0 bus: ON The ISDN panel's S 01 bus 

must be connected to the 

PBX system 

Voltage: NORMAL public network: 

The network terminal 

(NT) supplies a normal 

voltage which is enough 

for four telephones on the 

internal S 0 bus: 

The power supply voltage 

for the internal S 0 bus is 

supplied by the PBX system. 

This may cause the 

number of telephones that 

can be supplied with power 

to be limited (take note 

of the operating manual) 

Termination: ON The line pair are terminated 

with a 100-Ohm terminating 

resistance. 

2. How should the two sockets of the ISDN 

panel (NT 1) be wired together correctly? 

3. What is the result of your test? 

When the equipment is set-up as above with the 

two sockets correctly wired, then both telephones 

connected to the S 01 bus can establish a connection 

at the same time, either with each other (self-calling 

on the bus) or to telephones on the second internal 

S 0 bus. 

4. How do the terminal devices react when you 

disconnect the terminating resistance? Give a 

reason for your result. 

The lack of a terminating resistance does not adversely 

affect the functionality of the telephones. 

Reason: 

For short lines, the laws applying to characteristic 

resistance are not relevant. Therefore, the terminating 

resistance does not matter to the functioning 

of the bus in this case. 

Since, in practice, the length of the installed line is 

seldom known precisely, the terminating resistance 

must always be built-in anyway. 

5. How do the telephones behave when the power 

supply voltage is switched between normal and 

emergency operation? Give a reason for your 

result. 

If neither of the telephones is authorised for emergency 

current operation, then both will cease to 

function as soon as the current is switched to 

emergency operation. 

If there is an ISDN telephone connected to the private 

network that is set-up for emergency operation, 

then this phone will continue to fulfil its basic 

function during emergency operation. 

Note: 

A single ISDN line supports no more than one 

emergency-authorised telephone. 



If 1a and 1b are crossed over, it does not affect the 

functionality of the telephones at all. Both calls to 

other devices on the bus and the establishment of 

communication with a second internal S 0 bus are 

possible. 



If 2a and 2b are swapped, however, the bus ceases 

to be operable, i.e. none of the telephones on the 

bus will work. 

63

64 


Reason: 

The cable pair 2a and 2b carry data in the direction 

TE → NT. If they are swapped over, when two 

terminal devices attempt to transmit simultaneously, 

the first two bits of the transmission frame 

(the F and the L bits, see the chapter on layer 1) are 

cancelled out because one device is sending the F 

bit as a logical “1” of positive voltage and the other 

is sending an F bit of negative voltage. With the 

two devices cancelling out each other's signals in 

this way, the S 0 bus ceases to function. 

1.2.2 Resistance and wiring measurements 

1. Use the ISDN tester to measure the resistance 

on a correctly wired S 0 bus. 

1.1 How do you connect the test equipment to the 

S 01 bus in order to take measurements? 

For this test, the test adapter is employed along 

with the testing equipment. The test adapter has a 

Western socket on each side, one of which should 

be connected to the L-Test socket on the ISDN test 

equipment using one of the supplied S 0 cables 

while the other is connected via the other S 0 cable 

to the last socket on the ISDN panel (NT1). 

IS 

DN 

IS 

DN 

Caution ARGUS 

S-Bus :Disconec ISDN LINE TEST ADAPTER 

Voltage 

tBus from 

NT/ 

PABX 

IS 

DN 

ARGUS 10 1.20 

NT-Mode L123 

B12 

S0 P-MP DSS1 

^ ^ ^ 

AR 

GU 

S1 

2 3 

10 4 5 6 

7 8 9 

0 # 

* 



a) with “Termination off”? 

b) with “Termination on”? 

Termination on: 

Line test 

Line ok 

Impedance: 100 Ω 

Termination off: 

Line test 

Line ok 

No impedance 

Note: 

When “Line ok” is shown on the tester, it indicates 

that there is no short circuit. 

2. Perform the same test after creating a short 

circuit between 3 (2a) and 4 (1a). What does 

the ISDN tester show the results of the test to 

be? 

Line test 

Short circuit 

3, 4 

Note: 

Any possible combination can be checked at this 

point! 

3. Use the ISDN tester along with the ISDN panel 

(NT1) to test for swapped terminals. 

3.1 How do you connect the ISDN tester to the 

bus for the swapped-terminals test when you 

want to take a measurement of the S 01 bus between 

the first and last sockets? 

For the wiring test, the ISDN testing device is once 

again used in conjunction with the test adapter.


a) The L-Test socket on the ISDN tester is connected 

with the so-called S 0 cable directly to 

the first socket on the ISDN panel (NT1). 

b) The test adapter is connected via the second S 0 

cable to the last socket on the ISDN panel 

(NT1). 

ISDN 



a) with correct wiring? 

Line test 

Line ok 

S-Bus 

Voltage 

Impedance: 100 Ω 

b) when 1a and 1b are swapped over? 

Line test 

Lines crossed 

ISDN 

Caution ARGUS 

:Disconec ISDN LINE TEST ADAPTER 

tBus 

from NT/ 

PABX 

4> 100 MΩ/km) then it is not measurable, so the assumption 

of G = 0 is justified. 

65

66 


Result: 

Within the observed frequency range: 

R’≈ 117 Ω/km = Constant 

When f = 50 kHz increases in resistance due to 

skin effects become observable 

L’ ≈ 700 µH/km = constant 

G’ = 0 

C’ ≈ 100 nF/km = constant 

ωL < R 

1.3.2 Calculation of the characteristic impedance 

Graph of characteristic impedance to frequency according 

to empirical measurements. 


