Field Trial of Optical Fibre Cable-TV System Optical Fibre System for ...

ERICSSON 

REVIEW 

4 1985 

Field Trial of Optical Fibre Cable-TV System 

Optical Fibre System for Digital Cable-TV Transmission, ZAV 280/4 

Wavelength Division Multiplexing for Fibre-Optic Subscriber Lines 

Modems Series 7 

Computer Aided Production of Plastic Details 

Rectifier for Mobile Telephone Systems 

The Renewal of the London Underground Telecommunications Network 

Reliability of Transmission Equipment

ERICSSON REVIEW 

Number 4 1985 Volume 62 

Responsible publisher Gösta Lindberg 

Editor Gösta Neovius 

Editorial staff Martti Viitaniemi 

Address S-126 25 Stockholm, Sweden 

Subscription one year $ 16 

Published in Swedish, English, French and Spanish with four issues per year 

Copyright Telefonaktiebolaget LM Ericsson 

Contents 

154 • Field Trial of Optical Fibre Cable-TV System 

161 • Optical Fibre System for Digital Cable-TV Transmission, ZAV 280/4 

170 • Wavelength Division Multiplexing for Fibre-Optic Subscriber Lines 

175 • Moders Series7 

180 • Computer Aided Production of Plastic Details 

184 • Rectifier for Mobile Telephone Systems 

192 • The Renewal of the London Underground Telecommunications 

Network 

203 • Reliability of Transmission Equipment 

Cover 

Computer aid (CAD/CAM) is widely used in Ericsson's 

factories, now also in the manufacture of 

plastic details. See the article on page 180

Trial of Optical Fibre Cable-TV 

System 

Kurt Bergsten and Gerhard Gobi 

The Swedish Telecommunications Administration and Ericsson are jointly 

carrying out a field trial with optical fibre cable-TV. Ericsson has developed and 

supplied the digital transmission system used between the main centre and 

subcentres as well as the equipment for the analog subscriber lines. The optical 

fibre cable used in the trial was also made by Ericsson. 

The authors describe the objectives and scope of the field trial and consider the 

future development in this engineering field. 

UDC 535.394:621.397.74.001.55 

optical links 

testing 

digital communication systems 

cable television 

isdn 

frequency division multiplexing 

Fig. 1 

Broadband video, audio, data and text 

communication network 

HC 

UC 

Head end 

Subcentre 

Modern telecommunication networks 

are heading towards service integration, 

with the conversion into digital networks 

with transmission over glass fibre 

being a major step in this direction. Television 

programs will also be transmitted 

over such networks in future. The Swedish 

Telecommunications Administration 

and Ericsson decided to test digital 

transmission of picture and sound over 

optical fibres by carrying out a field trial 

of a newly developed digital, optical 

fibre transmission system within the 

framework of a cable-TV project for 

Skarpnäck, a new suburb near Stockholm. 

The Telecommunications Administration's 

activities in the field of cable television 

form part of its long-term work to 

establish service-integrated broadband 

local networks. Such networks, unlike 

present-day cable-TV networks, will be 

based on optical fibre technology and 

have a star-shaped structure in order to 

facilitate integration with other services, 

such as narrowband ISDN, videotelephony, 

television conferences and 

data transmission. These networks will 

also facilitate the introduction of interactive 

video services such as teleshopping, 

telebanking and access to video 

libraries via home terminals. It may 

therefore be foreseen that coaxial cables 

will gradually be superseded by 

fibre cables, starting at the higher levels 

of cable-TV networks, i.e. in trunk and 

primary networks. As costs go down 

fibre cables will be used increasingly at 

lower levels-and in the nineties it is likely 

that the fibre cable and digital techniques 

will be used right up to the subscriber's 

termination unit in the home. 

In 1982 the Swedish Telecommunications 

Administration and Ericsson started 

a joint development project for the 

implementation of the field trial. The 

project was aimed at field tests with optical 

fibre cable between a head end at the 

Farsta automatic exchange and a sub-

155 

KURT BERGSTEN 

Networks Department 

Central Administration of Swedish 

Telecommunications 

GERHARD GOBL 

Public Telecommunications Division 

Telefonaktiebolaget LM Ericsson 

Conventional cable-TV 

Coaxial cable and broadband amplifiers are 

used for conventional cable-TV systems. The 

networks have almost exclusively a tree and 

branch structure, which has proved to be very 

economical for the distribution of television 

and sound radio programs from a head end to 

the households. 

A conventional cable-TV network has the following 

levels: 

Trunk network, which is the network between 

head ends if several such head ends are linked 

together. The link between the satellite ground 

! station and a head end is also a part of the trunk 

i network. 

/l Primary and secondary network, comprising 

J the networks between the head end and the 

i points where the signals are delivered to the 

I distribution networks. 

I 

Distribution network, which is constructed in a 

residential area or premises. The distribution 

centre in a residential area at Skarpnäck 

about 10 km from the centre of Stockholm. 

The 3 km long cable is used to 

transmit the TV and sound radio programs 

which are fed in at the head end 

to a subcentre at Skarpnäck. The transmission 

is digital. In the subcentre the 

signals are converted back to analog 

form for further distribution over a conventional 

cable-TV system to the households 

in the area. The field trial has since 

been extended by an additional fibreoptic 

cable to offer the same programs 

as at Skarpnäck to a residential area at 

Enskededalen, about 4.5 km from the 

main centre at Farsta. Fig. 1 shows the 

fibre-optic network serving Skarpnäck 

and Enskededalen. The network may be 

seen as a first stage in the construction 

of a future broadband video, audio, data 

and text communication network. The 

figure also shows a planned extension 

whereby a satellite antenna on the Kaknäs 

Tower is connected to the Skarpnäck 

network. Programs received via 

INTELSAT and other satellites may then 

be distributed over the cable-TV network. 

Objectives 

The introduction of new products, e.g. 

transmission systems, in telecommunication 

networks is nearly always preceded 

by more or less extensive field 

trials aimed at detecting any weaknesses 

in the system in good time so that 

networks correspond generally to today's central 

antenna networks. 

Base network, is a common name for trunk, 

primary and secondary networks. 

The Swedish Telecommunications Administration 

is today constructing cable-TV networks at 

all these levels. The main emphasis is on base 

networks while distribution networks will be 

constructed principally in new residential 

areas. 

Broadband networks 

In a future service-integrated broadband network 

optical fibres will be used also for the 

subscriber lines. The networks will have a starshaped 

structure and every subscriber will be 

connected to a central point in the network 

where there is switching equipment for different 

services such as telephony, television 

and sound radio programs. In such a network 

only the programs selected by the subscriber at 

a particular time will be transmitted. 

they can be put right before the system 

is introduced on a wider scale. A field 

trial also provides valuable experience 

and ideas for future improvements. The 

aimof the field trial is to gain experience 

of 

- digitization of television and sound 

radio signals, comprising methods 

for digitization, encoding and reduction 

of redundancy in TV pictures 

- transmission engineering, comprising, 

modulation methods, the capacity 

and quality of the link, cost optimization, 

circuitry and choice of components 

- installation techniques, comprising 

the handling of fibre cables in base 

networks at branches, termination, 

splicing, fault tracing and repair 

- system engineering with network topology, 

integrated services, mix of 

distributive and interactive services, 

control and supervision as well as 

economy 

- operation and maintenance of fibreoptic 

cable-TV systems, comprising 

the collection, transmission and presentation 

of alarms. 

Selection of the route 

A residential area in Skarpnäck was 

chosen for the trial. In this area some 

4000 flats are being built during the 

period 1983-1987, and also about 150 

small houses. South of the area some 

light industries will be located. The 

Farsta automatic exchange about 3 km 

from Skarpnäck has been selected as 

the head end. Along the entire route between 

the Farsta exchange and Skarpnäck 

there are ducts to be shared by 

metallic telephony and fibre cables, resulting 

in major cost reductions. 

Along the route there are also other residential 

areas, e.g. Norra Sköndal and 

Södra Sköndal, which henceforth may 

receive TV and sound radio signals from 

the main centre by optical tapping of the 

fibre-optic cable. 

In 1984 an existing residential area in 

Enskededalen with some 2000 flats was 

included in the trial area. 

System structure 

Digital transmission 

The block diagram of the trial plant is 

shown in fig. 2. Two optical cables, each

Head end (HC] 

280 Mbit/s 

Farsta automatic exchange 

Enskede 

Subcentre (UC) 

Skarpnäck 

Fig. 2 

Block diagram of the digital trial plant between 

Farsta and Skarpnäck 

Fig. 3 

A branching point on the optical fibre cable 

between Farsta and Skarpnäck 

containing 12 fibres, have been laid between 

the Farsta automatic exchange 

and the subcentre at Skarpnäck. The cables 

are of standard size. The large number 

of fibres was selected with future 

experiments in mind. 

The fibre-optic cable has been provided 

with optical taps so that other residential 

areas along the route can be supplied 

with TV and sound radio programs 

from the main centre. There are three 

optical taps in tandem, see fig. 2. A 

branching point is shown in fig. 3. 

To gain experience of the effect of different 

branching methods on the transmission 

characteristics two different 

types of coupler have been installed, 

one mode-selective (fused type) and one 

non-mode-selective (lens and mirror). 

The digital fibre-optic transmission system 

ZAV280/4, which is described separately, 

is used for the transmission. 

ZAV280/4 transmits 4 television channels 

and 16 mono sound channels over 

each fibre. Each video signal requires 

67.2 Mbit/s and each sound channel 

480 kbit/s. The gross bit rate on each 

fibre is 280 Mbit/s. In the subcentre at 

Skarpnäck the digital TV and sound radio 

signals are converted to analog signals 

and placed in their respective frequency 

positions in the 47-400 MHz 

band which is transmitted to the households 

on coaxial cable. 

The fibres are of standard multimode 

graded index type with the dimensions 

50/125 /u.m. The bandwidth-length product 

is >500 MHzx km. The transmission 

wavelength is 850 nm. 

The trial plant was put into operation in 

October 1984 with transmission of 4 TV 

channels from the head end at Farsta to 

Skarpnäck. 

In February 1985 the capacity on the 

route was increased to comprise three 

280 Mbit/s line systems and six TV encoders/decoders 

for 70 Mbit/s. A couple 

of encoders/decoders for 140 Mbit/s 

have also been acquired for reference 

purposes. 

In March 1985 a 280 Mbit/s line system 

and four TV encoders/decoders for 

70 Mbit/s were installed on the route between 

the Farsta and Enskede automatic 

exchanges. 

The fibre-optic trunk line built up of 

modules comprising four television 

channels perfibre, which means that the 

plant can easily be extended for transmission 

of the 32 TV channels and 2* 

stereophonic sound channels for which 

the conventional cable-TV plant at 

Skarpnäck is designed. 

In the summer of 1985 the transmission 

capacity on the Farsta-Skarpnäck anc 

Farsta-Enskede routes amounted to 

digital TV channels and 12 digital sterec 

channels. 

Interiors from the main centre and subcentre 

are shown in fig. 4 and 5.

Fig. 4 

The head end at Farsta 

Fig. 5 

The subcentre at Skarpnäck 

Fig. 6 

The receiving antenna at the head end Farsta 

Program inputs 

Program inputs at the main centre are 

obtained from a receiving station at the 

Farsta automatic exchange. The station 

which is equipped with a 5-metre parabolic 

antenna receives broadcasts from 

the European Communications Satellite 

ECS on 12 GHz. 

Fig. 6 shows the receiving antennas at 

the main centre. 

The following programs are at present 

received from ECS and transmitted to 

the trial areas: 

- Sky Channel from UK 

- Music Box from the UK 

- TV5 from France. 

\«SC»*jÄiääii. 

There is also a smaller parabolic antenna 

on the roof of the automatic exchange 

for reception of television transmissions 

from the Russian TV satellite 

Ghorizont. The Russian program is also 

transmitted to the trial areas. 

Before the French and Russian TV programs 

are transmitted to the households 

they must first be converted from 

SECAM to PAL, as France and the Soviet 

Union use a different TV standard. This 

conversion is done at the head end. 

A local television channel is also available 

for clubs and other associations. 

This channel also provides text information 

about the programs to be transmitted 

on the various channels. 

The Swedish programs TV1 ochTV2,the 

sound radio programs P1-P5 and a 

community radio program are received 

at the subcentre at Skarpnäck and 

modulated into the 47-400 MHz band 

which is transmitted to the households 

on the conventional cable-TV network. 

Subscriber equipment 

In a future service-integrated fibre-optic 

broadband network fibre cables will 

also be used for subscriber terminations. 

The networks will have a starshaped 

structure and different services, 

such as "narrow-band ISDN", highspeed 

data, videotelephony, videoconference, 

will be transmitted over one

158 

Switching equipment 

Fig. 7 

Future service-integrated broadband network 

Individual optical fibres are run to each flat from 

centrally situated switching equipment for video, 

audio, data and text transmission. The subscriber 

can himself select via the switching unit what he 

wishes to see and pay for of the TV programs 

offered 

Fig. 9 

The demonstration room at Skarpnäck 

and the same fibre in addition to television 

and stereo radio programs. Digital 

techniques will be adopted both for 

transmission and switching (fig. 7). 

To give an idea of the form that a future 

subscriber network may take, a demonstration 

plant for integrated fibre-optic 

subscriber termination has been built 

up at Skarpnäck. The subscriber termination 

comprises a video conference 

room and a demonstration room arranged 

on the same premises at 

Skarpnäck. The premises are connected 

via optical fibres to the 

Skarpnäck subcentre, fig. 1. 

The demonstration room is designed as 

the living-room of the future. It contains 

a television set, stereo set, telephone, 

Datavision, video disc, compact disc 

player, large-screen TV, home computer 

equipment, etc. The room will later be 

furnished with equipment for still more 

futuristic services such as videotelephony, 

video library, and for shopping 

and banking via a home terminal. 

The block diagram of the demonstration 

plant, is shown in fig. 8. The equipment 

is described in greater detail in another 

article in this issue of Ericsson Review 2 . 

The demonstration room, fig. 9, is connected 

via optical fibres to video and 

audio switching equipment in the subcentre. 

In the demonstration room a 

small remote control unit (infrared 

transmitter) is used to select the desired 

TV and stereo program. A control signal 

is transmitted digitally over the fibre-optic 

subscriber line to the microprocessor 

in the switching equipment, which 

sets up the connection. Wavelength 

multiplexors enable a TV and a stereo 

program to be transmitted simultaneously 

over the subscriber fibre, which is 

also used for the transmission of control 

signals in the opposite direction. The 

video and audio signals are transmitted 

over the subscriber line using analog 

pulse frequency modulated systems. 

In the demonstration room there is also 

a telephone set connected via an optica 

fibre to the Farsta telephone exchange 

The video conference room, fig- 1 ° l! 

connected via two optical fibres to 

video switching unit at Farsta. The u r

TVT/HC 

Farsta 

p 

F 

M 

ii 

P 

F 

M 

Farsta 

automatic 

exchange 

Conference switch 

ET 

ET 

PfM 

\ /JV /Jâdio 

UC 

Skarpnäck 

PFM 

© ® 

Demonstration 

room 

Skarpnäck 

Video conference room 

Fig. 8 

Block diagram showing how the video conference 

and demonstration rooms are connected over 

optical fibres to the audio and video switching 

equipment at the subcentre 

forms part of a video network, working 

with analog circuits, that has been constructed 

in Stockholm. On fibre is used 

for the transmission of video and audio 

signals to the video conference room, 

the other for the transmission of the 

camera signal and sound in the opposite 

direction. For these signals, too, analog 

transmission with pulse frequency modulated 

systems is used. 

From the video conference room conferences 

can be held with every other 

video conference room connected to 

the Swedish Administration's video network 

or with any of the more than 20 

public video conference rooms that the 

Swedish Administration has in operation 

in Sweden. 

The Skarpnäck TV switching unit has a 

capacity of 30 incoming TV and as many 

stereo signals. There are ten outgoing 

lines available, so some other locations 

can be connected to the switching 

equipment, e.g. some of the schools in 

the area and the library. 

Experience gained and 

further development 

Picture coding 

The selected encoding method, adaptive 

differential PCM (ADPCM), provides 

an excellent picture quality. This was 

confirmed in a test made by the Swedish 

Telecommunications Administration 

and Swedish Television. A subjective 

quality assessment was made with six 

program sections using the method of 

comparing pairs of pictures with small 

differences in quality. The test subjects 

were divided into two groups, one consisting 

of eight video experts and the 

other of 16 "ordinary" TV viewers. The 

expert group recorded a certain deterioration 

of the picture, whereas the viewer 

group could see no significant deterioration. 

The opinion was that 70 Mbit/s 

ADPCM encoders can very well be used 

for the distribution of PAL video signals. 

The encoder also transmits teletext signals 

satisfactorily. 

This ADPCM encoder is intended for 

transmission over a medium with low bit 

error rate (BER), e.g. optical fibre circuits, 

and functions well down to a BER 

value of c. 10~ 7 . 

Passive branching 

The installation of the passive fibre couplers 

was done using the same routines 

as employed for other cable installations. 