1. Connect NT2 to the PBX system then plug an 

ISDN telephone into a free socket. 

a) What does the oscilloscope show? Settings: 

0.5 V/cm; 10 µs/cm. 

b) Make a sketch of the oscilloscope display. 

c) Compare your sketch of the display with the 

S 0 frame structure. Which bits can you identify 

straight away? 

Direction of transmission NT → TE (upper curve 

on oscilloscope display): 

The first, upward-moving pulse is the I bit of the 

arriving frame. It is followed by the F bit (frame 

bit) with which the next S 0 frame begins. According 

to definition, this bit is always a “positive 0”. 

The second, downward-moving pulse is the L bit 

or DC-bias compensation bit. According to definition, 

this is always a “negative 0”. The combination 

of the F bit and the L bit uniquely delineates 

the start of a frame. 

The third pulse, which also goes downwards, is 

the FA bit which causes a violation of the coding 

rules paired with the L bit. This violation makes it 

possible for the terminal device to uniquely determine 

the beginning of a frame. 

In the TE → NT direction (lower curve on oscilloscope 

display): 

The first, upward-moving pulse is the F bit (frame 

bit) with which the S 0 frame begins. According to 

definition, this bit is always a “positive 0”. 

The second, downward-moving pulse is the L bit 

or DC-bias compensation bit. According to definition, 

this is always a “negative 0”. The combination 

of the F bit and the L bit uniquely delineates 

the start of a frame. 

The third pulse, which also goes downwards, is 

the FA bit which causes violation of the coding 

rules paired with the second negative pulse (the 1st 

L bit). This violation makes it possible for the NT 

to uniquely determine the beginning of a frame. 

The fourth, upward-moving impulse is the 2nd L 

bit. This ensures the absence of DC bias in the 

partial frame containing the FA bit. 

2. Lift the telephone receiver. 

d) How does the oscilloscope display alter?


e) Where is the dial tone carried? 

The F and L bits representing the start of a frame 

are clearly recognisable from the oscilloscope display. 

Then there is an alteration in the oscilloscope 

display between the L bit and the FA bit when the 

PBX system sends the terminal device a dial tone. 

This tone is transmitted in the B channel and this 

causes the alteration in the display. 

Note: 

If the PBX systems assigns the B2 channel to the 

terminal device, then the alteration in the oscilloscope 

display occurs after the FA bit, which is 

where the B2 channel first appears in the S 0 frame. 

3. Add noise to the S 0 bus by turning the potentiometer 

(NOISE) slowly. (Switch ON.) 

f) What does this noise do? 

The oscilloscope display shows increased interference 

to the display in the form of pulses. Should 

any of these be larger than a predefined voltage 

value, the S 0 bus ceases to function. 

Note: This process can also be demonstrated by 

connecting one or two terminal devices to the NT 2 

outlet on the ISDN panel and attempting to establish 

a connection with one of them. Above a certain 

level of interference the connection is broken. 

4. Dial on the telephone the number of a terminal 

connected to the PBX system's second internal 

S 0 bus. Establish a connection between the two 

devices and conduct a call. 

g) In what ways does the oscilloscope display 

change when you send as constant a tone as 

possible, e.g. a whistling noise? 

If a sound is sent via the telephone microphone 

which is as near as possible to being a monotone, 

then it is easy to see a clear change in the oscilloscope 

display. If the terminal device is assigned 

the B1 channel, the change will appear between 

the first L bit and the FA bit. If the B2 channel is 

active, the change only appears after the FA bit because 

the first B2 bits in the frame are only sent 

after the 2nd L bit. 

h) Can you conduct an analysis of the D channel 

using the oscilloscope as it is presently 

set up? Give reasons for your answer. 

The oscilloscope is not suitable for analysing the 

D channel. This requires special measuring devices 

as the total amount of data to be transmitted is 

too great even for a storage oscilloscope. 

Example calculation: 

When block dialing, the layer-3 SETUP message 

is assumed to consist of 50 octets (1 Octet = 8 

bits). However, an S 0 frame contains only 4 D bits 

so that for each octet, two S 0 frames need to be 

transmitted. This means that, even for this one layer-3 

SETUP message, 100 S 0 frames need to be 

sent and each of these 100 frames would need to 

be individually inspected to locate the D bits. 

Clearly no one can afford to go to that much trouble 

in practice. 

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2 Services and Features in ISDN 

2.1 Automatic line test 

1. How can you conduct a simple automatic line 

test using the ISDN tester? 

The automatic line test can be performed either on 

the basic access ISDN connection or on the internal 

S 0 bus of the PBX system. The results of the 

line test on the internal S 0 bus depend on the range 

of services of the PBX system. 

To conduct an automatic line test, the ISDN tester 

is connected to an existing ISDN socket and 

turned on. After it is powered up, the menu S 0 interface 

and the entry automatic TE mode should 

be selected. The testing device now performs the 

test automatically and shows the results on the display 

after a short period. 

Explanation of the display: 

Argus 10 1.0 

B12 

S0 P-MP TE EDSS1 

Restart 

Menu 

Note: 

At the start of the line test LEDs L1, L2 and L3 

come on. The letter L stands for Layer and the 

LEDs thus apply to layer 1, layer 2 and layer 3. 

When the LEDs are on, it means that the layer is 

active, i.e. working. 

To find out the configuration or the protocol or 

whether the B channels are in use, the testing device 

has to establish a connection to the exchange 

via the D channel and thus all three layers need to 

be activated (see theory section). 

2. What results are shown on the display after 

completion of the test? 

The result of the test, as shown on the ISDN test- 

er's display, depends on the circumstances of the 

connection under test. The testing device checks 

whether the B channels are available and which 

configuration and protocol are in use on the connection. 

The test results are shown on the display. 

An example of possible test results are shown in 

the following solution. 

3. How do you interpret these results? 

One possible result of the automatic line test of an 

ISDN connection (internal S 0 bus or ISDN connection 

to the public telephone network) might be 

as follows: 

Argus 10 1.0 

B12 


Restart 

Menu 

The automatic line test shows that it has detected a 

basic access terminal in point to multipoint configuration 

using the D-channel protocol EDSS1 

(Euro-ISDN). Both B channels are available. 

4. How do the test results appear when a B channel 

is used during the course of the test? How 

can a B channel be occupied using the existing 

terminal devices for the duration of a line 

test? 

To occupy a B channel the easiest thing to do is to 

establish a connection between an ISDN telephone 

on the S 01 bus and one on the S 02 bus. 

Note: 

The direction of the call does not matter but the 

test cannot be performed until the connection between 

the two telephones is established.


Argus 10 1.0 

B2 


Restart 

Menu 

The line test detects a basic access terminal in 

point-to-point configuration using the protocol 

EDSS-1 (Euro-ISDN), but only the B2 channel is 

available as B1 is already being used for another 

connection. 

5. How do the test results appear when two B 

channels on the S 0 bus are used? How can you 

occupy both B channels on the internal S 0 bus, 

even though only three ISDN telephones in 

total are available to you? Can you achieve the 

exercise with only two telephones? 

The easiest way to occupy both B channels on the 

internal S 0 bus is to connect two ISDN telephones 

to the bus and establish a connection between the 

two of them. 

Note: 

The direction of the call does not matter but the 

test cannot be performed until the connection between 

the two telephones is established. 

L2 L3 

Argus 10 1.0 

B-- 


Restart Menu 

This basic access terminal (S 0 interface, point-tomultipoint 

configuration, Euro-ISDN Protocol) 

has no B channel available at this time. Such a 

case occurs, for example, when two subscribers 

have an active connection at the same time. 

6. How does the testing device react when it is not 

connected to the S 0 interface during the line 

test? 

In this case, the S 0 bus, upon which the line test is 

to be performed is disconnected from the PBX 

system. 

Argus 10 1.0 

No/wrong network 

Continue 

The automatic line test cannot be performed. The 

test equipment indicates the fact that it has been 

unable to detect a network as a fault. Possible reasons 

for this to occur include, for example, the 

ISDN tester not being connected to the S 0 bus at 

all or being connected to the wrong socket on the 

S 0 bus. 

2.2 Test of services 

2.2.1 Automatic testing of services 



In an automatic services test, the testing device 

uses the D-channel protocol to test whether 

connections can be established between the 

various services for terminal devices. The results 

are shown in simplified form on the tester's 

display. 

If a terminal device has been checked using a 

services test and found to be OK yet still fails to 

work, then the process of eliminating possible 

faults simply has to be continued. 

2. Which test menu do you call up on the ISDN 

tester for this? 

The ISDN tester's automatic service tests are started 

from the à Individual tests menu under the 

sub menu Service test. 

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tester? Describe the testing process step by 

step. 

The menu for the individual tests is called up 

by pressing the ü key. 

• The â button can be used to scroll through the 

services tests 

• The required service test is then selected via the 

ü key 

• Now the call number can be entered. 

Important: 

By scrolling with the â button, it is possible to 

switch back and forth between the entry field 

for one's own number and that for entering an 

external number. 

• The entry of a number is confirmed with the ü 

key. 

• Now the B channel can be selected. 

Note: 

Selection of a B channel means that the test 

equipment requests the use of the selected B 

channel from the exchange. Whether it is assigned 

this channel, however, depends on 

whether the channel is free. 

• Next the ü key is pressed to begin the test 

4. Carry out the services test by means of selfcalling. 

What are the objectives of the test 

when calling the test device's own number? 

Interpret the results of the test. 

Should anyone call their own number it is called a 

self-call. A self-call can be used to test whether a 

service is working properly for both incoming and 

outgoing calls. For an outgoing call, the exchange 

accepts the service and passes it on to the dialed 

subscriber. 

A self-call can also be used to determine whether 

the exchange has accepted this service with the 

incoming call and passed it onto the connection 

under test so that compatible terminal devices can 

accept it. 

A self-call can help the service test achieve an accurate 

result, as when the test equipment is both 

the instigator and the receiver of a call, it can categorically 

determine if the transmission is reaching 

its own connection terminal. 

A self-call on the internal S 0 bus works according 

to the same principle as a call to the public ISDN 

connection. It checks to see whether the PBX system 

for this bus can accept a service for both incoming 

and outgoing calls. 

Possible test results for a self-call: 

Service outgoing incoming 

Tel. analogue + + 

Tel. ISDN + + 

Fax G3 + + 

Fax G4 + + 

For these services communication can be established 

via the tested connection for both incoming 

and outgoing calls. 