The couplers were placed in their 

own splicing box, furnished with stub 

cables, which were then spliced to the 

fibre cables. The difference between 

mode-selective and non-mode-selective

160 

Fig. 10 

The teleconference room at Skarpnäck 

couplers could be observed, especially 

on the first coupler which is only a few 

tens of metres from the transmitter. The 

mode-selective (fused) coupler exhibited 

a lower effective coupling factor 

measured at the end of the cable owing 

to the fact that mainly higher modes are 

coupled into the secondary fibre and 

these are then subjected to higher attenuation. 

With the non-mode-selective 

coupler (lens/mirror) such phenomena 

are avoided. 

Evaluation of the couplers is continuing 

in the field trial in order to produce reliable 

data for the planning of networks 

with passive branching. 

Transmission distance 

The transmission of 280 Mbit/s over a 

multimode fibre is limited mainly by the 

mode dispersion of the fibre, but material 

dispersion cannot be entirely disregarded. 

With wideband fibers (more 

than 800 MHzxkm) a repeater spacing 

of 7-8 km is normally attained. This is 

sufficient for the primary network and in 

most cases also for the trunk network. 

Most administrations today install mainly 

single-mode fibres in their fibre-optic 

networks and a single-mode version of 

ZAV 280/4 is also available. The dispersion 

limitation is thereby eliminated and 

the range of the system will depend almost 

entirely on the attenuation of the 

route. The single-mode version of 

ZAV 280/4 operates at 1300 nm and will 

permit repeater spacings of at least 25- 

30 km. 

In the long distance network video 

transmission will take place mainly on 

565 Mbit/s line systems with 8 TV channels 

per fibre. 

Operation and maintenance of the 

hybrid network 

An important task for the field trial is to 

evaluate the methods for operation and 

maintenance of cable-TV networks. At 

Skarpnäck alarm collection is done by 

an alarm collection network which 

transmits the alarm signals to the Swedish 

Administration's supervisory centre, 

from which the necessary measures are 

ordered. 

Referenses 

1. Gobi, G. and Jacobson, S.: Optics! 

Fibre System for Digital Cable- iv 

Transmission, ZAV280/4. Ericsson 

Rev. 62 (1985):3, pp. 161-169. 

2. Hansson, A.-K. and Jacobson, »•• 

Wavelength Division Multiplexing w 

Fibre-Optic Subscriber lines. Encsso 

Rev. 62(1985):3, pp. 170-174.

Optical Fibre System for Digital 

Cable-TV Transmission, ZAV 280/4 

Gerhard Gobi and Sten Jacobson 

ZAV 280/4 is a system for digital transmission of television and stereo sound 

programs over optical fibre. It has been designed for use in large cable-TV 

distribution networks, at the upper network levels. The digital transmission 

ensures high picture and sound quality regardless of the size of the network. 

ZAV 280/4 was developed by Ericsson in collaboration with the Swedish 

Telecommunications Administration and the first field trial took place in a suburb 

of Stockholm (Skarpnäck). 

The authors describe the design and function of the system and also discuss the 

network structure. 

UDC 535.394:621.397.74 


optical links 

digital communication systems 

System ZAV 280/4 has been developed 

to provide high-quality transmission of 

picture and audio signals in large and 

medium-sized cable-TV networks. The 

high quality is ensured by means of digital 

transmission over optical fibre. 

In a cable-TV network, fig. 1, ZAV 280/4 

is used at the trunk and primary network 

levels, while conventional analog coaxial 

cable systems are used at the secondary 

and on premises network levels. 

Such a network structure is sometimes 

called a hybrid network because different 

transmission systems are used in 

different parts of the network. As can be 

seen from the system designation, 

ZAV 280/4, the system data rate is 

280 Mbit/s, which permits the transmission 

of four digital video signals and 16 

audio channels, corresponding to four 

television programs and four to six 

stereo audio programs (depending on 

whether the television programs have 

stereo or mono sound). The system also 

has a data transmission capacity of 

2x640 kbit/s. 

ZAV 280/4 has a modular structure 

based on the standard transmission rate 

of 139.264 Mbit/s, both as regards the 

electrical and the mechanical construction. 

ZAV 280/4 can therefore be 

used as encoding equipment with either 

140 or 565 Mbit/s line systems. The system 

is then equipped with CMI adapters 

instead of optical transmitters and receivers 

in order to provide a standard 

CCITT D4 interface. 

The system offers optical transmission 

over multi-mode fibres at a wavelength 

of 850 nm or over single-mode fibres at 

1300 nm, with a repeater span of 

5-7 km (with a fibre bandwidth of 500- 

800 MHz km) or 25-30 km respectively. 

Active (electrical) and passive (optical) 

branching equipment and regenerators 

are also available to ensure flexibility as 

regards network configuration. 

The basic version of ZAV 280/4 comprises 

two 19" magazines having a 

Hepeater and 

active branching 

Trunk network-^ 

Primary network 

Base^p 

band 

^7 

Local 

centre 

Repeater 

Trunk 

line 

wn 

GERHARD GOBL 



STEN JACOBSON 

Ericsson Fiber Optics AB 

Fig. 3 

Block diagram of the digital optical fibre cable-TV 

transmission system ZAV 280/4 

heightof six building modules (244 mm) 

at both the send and receive sides, fig. 2. 

System ZAV 280/4 has been developed 

in collaboration with the Swedish Telecommunications 

Administration. 

Equipment 

ZAV 280/4 is made up of a number of 

function blocks as in fig. 3. The equipment 

is accommodated in 19" magazines 

for rack mounting. 

The encoder (ZFV 495012/101 or 102) 

contains video encoders, audio encoders, 

data transmitter and multiplexer. 

The output is a 139.264 Mbit/s data 

stream (CMI or binary coded) which 

contains two video signals, eight audio 

signals, a 640 kbit/s data signal and synchronizing 

information. 

The encoder can also be furnished with 

a 280 Mbit/s optical transmitter which 

multiplexes data from two encoder 

blocks and transmits it over an optical 

fibre. It is then called a transmitter 

(ZFV 495011). 

The branching unit (ZFV 49505 or 

ZFV 49506) is a function block containing 

optical receivers, optical transmitters 

and adapter functions in various 

combinations. A branching unit enables 

the signal to be branched into at mosl 

six optical and one electrical output. 

The decoder (ZFV 495021/102 or 103) 

contains frame synchronization and demultiplexing 

for 139.264 Mbit/s in addition 

to a video decoder, audio decoder 

and data receiver. 

The decoder may also be furnished with 

a 280 Mbit/s optical receiver which demultiplexes 

the received signal and supplies 

two decoders with a 140 Mbit/s 

data stream each. It is then called a receiver 

(ZFV 495021/101). 

Picture 

encoder 

Picture 

decoder 

Sound 

encoder 

Data 

transmitter 

140 Mbit/s encoder 

Frame 

multiplexer 

D1 

D2 

Optical 

transmitter 

5 bits 

ADPCM 

8 bits 

'Linear" PCM 

Analog 

video signal 

1 

Reconstruction 

^ Fine Next 

" ^ Coarse state 

O 

+ 

Out 

Control 

logic 

i 

Line 

memory 

3T 

delay 

Factor 

Fig. 5, above left 

Two-channel A/D video converter 

Fig. 6, above centre 

Two-channel ADPCM video encoder 

Fig. 8, above right 

Audio decoder for four channels 

ent sample with 32-level resolution (corresponding 

to five bits). The quantizer 

has two modes and is switched between 

them on the basis of information from 

earlier values on the same line and also 

values from the preceding line (two-dimensional 

prediction). 

The encoder is adaptive, which means 

that it switches between the two quantizing 

ranges, fine and coarse, depending 

on the picture content. Control ol 

the adaptivity is optimized for maximal 

subjective quality, i.e. is determined by 

how the eye perceives the picture. 

Fig. 7a 

Block diagram of PCM audio encoder 

Analog 

audio signal 

14 bits 

Linear PCM 

Serial 

audio dala 

In 1 (H)O-j— 

14 

-f- 

15 

I 7 MSB 

Generation 


parity bit 

PISO 

-I*- Out 

i 7 MSB 

Generation 


parity bit 

In 2(V) o -U^ 

X) 

14 

Data clock 

Sampling clocks 

f = 960 kHz 

f« = 32 kHz 

Fig. 7b 

Block diagram of PCM audio decoder 

Serial 

audio data 

n O I • SIPO 

Data clock 

11 /ISB 

7ti rfSB 

15 

Parity 

check 

Parity 

check 

t 

15 

t m 

1 

1 

Buffer 

Buffer 

14 bits 

Linear PCM 

1 

k 

D / 

A 

/ 

Sampling clocks 

I 

/ H 

i 

Sin x 

X 

Sin x 

X 

oo 

oo 

Analog 

audio signal 

H 

| 

1 

I 1 

i 

j 

I 

1 

| 

i — ^ 

I 

1 

„ Out 11 

Out 2 • 

f = 960 kHz 

t = 32 kHz

Fig. 9a 

Frame structure for the multiplexer 

Frame sync. 