The following may be displayed as a result: 

+ the service is available on this bus 

– the service is not available on this bus 

* no unambiguous information on this service 

5. Conduct a test of services by calling a remote 

number (=establishing connection to the S 02 

bus). What are the objectives of the test when 

calling a different number to that of the test 

device? Interpret the results of the test. 

If a remote number is called in the testing process, 

there can only be a definite indication of 

whether a service is available (or functioning) 

for the local terminal. 

For the dialed subscriber to be able to identify 

the service, a compatible terminal device has to 

be connected and able to indicate its presence at 

the dialed number. If the dialed subscriber has 

no compatible terminal device of if a compatible 

device is connected there but installed under 

a different number, the exchange may receive 

no acknowledgement from the dialed system. 

Thus the test device making the call is informed 

by the exchange that the dialed subscriber has 

not responded. An unambiguous conclusion 

about the availability of the service cannot 

therefore be drawn. 

To recognise a service as being available, the 

ISDN tester requires the layer-3 message 

ALERT to be sent from the receiving terminal 

(meaning, "it's ringing"). This ALERT message


can only be sent by a terminal device which is 

capable of using the service signalled in the incoming 

messages. If the ISDN tester does not 

receive the ALERT message, it ends the connection 

and normally no charges are run up. 

Internal S 0 bus 

If the service test is conducted on the internal S 0 

bus, an extension number belonging to another internal 

S 0 bus is entered as the call number of the 

remote subscriber. It is recommended that at least 

one terminal device be connected to this second S 0 

bus so that the result also includes an available 

service on the receiving side. The service test is 

then conducted exactly as for the public ISDN 

connection. 

To obtain an unambiguous result concerning the 

services available to the remote subscriber, a second 

ISDN tester in TE-simulation mode can be installed 

there which should be able to react to all 

incoming calls (with various services). 

Possible test results for a self-call: 

Service outgoing incoming 

calling called 

subscriber subscriber 

Tel. analogue + + 

Tel. ISDN + + 

Services “work” on outgoing calls from the caller 

and incoming calls to the remote subscriber (unambiguous 

information) 

Fax G3 + * 

Fax G4 + * 

Services “work” on outgoing calls from the caller. 

It is not unambiguously known whether services 

are supported for incoming calls to the remote subscriber. 

+ the service is available on the bus 

– the service is not available on the bus 

* no unambiguous information on this service 

6. Why do the LEDs L1, L2 und L3 turn on or 

flash during the course of the test? 

The automatic line test uses the exchange of layer- 

3 messages on the D channel. To test the reactions 

of the exchange and the remote subscriber, a layer-3 

connection is established for every service 

and the layer-3 disconnection procedure is conducted 

after every test is finished. In order to activate 

layer 3, layers 1 and 2 must already be active. 

When the LEDs L1, L2 and L3 are lighting 

up or flashing, they are indicating this activity on 

the layers. 

2.2.2 Testing of an individual service 

Answers 



The ISDN tester is used as a TE simulator, i.e. it 

emulates a terminal device using the group-3 fax 

service. A single service can be checked by having 

the ISDN tester establish a connection to a remote 

subscriber. If this outgoing call is accepted by the 

exchange and forwarded to the 2nd subscriber then 

this service is operating successfully at the existing 

connection. 

The remote subscriber can be connected to the second 

internal S 0 bus for the purposes of the training 

system. The terminal device used for this must be 

either a group-3 fax machine or alternatively a second 

ISDN test device. Note that an analog device 

must be connected to the S 0 bus by means of an 

a/b adapter. 

2. Which test menu on the do you call up on the 

ISDN tester for this? 

To establish a connection to a remote subscriber 

for a specific service, the submenu Connection 

under Individual tests is used. 


tester? Describe the testing process step by step. 

The steps in the process are as follows: 

à Individual tests ü key 

à Connection ü key 

enter remote call number and confirm with the ü 

key 

select the service using the à key 

confirm selection with the ü key 

Select the required B channel by entering 1 or 2. 

dialing begins when the ü key is pressed. 

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4. What information are you given by the device's 

display during establishment of the connection 

(= dialing)? 

Device display 

Dialing 

Own call number 

To: dialed number 

FAX G3 

B channel 

The ISDN tester's display shows... 

• whether the call is being dialed (dialing), 

• the service with the connection is being established 

(Fax G3 = Group-3 Fax), 

• the number being sent as the tester's call 

number (providing the number has previously 

been entered into the tester), 

• which B channel is in use (B channel) 

• the number being dialed. 

5. What is the result of the test when a connection 

is successfully established? How do you interpret 

this result? 

Testing device display 

Connection 

Own call number 

To: dialed number 

FAX G3 

B channel 

Explanation: 

A connection is established with the group-3 fax 

service, i.e. the dialed subscriber has a terminal 

device which has accepted the call. The testing 

device also shows the B channel in use on the local 

line, the local number (provided it has been 

entered into the equipment) and the dialed 

number. 

Results in this case: 

The service is available on both this line and on 

the called subscriber's. 

Summary: 

If a fault remains, the process of elimination of 

errors should continue with a check on the hard- 

ware and software installation of the PC card, the 

terminal adapter (TA a/b) or the PBX system and 

its configuration (protocol, local call number etc.). 

6. What is the result of the test when there is no 

terminal device to receive the call at the dialed 

number? How do you interpret this result? 