12 

Row 1 

120 

4 

2 

120 

4 

(4352 bits) 

35 

120 

4 

Bit r 

Video field Bit 120 

Bit 124 

Fig. 9b 

The arrangement of the video field in the frame 

Bit 

o 

Video channel 

Word 

1212 12 12 12 1212 12 12 12 12 12 12 12 12 

12 

Column 

Row 1 

7 

14 

21 

1 2 3 

i 

Audio 

Data 

Audio 

Data 

Audio 

Data 

Audio 

4 

The Swedish Telecommunications Administration, 

together with Swedish 

Television, has evaluated the encoder in 

a subjective test. The subjective picture 

quality of the encoder was judged to lie 

in the interval 4.5-4.7 (on a 5-degree 

scale for subjective quality), which implies 

insignificant degradation of the 

picture quality. 

The complexity of the encoder will be 

seen from figs. 5 and 6, which show the 

A/D converter and ADPCM encoder for 

two channels on two printed circuit 

boards of type ROF (82x60 modules of 

2.54 mm). The decoder for two channels, 

on the other hand, is accommodated 

on a single board. 

Audio codec 

The chief system parameters of the audio 

codec comply with the international 

standard for digital audio transmission, 

figs. 7a and b. 

bit-interleaved and shifted out to the 

multiplexor in the form of a 960 kbit/s 

(480 kbit/s for each channel) serial data 

stream. 

The decoder's filter contains a (sine x)/x 

correction to produce a flat frequency 

characteristic in the pass band. On the 

decoder side the circuits can be made 

more compact than on the encoder side, 

fig. 8. This has meant that the modularity 

is four channels per printed circuit 

board on the decoder side and two per 

board on the encoder side. 

Multiplexing 

Frame structure 

The multiplexor in ZAV 280/4 combines 

two video signals, eight audio signals 

and two data signals into a serial stream 

with the data rate of 139.264 Mbit/s. This 

is done using the frame structure shown 

in fig. 9. 

28 

35 

Data 

Audio 

Data 

Fig. 9c 

The arrangement of the audio/data field in the 

frame 

Fig. 9d 

The structure of the frame alignment word 

Word 1 1 1 1 1 0 1 0 0 0 0 0 

Pulse shapei 

U~l 

The sampling frequency of 32 kHz at 

15 kHz audio bandwidth was chosen for 

ZAV 280/4 in accordance with CCITT 

Recommendation 606. This imposes exacting 

demands on the filtering. In 

ZAV 280/4 this problem is solved with a 

phase-linear active filter of the eleventh 

order developed by RIFA. The audio encoder 

also uses 14-bit linear PCM and a 

simple error protection function, a parity 

bit for the seven most significant bits, 

the preceding sample being repeated if 

an error has occurred in the transmission. 

This method provides good protection 

up to a bit error rate of approximately 

10~ 5 and has the desired effect that the 

sound quality is fully satisfactory even if 

the picture, for example due to a fault in 

the transmission equipment, is heavily 

disturbed. 

The audio encoder's data consist of 15- 

bit words from two channels, which are 

The frame frequency is 32 kHz and a 

frame contains 4 352 bits, which gives 

the data rate of 139.264 Mbit/s. 

The frame consists of a 12-bit frame 

alignment word, fig. 9d, and 35 lines of 

124 bitseach,fig.9a. Each line in its turn 

consists of 120 bits for video and four 

bits for audio or data. 

The video field, fig. 9. is organized as 12 

packages of 10 bits, the 10 bits corresponding 

to a video sample of five bits 

for each of the two video channels in a 

frame. In a 10-bit video package video 

data are rearranged so that they lie alternately 

with one bit from channel 1 and 

the next from channel 2. 

The audio/data field consists of the last 

four bits in each line, forming a field of 

35x4 = 140 bits, fig. 9c. Each of the four 

columns contains 30 audio bits corresponding 

to two sound samples from an 

audio encoder board and five data bits.

Digital 

input signals 

Digital 

output signals 

Picture 

Sync word 

Channellselection 

H 

Sync 

algorithm 

Picture 

Data 

Data 

Clock 

Transmission 

equipment 

Data 

Clock 

Signal 

converter 

_^ 

Dalj 

Souna 

'Sync 

Control 

Frame multiplexer 

Frame handler 

Frame 

demultiplexer 

Fig. 10 

Frame multiplexer and frame demultiplexer tor 

ZAV 280/4 

Fig. 11 

State diagram for the sync algorithm used in the 

frame demultiplexers 

Wrong sync word 

Right sync word 

Sync word found 

Audio and data are evenly distributed in 

the columns, so that throughout there 

are six audio bits and 1 data bit vertically. 

Frame handling 

On the receiver side of the system, synchronization 

of the data stream and 

identification of the frame structure 

must be effected to permit demultiplexing. 

All of this is done in the frame handler, 

fig. 10. 

Frame synchronization is done with an 

algorithm of the same type as the one 

used in line systems and as recommended 

by CCITT, fig. 11. The algorithm 

implies that the synchronized basic 

state is not released until an error in the 

frame alignment word has occurred four 

times. Only then is the synchronization 

released and a search is started for a 

new frame alignment word in the data 

stream. If one is found, the stream is 

locked again but a return to the synchronized 

basic state is not made until it 

has been confirmed twice again that the 

frame alignment word is the correct one. 

An algorithm of this type ensures very 

stable synchronization. In practice it 

means that, apart from the start-up procedure 

when the equipment is switched 

on, the system never loses its synchronization 

as long as the equipment functions. 

The frame handler generates al 

the control signals which identify the 

frame structure and indicate the timing 

to the decoder units and demultiplexoi 

The frame handler also controls the 

channel selection in the optical receiver 

so that the receiver changes channel 

(see section Receiver) if frame synchronization 

has not been obtained withina 

given time. 

Finally the data stream is descrambleo 

with a reset scrambler to recreate the 

correct data stream as it was before the 

corresponding scrambling in the multiplexer. 

No scrambling takes placeon 

the frame alignment word (12 bits). 

The object of the scrambling together 

with the rearrangement of bits in the 

frame structure is to ensure a balanced 

data stream without long sequences of 

ones or zeroes. 

The frame handler consists of a fou' 

layer printed circuit board and is equipped 

with high-speed logic of ECLtype 

MUX/DEMUX 

The multiplexer and demultiplexer, respectively, 

combine and separate the 

video/audio channels and data. 

The multiplexer assembles the fra" 16 

structure from data and timing information 

and scrambles the serial # 

stream except the frame alignme ṟ 

word, in a reset scrambler. 

To take up time differences which a f) 

when video data are placed in the fîn 

bursts (it takes time to insert a"»'; 

data and frame alignment bits) ' 

(First In First Out) buffers are usedthe 

inputs to the multiplexer.

280 Mbit/s 

optical 

interface 

280 Mbit/s 

electrical 

interface 

140 Mbit/s 

electrical 

interface 

BIAS = 

•

Power 

ZAV 280/4 has individual power supply 

to each magazine. This ensures full 

modularity at the magazine (evel anda 

reasonable size of the knock-out unit in 

the system. The maximum unit thuscorresponds 

to four TV-channels. 

The primary system voltage is -48Vd.c. 

If this voltage is not available, rectifiers 

are used which convert 220 V or 110V 

a.c. to -48 V d.c. Uninterruptible power 

can be arranged at -48 V if needed, but 

consists of equipment separate from the 

ZAV 280/4 system. 

Fig. 14 

ZAV 280/4 magazines in the Swedish Telecommunications 

Administration's 19" cabinet. The 

magazines are seen at the rear 

Fig. 15 

T/BYB rack for digital transmission equipment 

JllUlliilllllllliilllllli 

lllllllllllllllllllllllllH 

lllllllllllllllllllllllllll 



[miilHiumiUöUiH 

IiuiiBilliUUlÄ 

streams. If, on the other hand, the receiver 

is in the wrong channel position 

both output signals will be inverted, one 

due to inversion on the send side and 

the other one due to inversion on the 

receiver side, so that the frame handler 

cannot find any frame alignment word. 

The controlling frame handler then 

gives a change channel signal (CS) to 

the receiver, which changes channel by 

shifting the data by one bit slot in relation 

to the clock. 

In the short wave case the receiver preamplifier 

consists of an avalanche photo 

diode (APD) followed by an AGC-amplifier 

(Automatic Gain Control) and equalizer. 

The reverse voltage across the 

APD, the gain and equalization are regulated 

dynamically for optimal reception. 

Timing recovery is done with a phaselocked 

circuit containing, among other 

items, a voltage controlled oscillator 

(VCO). On start-up of the receiver it 

sweeps the VCO frequency to phaselock 

the receiver on the incoming timing 

information from the received optical 

signal. The timing circuit then controls 

the sampling of incoming data. 

After sampling, the regenerated 

280 Mbit/s bit stream is demultiplexed 

by clocking alternate bits to channels 1 

and 2 respectively. If necessary, channel 

switching is initiated as described 

above. 

The secondary system voltages are 

±5 Vand ±15 V. These are generated by 

d.c./d.c. converters in the magazines 

Each magazine contains a d.c./d.c. converter 

for ±5V (2x35 W) and one for 

±15 V(2x15 W). 

Construction practice 

ZAV 280/4 is built in Ericsson's standard 

construction practice BYB101. The 

magazine are of type BFD 329 and are 

19" wide, so that, with brackets, they can 

be mounted in any standard 19" rack 

system (fig. 14). 

When the system is delivered with racks 

from Ericsson, the new construction 

practice for transmission equipment 

T/BYB is used. T/BYB is a cost-optimized 

further development of the MS 

BYB construction practice. 

Alarms 

Each magazine in ZAV 280/4 contains an 

alarm board. This printed board assembly 

collects all alarms in the magazine 

concentrates them into A and B alarms 

and adapts the outputs to an external 

standardized alarm interface using transistor 

contacts. 

The board is equipped with LEDsonthe 

front which indicate A, B and P alarms 

and there is also an outlet for system 

alarm (SA). The system alarm can b' 

connected to a separate LED module 

which then identifies the faulty magazine.

Technical data for 

TV channels 

ZAV 280/4 

4 

16 

Audio channels 

Data channels for 640 kbit/s 2 

Transmission 

Fibre 

Transmission rate, Mbit/s 

Wavelength, nm 

Repeater spacing, km 

Picture coding 

Number of bits 

Video bandwidth, Mbit/s 

Audio coding 

Number of bits 

Audio bandwidth, kbit/s 

Input/output 

Baseband, kHz 

Signal-to-noise ratio, dB 

Differential gain, % 

Differential phase shift, degrees 

Group delay, ns 

Non-linear distortion, % 

Dynamic range, dB 

Crosstalk, dB 

Multi- Singlemode 

mode 

280 280 

850 1300 

5-8 >25 

ADPCM 

5 

67.2 

PCM 

14+1 

480 

Video Audio 

5300 15 

>52 >75 

Wavelength Division Multiplexing for 

Fibre-Optic Subscriber Lines 

Anna-Karin Hansson and Sten Jacobson 

As a part of the field trial with cable-TV transmission over optical fibre at 

Skarpnäck near Stockholm, a demonstration equipment for subscriber line using 

this medium has been developed by Ericsson. The equipment demonstrates only 

one of several possible solutions for subscriber connections in future interactive 

networks. The subscriber lines are arranged in a star-shaped structure around a 

central program exchange and the technical solution demonstrated in the trial 

employs optical wavelength division multiplex, WDM, to provide a complete 

multi-service subscriber connection using only one optical fibre. 

The authors describe the design of the system with emphasis on the WDM 

system specially developed for this field trial. 

The installation handles 30 TV channels 

and 20 stereo sound channels and may 

be expanded to a maximum of 10 subscribers. 

None of these limitations areof 

a theoretical nature, however, but only a 

practical limitation of the hardware used 

in the field trial. 

System design 

The technical solution chosen for the 

system is based on pulse-frequency 

modulated (PFM) optical links, threechannel 

two-way WDM and a data channel 

for remote control of the exchange. 

UDC 535.394:621.395.4.001.55 

frequency division multiplexing 

optical links 


testing 

Fig. 1 

The demonstration installation for a fibre-optic 

subscriber line at Skarpnäck. 

The WDM system connects the two subscribers in 

the living room and the TV conference room to 

the sub-centre, where an exchange for TV and 

stereo programs is located. The exchange is at 

the centre of the star-shaped subscriber network 

The purpose of the first trial at 

Skarpnäck nearStockholm is notonlyto 

provide practical experience, but also to 

demonstrate the possibilities offered by 

fibre-optics in subscriber lines for cable- 

TV, and in a broader perspective, in the 

interactive broadband network of the future. 

In order to provide the greatest possible 

technical experience the solution 

should include optical single-fibre 

transmission, a star-shaped network 

and a central program exchange, remotely 

controlled by the subscribers. 

The task of the system in the field trial, 

fig. 1, is to connect two subscribers in 

the building of the Telecommunications 

Administration to a sub-centre ca. 1 km 

away. 

PFM links for TV and stereo 

channels 

In the field trial both TV and stereo signals are 

transmitted by means of a modified standard 

product. ZAV 103. 

ZAV 103 transmits a video signal, together with 

its accompanying audio signal, via an optical 

fibre. In its basic version the link uses a spectrally 

filtered LED with 890 nm wavelength as 

the transmitter and an APD receiver. 

The transmission distance, which is limited by 

material dispersion, is 6 km with a maximum 

fibre attenuation of 3 dB/km and a fibre bandwidth 

of 500 MHzkm. 

At maximum transmission distance the link has 

the following performance data: 

Video 

Weighted S/N ratio 

Differential gain 

Differential phase 

IN/OUT impedance 

Video signal level peak-to-peak 

>48dB 

171 

ANNA-KARIN HANSSON 



STEN JACOBSON 

Ericsson Fiber Optics AB 

Fig. 3a, top 

The subscriber unit. 

The channels selected for TV and stereo programs 

are indicated on the front panel. It also 

contains a window that allows the built-in IR 

receiver to receive the signal from the subscriber's 

remote control unit 

Fig. 3b, above 

The subscriber unit 

The unit contains the WDM couplers and their 

fibre connections (upper right), the optical receivers 

and demodulators for the TV and stereo 

programs, the control unit for remote control and 

exchange control and also an optical transmitter 

for the data channel 

Fig. 2 shows the block diagram of the 

demonstration set-up. 

The pulse-frequency modulated optical 

links used are modified versions of the 

Ericsson standard product ZAV103, 

which is an optical video links for the 

transmission of TV-signals (video and 

audio) via an optical fibre. See the adjacent 

box for data on PFM links for TV 

and stereo channels. 

The data channel serves to transmit control 

data from the subscriber's remote 

control unit to the internal control data 

bus in the exchange. The control data 

from the remote control unit, which has 

a symbol rate of 300 baud, is converted 

in several stages to a data stream of approximately 

300 kbit/s in the optical link, 

and is subsequently converted into exchange 

data via a built-in RS 232 port in 

the exchange. 

The TV and stereo programs are transmitted 

to the subscriber by means of two 

optical channels in the forward direction 

of the WDM system, and the control 

data channel is transmitted to the subcentre 

in a third optical channel in the 

backward direction of the WDM system. 

The WDM system is described in greater 

detail below. Fig. 3 shows the completed 

subscriber unit. 

The WDM system 

Fig. 4 shows the optical three-channel 

WDM system developed for the field trial. 

In order to demonstrate that WDM can 

be a technically and economically viable 

alternative, care has been taken to use 

components which allow the performance 

requirements of the completed 

system to be met at the lowest possible 

cost. 

The light sources chosen are three commercially 

available LEDs, all working at 

wavelengths below 940 nm, i.e. the "first 

window". The detectors used are one 

PIN diode and two APDs, also commercially 

available. 

The method chosen for the WDM couplers 

requires fewer components and is 

more suitable for serial production than 

most other types of coupler. It should 

therefore also be more economic in 

large-scale production. 

The system permits transmission distances 

of up to 1.5km with a fibre attenuation 

of 3 dB/km and a maximum 

coupler loss of 12dB (corresponding to 

3 dB per coupler in the system as shown 

in fig. 4). The main limiting factor is the 

output power of the LEDs. In the field 

trial the transmission distance is ca. 

1 km while the coupler losses are well 

below 3dB per channel, leaving plenty 

of margin for the output power of the 

light emitting diodes. 

A more critical parameter, however, is 

the crosstalk between the channels, 

since the LEDs have a fairly wide spectrum 

and their centre wavelenghts are 

relatively close to each other. The sys- 

Fig. 2 

Block diagram of the demonstration installation. 