Rejection 

Cause 18 

No answer 

Explanation: 

The testing equipment displays that the connection 

was refused. The number 18 is shown to identify 

the cause of the failure and the test device also 

displays that it has interpreted this to mean, "no 

answer". 

In the original Euro-ISDN specification for the 

D-channel protocol on layer 3 (Note: ETS 300 

102-1; ETS = European Telecommunication 

Standard) a list of possible causes for failure is 

defined. Each cause is assigned a number which is 

what the test device is showing here. A list of the 

most important causes is attached to the theory 

section as an appendix. (see 9.2). 

The ETS specification defines cause 18 to be “no 

user responding”, i.e. the dialed subscriber 

number has not acknowledged the presence of any 

suitable equipment to accept the call. 

Result: 

It can be seen that the outgoing call has reached 

the dialed subscriber because the number 18 for 

the cause of failure has been returned from there. 

The service is thus available at the local connection 

since the exchange has passed on the call to 

the network. 

Summary: 

In order to continue with the process of eliminating 

errors, it is sensible to check, amongst other things, 

the hardware and software installation of the PC 

card, of the terminal adapter or the PBX system 

and the configuration of the protocol, the local call 

number, etc.)


2.3 Features 

1. What is the original designation for Hold in the 

Euro-ISDN Specification? 

The original designation for hold is Call Hold, so 

HOLD for short. 

2. Why must you know the technical abbreviations 

and the original descriptions of a feature 

in practice? 

It is essential to know the abbreviations used in 

ISDN and their precise meanings in order to properly 

go about the measurement of ISDN systems 

or devices since these terms frequently appear in 

the menus of many devices. 

3. Where is the Hold feature described in detail? 

Every feature is described in an ETS specification 

of its own. Apart from a precise definition , the 

specification also describes the procedures and 

protocol messages necessary for that feature. The 

HOLD feature is defined in ETS standard 300 141. 

Note: 

An overview of the features available in Euro 

ISDN appears in the book “Euro-ISDN für Installateure 

und Servicetechniker” (Spath/Heermeyer, 

page. 56). 

4. Describe the process of holding? 

Hold means that a subscriber has two connections 

established and can switch back and forth 

between the two of them (toggling). In this case, 

one B-channel is occupied by the call in progress 

while the second call is held at the exchange. The 

hold feature can take two different forms in practice: 

1. A second call arrives while the subscriber is already 

conducting a call. The subscriber wishes to 

accept this second call without breaking the connection 

to the first. By selecting the Hold feature 

from the menu on the telephone, the subscriber 

passes the existing call to the exchange where it 

will be held for a space of three minutes. The 

incoming call can then be accepted. 

If the subscriber wishes to continue the first conversation, 

there is a choice of ending the second 

connection so that the exchange continues to deal 

with the first (take note of how this works on the 

telephone in question!) or allowing the second 

call to be held at the exchange while returning to 

the first. In this case the subscriber can switch 

back and forth between the two calls. 

2. The second way the Hold feature can be used 

occurs when the subscriber is already conducting 

a call but then wishes to establish a second connection 

at the same time (consultation). 

In technical terms the activation and deactivation 

of the Hold feature is achieved by means of a 

specific protocol sequence on the D channel (layer 

3). 

Note: 

Telephone charges continue to be calculated even 

when a call is held. 

5. State how many B channels are used by the 

Hold feature. 

Only one B channel is used by the Hold feature. 

While the subscriber employs the B channel for 

the current call, the second is held at the exchange. 

6. How do you test the availability of the hold feature 

using the ISDN tester? Which menu do you 

use for the individual test? Describe the operating 

method step by step. 

Tests for individual features can be chosen from 

the Individual tests menu by selecting the submenu 

Features 

Operating steps: 

à Individual tests, confirm with ü key; 

à Features, confirm with ü key; 

own call number: enter, confirm with ü key; 

à Select HOLD test, confirm with ü key; 

A test of the Hold feature is then carried out automatically. 

7. What result is shown when Hold is working 

properly on the line (and if it isn't)? 

HOLD + 

The HOLD feature is available and working on 

this line. 

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Result if not functioning: 

HOLD – 

The HOLD feature is not available or not working 

on this line. 

8. What reasons could there be for Hold not to 

work? 

Examples: 

a) the subscriber is not operating the terminal device 

correctly 

b) the feature is not supported on the line and has 

not been ordered. 

c) it was accidentally forgotten to enable the feature 

at the exchange. 

2.3.2 The features Calling Line Identification 

Presentation CLIP 

and Calling Line Identification Presentation 

CLIR 

1. What are the technical abbreviations for 

Calling Line Identification Presentation and 

Calling Line Identification Restriction? 

The acronym for Calling Line Identification Presentation 

is of course CLIP; and for Calling Line 

Identification Restriction, it is CLIR. 

It is essential to know the abbreviations used in 

ISDN and their precise meanings in order to properly 

go about the measurement of ISDN systems 

or devices since these terms frequently appear in 

the menus of many devices. 

Every feature is described in an ETS specification 

of its own. Apart from a precise definition, the 

specification also describes the procedures and 

protocol messages necessary for that feature. The 

CLIR feature is defined in ETS standard 300 093. 

Note: 

An overview of the features available in Euro 

ISDN appears in the book “Euro-ISDN für Installateure 

und Servicetechniker” (Spath/Heermeyer, 

page. 56). 

2. Explain to the customer how Calling Line 


Identification Restriction work? 

a) Calling Line Identification Presentation 

(CLIP): 

The call number of subscriber A (= the subscriber 

making the call) is automatically 

conveyed to the dialed subscriber B during 

establishment of the connection and may, 

for example, be shown on the display of the 

ISDN telephone at the receiver end. 

b) Calling Line Identification Restriction 

(CLIR): 

If the caller has activated Calling Line Identification 

Restriction, either permanently or 

only for certain calls, the call number is 

only transmitted as far as the dialed subscriber's 

exchange. The exchange recognises 

the suppression of the caller's number 

and doesn't pass it on to the dialed subscribers 

connection. 

3. How does it affect Calling Line Identification 

Presentation on the B channel when... 

a) Subscriber A has entered an incorrect MSN 

into their telephone? 

b) Subscriber A has not entered an MSN into 

their telephone? 

a) incorrect MSN 

Public network: 

The telephone sends the number that has been entered 

into it to the exchange during establishment 

of a connection. In this case it would be the wrong 

MSN. The exchange compares the transmitted 

number with the block assigned to the subscriber 

and determines that the number is not present in 

this block so it ignores the received MSN and replaces 

it with the assigned default number for the 

connection and continues establishing a connection. 

When CLIP is activated, the dialed call is displayed 

a number which does belong to the caller's 

connection. 

Default number: 

A default number is assigned by the network provider 

as the subscriber's “main” telephone number 

and is always used when the received MSN is incorrect 

or no MSN is supplied.


PBX system: 

Basically the behaviour of the PBX system is dependent 

on the manufacturer's settings or the subscriber's 

individual configuration. Take note of the 

remarks in the manufacturer's manual. 

If an incorrect call number is entered into a telephone 

connected to an internal S 0 bus, there are 

two possible ways that the PBX system could react, 

depending on the aforementioned factors: 

1. The PBX system could ignore the incorrect 

number and transmit the correct extension 

number to the dialed terminal device on the 

second internal S 0 bus. 

2. The PBX system refuses the call from the terminal 

device. In this case D-channel data 

should be studied to find out the precise 

cause for this. 

b) No MSN 

Public network: 

If the exchange receives no MSN with the outgoing 

call, it adds the default number to the call and 

continues establishing the connection. 

PBX system: 

The statements made under a) about the manufacturer's 

settings and the subscriber's configuration 

with regard to the way the PBX system reacts also 

apply here. 

4. How can you use the ISDN tester to check 

whether Calling Line Identification Presentation 

or Calling Line Identification Restriction 

are activated on the customer's connection? 

Describe the method of operation step 

by step. 


à Individual tests. Confirm with ü key; 

à Features, Confirm with ü key; 

own call number: enter, Confirm with ü key; 

à Select CLIP test, Confirm with ü key; 

The CLIP test is then conducted automatically. On 

this ISDN tester, the test procedure also contains a 

test of the CLIR, COLP and COLR features. 

Explanation: 

COLP: Connected Line Identification Presentation 

Transmission of the dialed subscriber's 

number to the caller 

COLR: Connected Line Identification Restriction 

Suppression of the transmission 

of the dialed subscriber's number 

to the caller 

5. How do you interpret the results? 

Features that have been tested are marked on the 

ISDN tester by a following “+” if the feature is 

activated or “-” if the feature is deactivated or unavailable. 

The results might then look like this: 

CLIP 

CLIR 

COLP 

COLR 

This result shows that Calling Line Identification 

Presentation (CLIP) is activated but Calling Line 

Identification Restriction (CLIR) is not, i.e. the 

subscriber receiving the call would normally see 

the number of the caller on the display of an ISDN 

telephone. 

The result also shows that the number of the subscriber 

accepting the call has been transmitted 

back to the caller (COLP). Restriction (suppression) 

(COLR) of this number is not activated. 

6. What reasons could there be for Calling Line 

Identification Restriction not to work? 

Examples: 

a) the feature is not supported on the line and has 

not been requested. 

b) the subscriber is not operating the terminal device 

correctly 

c) the subscriber's terminal device has faulty software 

d) it was accidentally forgotten to enable the feature 

at the exchange. 

+ 

– 

+ 

– 

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3 D-channel protocol 

3.1 Establishment and ending of connections 


Answers 



The ISDN tester is connected to the S 0 bus with 

the help of the S 0 connection cable. This is inserted 

into the S 0 bus socket; connection to the PC is 

made using the supplied cable via the L-Test socket. 

2. Which function of the ISDN tester do you have 

to activate to enable communication with the 

PC? 

So that the D-channel data can be transmitted from 

the ISDN tester to the PC, the testing device must 

be switched into Trace mode. 

The precise operating process is as follows: 

• Install WinPlus software onto the PC 

• Connect ISDN tester to the S 0 bus and the PC 

• Start the WinPlus Software 

• Start Trace mode on the test device: Submenu 

under the menu Communication 

Call Trace mode and switch on so that the 

LEDs are illuminated. 

• Conduct the test procedures 

Note: 

Two software packages are available for the ISDN 

tester: WinPlus and WinAnalyse. The WinPlus 

Software concentrates on decoding the sequence 

of messages whilst WinAnalyse also permits the 

display of the message contents (Information elements). 

During the first steps of the learning process, 

it is sufficient to see the sequence of messages 

using WinPlus Software. 