Wavelength division multiplex and pulse-frequency 

modulated links are the main building blocks 

In the system. Only one optical fibre is needed 

per subscriber 

Video conference room 

Cable TV subscriber

172 

Subscriber side 

BP-890 

Controll lå TV and 

data , | [stereo 

lit; programs 

730 87o 890 ' en9th 

X1 \2 \3 

730 810 890 

\1 \2 \3 

, Wavelength 

1 km 

Fig. 4 

Three-channel WDM system for fibre-optic subscriber 

tines. 

The three channels are obtained by cascading 

two two-channel WDM couplers with different 

characteristics. Optical bandpass filters are used 

to improve the channel separation in the system 

Control] 

data 1 

\1 

PIN 

Stereo | 

I 

:-:•! 

\2 

LED-810 nm 

Exchange side 

I BP-890 

\3 

LED-890 nm 

tern requires at least 20dB optical 

crosstalk attenuation, which together 

with the attenuation requirements puts 

stringent demands on the optical filters 

used in the system. 

For the data channel which uses the 

770 nm wavelength, the receiver can be 

made extremely sensitive (in practice 

-70dBm), which means that the output 

power of the 730-nm LED can be reduced 

from the maximum of 30u. to 2\i 

without any problems. Thereby, the 

crosstalk from the data channel into the 

TV and stereo channels at 890 and 

810nm is considerably reduced. 

The principle of the WDM coupler is 

shown in fig. 6. The figure shows the demultiplexing 

function, but the basic 

principle is the same also for the multiplexing 

and duplex signalling functions. 

Two optical signals of different wavelength 

arrive at the coupler on one and 

the same fibre. The GRIN lens (GRaded 

INdex lens) acts as a collimator, i.e. the 

rays from the fibre form a point on one of 

the end surfaces of the lens and are converted 

to a parallel beam at the other 

end. 

The interference filter between the 

lenses is chosen so that one wavelength 

is transmitted and the other is reflected 

Each one of the two signals is then 

focused separately into point sources 

and sent out on the appropriate fibre. 

100 - 

Fig. 5 

Optical spectra for the three LEDs. 

The LEDs have wide spectra and the channels are 

also very close together, which means that optical 

bandpass filters must be used, see fig. 4 

•^HB 730 nm 

IH^BI 

610 nm 

^ H M 890 nm 

700 750 800 850 900

GRIN lenses 

\l, \o 

Fig. 6 

The basic principle 01 the two-channel WDM 

couplers as used in the demonstration equipment. 

Cylindrical GRIN lenses and an interference filter 

are used to transmit or reflect the light, depending 

on the wavelength. In this way, an optical 

wavelength-dependent multiplexing and demultiplexing 

function is achieved 

Interferencefilter 

The three-channel multiplexing desired 

for the field trial is achieved by connecting 

two of these couplers in series 

and using filters for different wavelenghts. 

The most critical component in the coupler 

is the interference filter. This filter 

consists of a thin glass sheet onto which 

several layers of coating have been applied 

by means of vapour deposition in 

order to achieve the desired characteristics. 

The performance of the coupler is 

highly dependent upon the quality of the 

filter and its matching the wavelengths 

of the light sources. The LEDs used in 

the system have a relatively wide spectrum. 

The half-value width is 35-50 mm 

and the distance between the wavelength 

peaks is only 80 mm. Consideration 

must also be given to the fact that 

the wavelength peaks of the LEDs 

change by 0.3 nm/°C which gives a variation 

of 13.5 nm over the temperature 

range of 0-45°C. Because of this, the 

coupler filters are not sufficient to separate 

the two signals, and they must 

therefore be optically bandpass-filtered 

in order to reduce the spectral width in 

the channels. 

The transmission characteristics of the 

filters used in the couplers are shown in 

fig. 7. As can be seen in the figure, almost 

all signals at the higher wavelength 

are reflected, while at the lower 

wavelength, only 90% are transmitted. 

The remaining 10% will be reflected into 

the wrong channel and cause crosstalk. 

The reflected signal must therefore be 

bandpass-filtered at the detector. 

Fig. 6 also shows that the LEDs with the 

two highest wavelengths have very wide 

spectra, so that in fact their spectra 

overlap each other considerably. It is 

therefore necessary to bandpass-filter 

the signals directly after the LEDs, using 

bandpass filters of the same type as in 

the detector above, fig. 8. 

In both cases the bandpass filters are 

placed in the LED housing between the 

LED and the fibre connector, fig. 9. 

When manufacturing the couplers, the 

only operation requiring high mechanical 

precision is the attaching of the 

fibres to the end surfaces of the GRIN 

lenses. This is done using motor-driven 

high-precision positioners to optimize 

the positions of the fibres. The precision 

of this adjustment and the characteristics 

of the filter are the major factors 

determining the performance of the 

coupler. The remaining assembly operations 

may therefore be carried out with 

the aid of simple fixtures. The parts are 

assembled using carefully chosen glues 

which are transparent at the wavelengths 

concerned and the couplers are 

embedded in polyurethane for protection 

against mechanical stress. 

Fig. 10 shows the finished WDM coupler 

together with a coupler before the polyurethane 

is filled in. 

100 - 

Fig. 7 

Transmission characteristics of the two types of 

interference filter used in the WDM couples. 

Filter type 1 transmits only the 730-nm channel 

and reflects the 820- and 890-nm channels. Type 2 

transmits the 730- and 820-nm channels and 

reflects the 890-nm channel 

Fig. 8 

Transmission characteristics of the bandpass 

filters used in the system. 

The filtering improves the crosstalk attenuation 

between the channels in the system, but also 

introduces extra losses which requires increased 

output power from the LEDs 

100- 

% 

BP-820 

50 

BP-890 

700 750 800 850 900 

\(nm) 

When measuring the performance of the 

couplers in the laboratory, it was found 

that the attenuation values for the individual 

couplers were 0.5-1,0 dB for the 

reflected channel and 1.0-1.5 dB for the 

transmitted channel. The measurements 

were made under steady-state 

conditions using monochromatic light. 

The crosstalk attenuation is better than 

25 dB for the transmitted channel, while 

the reflected channel only gives about 

10 dB isolation. This is a consequence of 

the above-mentioned filter characteristics 

and is remedied by optical bandpass 

filtering at the detector. 

Fig. 9 

Bandpass-filtered LED and APD in their housing. 

In the housing (drilled open) the optical bandpass 

filter can be seen as a thin disc between the 

active component and the sleeve of the optical 

fibre connector 

Fig. 10 

The finished WDM coupler is shown at the top of 

the picture. The lower part of the picture shows 

the coupler before the polyurethane is injected. 

The interference filter in the middle is clearly seen 

surrounded by the two GRIN lenses. The red 

tubes protect the fibres, which are glued to the 

end surfaces of the GRIN lenses 

During temperature cycling within the 

temperature range 0-45°C, no noticeable 

changes in the attenuation values 

of the couplers have been measured. A 

complete three-channel WDM system is 

obtained by connecting the couplers together 

two by two as shown in fig. 4. 

In the field trial installation at Skarpnäck 

it has been demonstrated that WDM systems 

for subscriber connections over 

optical fibre is a promising technique 

from a technical as well as an economical 

point of view. This technique is also 

very suitable for mass production. Optical 

wavelength multiplexing should 

therefore be one of the major alternatives 

when designing the fibre-optic 

subscriber network of the future, and it 

will compete successfully with other 

forms of time and space multiplexing for 

equivalent grades of service. 

References 

1. Bergsten, K. and Gobi, G: Field Trial 

of Optical Fibre Cable-TV Systern. 

Ericsson Rev. 62 (1985):4, pp. W 

160. . , 

2. Gobi, G. and Jacobson, S.: 0P" c JJ 

Fibre Systems for Digital Cable-1* 

Transmission, ZAV 280/4. Ericsson 

Rev. 62(1985):4, pp. 161-169.

Modems Series 7 

Christer Erlandson 

Ericsson has developed a new generation of data modems designated Series 7. 

The modem units have a new interchangeable modular structure. Different desktop 

cases are available. Alternatively the modem can be installed in a magazine, 

which is to be mounted in a 19" cabinet. There are several different power supply 

alternatives. The modular structure simplifies the stocking of spares for the 

modems and also the operation and maintenance of the data circuits. 

The author explains why a special modular structure was chosen for Ericsson's 

new modems, describes briefly the construction and the units in Series 7 and 

concludes with a more detailed description of the modem for 2400bit/s duplex 

transmission. 

UDC 681.327.8.001.6 

modems 

design engineering 

Ericsson has been developing and 

manufacturing modems for data transmission 

since the middle of the 1960s 

and has also participated actively in the 

international standardization work carried 

out by CCITT. Ericsson has concentrated 

on modems meeting the requirements 

of CCITT series V recommendations, 

and has therefore become one of 

the major suppliers to telecommunications 

administrations. 