3. What does Trace mode mean? 

In terms of measuring technology, the word trace 

means the recording of steps within a process. In 

Trace mode the ISDN testing device records the 

D-channel data. The limitation is that only the Dchannel 

data transferred between the testing device 

and the destination, e.g. the exchange, can be 

traced. 

In order to trace all D-channel data appearing on 

an S 0 bus – i.e. the data transmitted and received 

from all the terminal devices on the bus, an alter- 

native mode is employed which is called "passive 

monitor operation". 

4. How do you establish a connection? 

Select the submenu Connection from the menu 

Individual tests. Enter the call number for the remote 

subscriber and the requested service then establish 

a connection. 

5. What is the sequence of data on the D channel in 

layer 3 when subscriber B accepts the telephone 

call? Create a diagram containing all the Dchannel 

messages with their directions of transmission. 

The testing device only traces the D-channel data 

appearing at the caller's end. To elucidate the interplay 

between the outgoing (caller) and incoming 

(destination) calls, however, this diagram 

shows both sides of the connection. 

6. What are the layer-3 messages that appear and 

what are their meanings? 

The protocol sequence sketched above assumes 

that 

– the call was block-dialed,


– there is a free B channel 

– the dialed subscriber B has a telephone at the 

dialed number 

– the dialed subscriber B accepts the call. 

SETUP The SETUP message is sent when a subscriber 

or the exchange wishes to establish 

a connection using the B channel. 

CALL PROC 

The CALL PROCEEDING message indicates, 

the dialed number has been detected 

as complete and the connection 

has begun to be established. No further 

information will be accepted. 

ALERT The ALERT message is sent to signal 

that dialed subscriber is ready to receive 

a call (the telephone rings). 

CONN The CONNECT message indicates that 

the dialed subscriber has accepted the 

call (e.g. by lifting the telephone receiver). 

CONN ACK 

CONNECT ACKNOWLEDGE confirms 

that CONNECT has been received. 


when subscriber B has no telephone connected 

under the dialed number? (Draw a diagram!) 

If no device belonging to the dialed subscriber responds 

to the SETUP message, the destination 

exchange informs the caller's exchange after the 

ringing stops so that it can instigate the ending of 

the connection to the caller. 



This time, in addition to the messages for establishing 

the connection, messages are sent which 

are reserved for the ending of a connection 

DISC DISCONNECT requests the “D-channel 

partner” to end the connection. 

REL RELEASE indicates that the caller has 

switched itself off from the B-channel 

and released both the B channel and the 

layer-3 address (Call Reference). 

The caller requests the “D-channel partner” 

to do the same. 

REL COM 

RELEASE COMPLETE is sent by the 

caller to indicate that the B channel and 

the Call Reference have been released. 

If the desired connection cannot be established, 

one or other of these messages will contain the 

cause for the ending of the connection. 

3.2 Testing a feature with the help of 

D-channel information 



The ISDN tester is connected to the S 0 bus with 

the help of the S 0 connection cable. This is inserted 

into the S 0 bus socket; connection to the PC is 

made using the supplied cable via the L-test socket. 

2. Describe, step by step, the actions needed to 

record the D-channel data using WinPlus. 

The precise operating process is as follows: 

• Install WinPlus software onto the PC 

• Connect ISDN tester to the S 0 bus and the PC 

• Start the WinPlus Software 

• Start Trace mode on the test device: Submenu 

under the menu Communication 

Call Trace mode and switch on so that the 

LEDs are illuminated. 

• Conduct the test 

Note: 

Two software packages are available for the ISDN 

tester: WinPlus und WinAnalyse. The WinPlus 

Software concentrates on decoding the sequence 

of messages whilst WinAnalyse also permits the 

display of the message contents (Information ele- 

77

78 


ments). During the first steps of the learning process, 

it is sufficient to see the sequence of messages 

using WinPlus Software. 

3. How do you test the Hold function using the 

ISDN tester? 

Tests for individual features can be chosen from 

the Individual tests menu by selecting the submenu 

Features 


à Individual tests. Confirm with ü key; 

à Features, Confirm with ü key; 

own call number: enter, Confirm with ü key; 

à Select HOLD test, Confirm with ü key; 

A test of the Hold feature is then carried out automatically. 

4. What result is shown on the ISDN tester? 

Results of test when successful: 

HOLD + 

The feature HOLD is available and working on 

this line. 

Results of test when unsuccessful: 

HOLD – 

The feature HOLD is not available and not working 

on this line. 

5. What layer-3 D-channel data is sent during the 

test procedure? Prepare a sequence diagram. 

Activation of the HOLD feature: 

Acknowledgement of holding procedure: 

Hold feature declined by the exchange: 

Acknowledgement declined by the exchange:




HOLD The terminal device sends the HOLD 

message to request that the active call be 

held and the B channel currently in use 

for it be made available for the establishment 

of a further connection. 

HOLD ACK 

HOLD ACKNOWLEDGE is a confirmation 

from the exchange that it is holding 

the active connection and has released 

the B channel for a further connection. 

HOLD REJ 

In sending HOLD REJECT the exchange 

declines the request to hold the 

active connection. 

RETRIEVE 

RETRIEVE is sent by the terminal device 

to request return of the call held by 

the exchange. 

RETRIEVE ACK 

In the RETRIEVE ACKNOWLEDGE 

message, the exchange passes the held 

call back to the terminal device and assigns 

it to its original B-channel. 