Hitherto Ericsson's modems have been 

designed as single units, standing on 

tables or shelves, but the Series 7 modems 

can also be installed in magazines 

for rack mounting in standard cabinets. 

The main reasons for abandoning the 

standard Ericsson construction practice 

BYB used for the previous modem 

generation are that two modems with 

horizontal BYB printed board assemblies 

would not fit into the 19" cabinets 

used in most computer centres, and that 

CHRISTER ERLANDSON 

Ericsson Information Systems AB 

the circuit board area is too small to hold 

a complete modem. 

The Series 7 modems have been developed 

to meet the demands from telecommunications 

administrations as 

well as from large and medium size end 

users in all countries where Ericsson is 

active. 

The new modular structure was developed 

for data transmission products, 

often mounted in 19" cabinets, which is 

flexible and very useful also for other 

products with similar mechanical requirements. 

The modular structure 

Requirements 

Data modems are usually supplied as 

single units to be placed on a table. They 

have a mains lead and a telephone cord 

for connection to a wall socket. 

In computer centres equipped with 

many modems these are often installed 

in cabinets with doors. The modem is 

then placed on a shelf, the mains is connected 

to a power distribution panel and 

the telephone cord to a telephone socket 

panel. The long cables are left dangling 

at the rear of the cabinet. 

Fig. 1 

The Series 7 products consist of functional units 

that can be mounted in different cases

Most computer cabinets are 19" wide 

and the height is divided into a number 

of standard modules. For efficient utilization 

of the cabinet the modems 

should have a width equal to the whole 

or half of the cabinet width, and a height 

equal to a number of standard modules. 

The size of the printed boards in the BYB 

structure does not meet these requirements 

and Ericsson has therefore developed 

the Series7 modular structure. 

Objective 

The objective is to obtain a flexible 

structure that facilitates both installation 

and operation of data communication 

equipment, fig. 1. 

The different parts of the modem, the 

modulator, demodulator, control circuits 

and line interface circuits are assembled 

into a functional unit, figs. 2-3. 

This unit can be installed in different 

cases as required. 

Special cases are available for the modems 

when they are to be placed on a 

table or mounted in a cabinet. There are 

two types of the desk-top cases, a small 

one that holds one, horizontal functional 

unit, and a larger one with room 

for three units placed upright. If many 

modems have to be accommodated in a 

small space, a magazine should be 

used. It holds eight upright units and is 

mounted in a 19" rack or cabinet. 

Advantages 

The uniform mechanical structure simplifies 

the stocking of spares. The same 

type of case is used for several different 

modems. The functional unit used for 

one type of modem is the same whether 

mounted in a desk-top case or in a magazine. 

The uniform appearance makes for an 

aesthetically attractive product family. 

The fact that all functional units can be 

mounted in magazines saves valuable 

space in computer centres. The prewiring 

of power supply and telephone lines 

also avoids a tangle of loose cables in 

the cabinets. 

The different modems have the same 

type of connection to the back plane and 

can therefore be mixed arbitrarily in the 

magazine. This means that the operator 

can very quickly exchange the type of 

modem when so required. 

All modems have operator controls on 

the front, for example for changing data 

rate and for fault location using a test 

pattern and different loop settings. 

Mechanical construction 

Printed board assembly 

The functional unit consists of a printed 

board assembly with an area of approx. 

200x300 mm and a total height of 

43 mm. When necessary, a daughter 

if^^^^^^^ 

ti^^^^. jg»^ i \yjk 

•^"^^^^H 

• 

5f i&f^lZ-ZM* 

^^^^^^^^/ tr '**^ maÊZ 

^^^^^^^^^^' ' .^^ i^B 

•'** "&*'•*}*' ! 

Ä:; T l 

£**WM 

w 

m !% 

1 - ;*

Fig. 4 

Magazine for 8 modem units, rear view 

Mechanical dimensions, mm 

Width Depth Height 

Small desk-top case 212 420/450 60 

Large desk-top case 130 390 250 

Magazine 483 358 222 

Advantages of the modular structure 

- Reduced cost ol spare parts 

- Small space requirements 

- Uniform appearance 

- Easy fault location 

- Easy exchange of transmission rate 

board can also be added within this 

space. The functional unit is connected 

by means of one or two euroconnectors 

at the rear. 

The board width is in accordance with 

the IEC297-3 recommendation, whereas 

the length is determined by market 

requirements. 

If necessary, the functional units can be 

reduced to half the total height. 

Desk-top case 

There are two types of the desk-top 

case, with different numbers of functional 

units and different power units. 

The two types contain a back plane for 

power distribution and connection of, 

for example, data terminals and telephone 

lines. The modems can also be 

equipped with overvoltage protection. 

The small unit can be equipped with a 

5W or 10 W power supply for 220/110 V 

or 240/120 V. A battery powered 48 V DC/ 

DC converter is also provided. 

The power supply for the large desk-top 

unit provides 18W and is powered by 

220/110 V or 240/120 V. 

Magazine 

The magazine in the Series7 modular 

structure has a height of 5 modules, 

each of 44.45 mm. Power distribution 

and the connection of terminals and 

telephone lines is located on the back 

plane. This means that a functional unit 

can always be changed from the front 

without the terminal connector at the 

rear having to be adjusted. If overvoltage 

protection is required for the telephone 

lines the protectors are also located 

on the back plane, fig. 4. 

The power supply can be arranged in 

different ways. The modem AC power 

unit can be switched between 115 V and 

220/240VAC. With DC supply, the 

modem is equipped with a power unit 

for 24 V or 48V. The magazine can also 

be connected directly to +5V, +12V 

and -12V DC. 

Cabinet 

The cabinet is 19" wide, 1 835 mm high 

and has a smoke-glass door. When used 

for desk-top modems the cabinet is 

equipped with shelves and two distribution 

frames at the rear, for the power 

supply and connection of telephone 

lines. 

When the cabinet is equipped with modems 

in magazines it needs no extra internal 

parts since the power and telephone 

lines are wired direct to the magazines. 

Products 

The Series7 modems can be divided 

into product families which have similar 

applications and characteristics. 

Modems for leased lines 

This product family comprises modems 

for telephone channels and baseband 

modems, fig. 5. 

A combination modem for 0-300bit/s 

duplex and 0-1200bit/s half-duplex 

transmission is available for telephone 

channels, as well as modems for 2400 

and 4800bit/s. A four-channel multiplexer 

is also available as an option for 

the 9600 bit/s product. 

For physical circuits a baseband modem 

is provided which can be switched from 

600 to 19 200 bit/s. 

All modems have built-in functions for 

fault location on the data circuit. Faults 

are located by the transmission and reception 

of data patterns and loop settings 

on the local as well as the remote 

modem. These functions are accessible 

from the modem front to facilitate fault 

location. The loop settings may also be 

controlled via the terminal interface.

178 

Fig. 5 

Modems in Series 7 

Modems for switched lines 

The modem for 4800bit/s is a combination 

modem intended for both leased 

and switched lines. Parameters, such as 

transmit level and receive level ranges, 

can be preset separately for the two applications. 

If a fault should occur on the 

leased line, the modem can easily be 

switched for operation over an alternative 

line through the public switched 

network. There is a switch on the 

modem front panel for this purpose. 

The duplex modem for 2 400 bit/s has an 

alternative rate of 1200 bit/s. It can 

transmit data in two directions simultaneously 

over a two-wire circuit. The 

modem is equipped with automatic calling 

and answer. This modem is described 

in greater detail below. 

Modem for 2 400 bit's duplex 

transmission 

The modem is designed in accordance 

with CCITT Recommendation V.22bis, 

which means that it can send and receive 

data simultaneously over a twowire 

circuit. This is particularly important 

on switched lines, where with 

2 400 bit/s it has hitherto only been possible 

to transmit data in the half-duplex 

mode. Since the most common protocols 

are half-duplex the main advantage 

of a duplex modem will in practice 

be shorter turn-around times. The terminal 

and computer determine the turnaround 

time in this case. 

The duplex circuit is obtained by dividing 

the telephone channel into two frequency 

bands with a centre frequency of 

1.2 kHz and 2.4 kHz respectively. When 

the circuit has been set up, the handshaking 

procedure, with which each 

data transmission starts, is arranged so 

that the calling modem sends data over 

the lower channel and the answering 

modem over the higher channel. 

The modem can interwork with modems 

for 1 200 bit/s in accordance with CCITT 

Recommendation V.22. In such cases 

the modem is automatically switched 

over to 1 200 bit/s by the handshaking 

procedure. 