RETRIEVE REJ 

In sending RETRIEVE REJECT to the 

terminal device, the exchange declines 

the request to return to a held call. This 

message can be sent for various reasons, 

e.g. because the call was held at the exchange 

for longer than the allowed time 

so that connection was cut off. 

79

Index 

80 


A 

ARGUS 10 ------------------------------------ 52, 54 

B 

B1-, B2 bits -------------------------------------- 24 

Basic access ----------------------------------------- 9 

Block dialling ---------------------------- 41, 67, 76 

C 

Cable pair --------------------------------- 17, 18, 55 

Called party number ----------------------------- 39 

Calling Identification Display ----- 14, 43, 58, 74 

Calling Line Identification 

Display -------------------------------------------- 75 

Calling Line Identification 

restriction ---------------------------- 14, 43, 58, 74 

Calling party number ------------------- 39, 41, 42 

Characteristic impedance ----------------------- 55 

CLIP -------------------------------------------- 15, 40 

CLIR ------------------------------------------- 15, 40 

Coding ---------------------------------------- 60, 61 

COLP---------------------------------- 15, 47, 53, 75 

COLR ---------------------------------15, 47, 53, 75 

Compressor---------------------------------------- 61 

Connection menü --------------------------------- 53 

Connection mode --------------------------------- 52 

D 

D bit -------------------------------------------- 24, 67 

D channel (Signal channel) ----------------------- 9 

D channel access control -------------------- 22, 25 

DDI ---------------------------------------- 14, 39, 47 

DISC ---------------------------------- 32, 37, 47, 77 

E 

E bit -------------------------------------------- 24, 47 

Emergency operation --------------- 17, 54, 55, 63 

Ending of connections ------------------ 32, 34, 35 

Establishment of connections --------- 15, 22, 26 

Extended passive bus ------------------------ 17, 19 

Features -------------------------------------------- 13 

H 

HDLC protocol ----------------------------------- 27 

Hold --------------------------------------- 16, 34, 43 

HOLD ---------------------------- 43, 47, 53, 73, 78 

I 

I format ----------------------------------- 27, 28, 33 

Individual dialling ---------------------------- 40, 53 

INFO S 0 ----------------------------------------------------------- 26, 48 

INFO S1 --------------------------------------- 26, 48 

INFO S2 --------------------------------------- 26, 48 

INFO S3 --------------------------------------- 26, 48 

INFO S4 --------------------------------------- 26, 48 

Information elements --------------- 34, 38, 40, 76 

Installation experiments ------------------------- 54 

Integration of services ----------------------------- 9 

Interfaces ----------------- 9, 11, 12, 16, 17, 24, 53 

Internal S 0 bus ------------------------------------ 11 

ISDN panel ---------------------- 50, 54, 55, 63, 64 

L 

L bit ------------------------------ 24, 48, 64, 66, 67 

Layer-1 activation procedure --------------- 25, 26 

Layer-1 deactivation procedure ---------------- 26 

Layers -------------------------------- 21, 52, 68, 71 

Line coding ---------------------------------------- 62 

Line quantities per unit length ------------- 55, 65 

Line testing ------------------------------- 57, 68, 69 

M 

MoU -------------------------------------------------- 8 

MSN ------------------------------------------------ 14 

Multiple device connection --------------------- 10 

N 

Network termination/terminal ------------------ 10 

O 

OSI reference model -------------------- 21, 22, 31 

P 

PAM ------------------------------------------- 50, 60 

passive monitor operation ----------------------- 53 

PBX system --------------------------------------- 10 

Phantom circuit ------------------------------ 17, 18 

Physical layer ------------------------------------- 21 

Point-to-multipoint -------------------------------- 9 

Point-to-multipoint configuration -------------- 10 

Point-to-point -------------------------------------- 9 

Point-to-point configuration ------------------- 10 

presentation indicator ----------------------- 42, 43 

Primary multiplex (rate) access ------------------ 9


Q 

Quantisation --------------------------------------- 60 

S 

S format ----------------------------------- 27, 28, 31 

S 0 frame structure ----------------------- 23, 55, 66 

SABME ---------------------------------------- 31, 48 

Sampling ------------------------------------------- 60 

SAPI --------------------------------------- 29, 33, 48 

Data link layer --------------------------- 21, 27, 48 

Service features menu --------------------------- 53 

Service test menu--------------------------------- 53 

Standard operation ---------------------- 17, 55, 63 

Subscriber interfaces ----------------------------- 10 

supplementary services -------------------------- 13 

Swapped terminals ----------------- 54, 55, 63, 64 

System Configuration Program ---------------- 52 

T 

TE Mode menu ----------------------------------- 53 

TEI -------------------------- 28, 30, 31, 33, 48, 49 

Toggling ---------------------------------- 16, 58, 73 

Trace mode ------------------------------- 53, 59, 76 

Transport layer-------------------------------- 22, 31 

U 

U format ---------------------------------- 27, 32, 33 

UA 31, 32, 49 

UAE socket --------------------------------------- 20 

V 

Violations of coding rules ------------- 24, 54, 66 

W 

WinAnalyse -------------------------- 53, 59, 76, 77 

WinPlus ------------------------------- 53, 59, 77, 78 

Wiring errors -------------------------------------- 20 

81

82 

MTS 7.1.2.1 Fundamentals of ISDN Technology Answers

option TPS 7.1.2.2 - Inele

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