Automatic calling and answer are two 

essential functions in a modem for 

switched lines. The modem meets the 

new CCITT Recommendation V.25bis, 

which permits a call to be set up from a 

terminal and controlled via the normal 

terminal interface. The control wires can 

be operated in a different way from normal 

data transmission in order to allow 

the wires for the transmit and receive 

data to transmit commands and telephone 

numbers between the terminal 

and the modem. 

t|! .

179 

Products 

Designation 

ZAT 1200-7 

ZAT 2400-7 

ZAT 2400/2400-7 

ZAT 4800-7 

ZAT 9600-7 

ZAT 6-192-7 

CCITT recommendation 

V.21, V.23 

V.26 

V.22, V.22bis 

V.27 bis/ter 

V.29 

none 

The number to be called can either be 

transmitted from the terminal when the 

call is to be made or be selected from 

several numbers that have been previously 

stored in the modem. 

The modem then makes the call, using 

pulse or tone signalling, and the called 

number is displayed on the front. In 

order to be able to make calls via a PBX 

the modem waits for the dialling tone 

from the public network. If the connection 

cannot be set up the modem will 

inform the terminal of the reason. This 

information will also be displayed as text 

on the modem front panel. 

board can be used, since ASCII characters 

are used for the communication between 

the terminal and the modem. 

Summary 

The data modems in the Series 7 generation 

have been developed and designed 

using a uniform modular structure. It 

comprises desk-top units as well as 

units mounted in magazines for installation 

in 19" racks or cabinets. 

Several power supply alternatives are 

available for all versions. 

Other special functions essential to the 

user can also be controlled from the terminal. 

The control from a terminal is simplified 

by a communication program for 

V.25 bis. This is not a prerequisite, 

however, and an ordinary asynchronous 

terminal controlled direct from the key- 

The uniform construction simplifies the 

stocking of spares. Different types of 

modems can be mixed in one magazine 

which saves space in computer centres 

and enables the operator to quickly exchange 

the modem type when a customer 

so requires. If a fault should occur on 

the transmission link it can quickly be 

located with the built-in test functions. 

References 

1. Recommendations, series V, CCITT 

Yellow Book 1981, Vol. VIII and Red 

Book 1985, Vol. VIII.

Computer Aided Production of 

Plastic Details 

Per Germundsjö and Anders Valentinsson 

Ericsson's factories have used computer aids (CAD/CAM) for many years in the 

production of electronic equipment. These aids are now also being used to an 

increasing extent in the manufacture of mechanical equipment. A good example 

is the computer aid for the production of plastic details at the factory in 

Kristianstad, south Sweden. 

The authors describe the methods, equipment and software for the tool design, 

tool making and injection moulding. 

UDC 681.3:678.06.002.5 

cad 

polymers 

machine tools 

Traditionally the production of plastic 

details for a telephone set, for example, 

is carried out with the aid of models, 

drawings and other production documents. 

The basic data must comprise all 

information needed for the tool design, 

tool making and production. The mod- 

Design 

els must be made with allowance for 

shrinkage and draught, and it is both 

time-consuming and expensive to prepare 

the models. More efficient methods 

were needed, and in 1979 an investigation 

into computer-aided methods was 

started at the Kristianstad factory. During 

the period 1980-82 a few individual 

projects were completed using computer 

aid in the manufactureof mechanical 

details. By now the CAD/CAM techniques 

for the documentation and manufacture 

of mechanical details are used 

throughout the factory. 

Information flow 

Information concerning the following 

stagesof the design and manufactureof 

any item made of plastic is stored in the 

computer; 

- Design 

- Product engineering 

-Tool design 

-Tool making 

- Injection moulding. 

Product 

engineering 

Model, 

prototype 

pieces 

The design and product engineering 

work is usually carried out by other units 

within Ericsson, The description below 

will therefore deal mainly with the other 

three stages, fig. 1. 

Fig. 1 

Information flow in the development and production 

of plastic details 

Product 

information 

A 

Tool design 

Preparation 

/ / / / Numerically 

/ / controlled 

/ machines 

/ 

It is essential that information can be 

obtained from one stage and transferred 

to the next. The transfer is no problem if 

all stages have the same CAD/CAM system. 

The Kristianstad factory has several different 

clients within the Group and a 

number of external customers. Unfortunately 

all do not have the same CAD/ 

CAM system. The exchange of product 

information between different systems 

has been facilitated by the use of the 

Initial Graphics Exchange System. 

IGES, developed in the US. In additions 

translation module has been used which 

has been developed by CAD Stockholm 

AB. The exchange of simple data is 

no problem nowadays, but the transfer 

of data for curved surfaces, for example 

still requires a certain amount of manual 

work. These problems will aslo be 

solved eventually. 

When the designers do not supply'^ 

product information in a form suitable 

for automatic transfer, the data from ft

181 

drawings must at present be fed into the 

local system in the Kristianstad factory 

manually for further processing, fig. 2. 

Tool design using computer 

aid 

A moulding tool is built up of standard 

components. They are modified by 

means of different processes to suit the 

application in question. The parts that 

shape the plastic detail are usually loose 

steel inserts that are fitted into the 

moulding tool. The parts that determine 

the geometry of the plastic detail have to 

be modified because of the shrinkage of 

the plastic material, which means a conversion 

of the measurements of the plastic 

detail. 

With computer aid this design work is an 

interactive process. The relevant standard 

components, which are stored in a 

data base, are retrieved and built up into 

a mould. Mould inserts are designed on 

the basis of the product geometry built 

up by the designer, with compensation 

for the shrinkage of the plastic material. 

Drawings are produced to the extent required 

for each individual design. The 

system has standard forms for this purpose 

which are displayed on the screen. 

This is laso an interactive process, as are 

the defining of positions and generation 

of a parts list. 

Component library 

The standard components required for 

the tool design are stored in a data base, 

which has been provided by one of our 

suppliers. It contains 25 000 items, for 

example screws, guide pins, ejectors 

and mould plates. The graphic display is 

two-dimensional. An order list is created 

automatically when components are selected. 

Calculations 

Strength calculations 

Strength calculations are often necessary 

in order to be able to judge whether 

the tool can function satisfactorily. Injection 

moulding pressure in the order 

of 1000bar/cm 2 can easily cause displacements 

in the tool. With tolerances 

of a few hundredths of a millimetre the 

tool must be self-supporting and be deformed 

by a maximum of a few thousandths 

of a mm. The CAD/CAM method, 

together with the finite element 

method, has meant that the mechanical 

function of the moulding tool can be 

assessed more easily and earlier, during 

the design stage. 

The finite element method (FEM) is a 

general means of approximating differential 

equations in connection with 

strength calculations, The method is 

based on a grind, whereby the object to 

be investigated is divided into a number

Fig. 4 

Product geometry displayed on a screen and 

used to generate programs for numerical control 

Fig.3 

Manufacturing a model tor verification 

of finite elements. The calcualtion is 

then carried out for each individual element, 

for which differential equations 

can be set up and solved. The separate 

solutions are combined into an approximation 

for the whole object. 

Temperature calculations 

In order to obtain a product that is the 

optimum with respect to technology and 

economy it is necesary to devote great 

care to the tempering system of the tool. 

During the interrupt stage the heat from 

the plastic melt must be dispersed by a 

tempering medium consisting of water 

or oil. The positions of the channels are 

extremely important in order to obtain 

the optimum result. FEM permits advanced 

analysis, but this is a laborious 

task. Instead the Kristianstad factory 

usesaprogram, MOLD TEMP, which dimensions 

and positions the channels 

efficiently. However, the calculations require 

great simplification of the shape of 

the mould space. 

Mould fill calculation 

The position of the intake, suitable material 

thickness, and suitable product 

design are obtained by means of mould 

fill simulation, for examle by means ol 

the program MOLS FILL. The piece is 

divided into sections, which in their turn 

are divided into elements. The calculation 

provides information regarding 

pressure drop and gives temperature 

and speed profiles. The results of the 

calculation provide a good idea of what 

product quality can be expected. 

Fig. 5 

Manufacture of a graphite spark tooling electrode 

using numerical control based on the geometry 

shown in fig. 4 

Fig. 6 

The finished product, an intercom ERICOM Direct 

DEE 401 01

Product 


Q 

CAD/CAM 

computer 

183 

Graphic 

work stations 

Local computer 

Miller 

Multioperation 

machine 

Wire spark 

machine 

Miller 

Spark 

sinker 

Spark 

sinker 

Injection moulding 

machines 

tooling are in two and a half and three 

dimensions. The tooling of standard 

components such as mould plates comprises 

mostly drilling and milling, which 

is programmed in two and a half dimensions. 

The tooling of shaping parts for 

the models as well as for tools and electrodes 

for spark sinkers usually requires 

three dimensions, figs. 4 and 5. The latter 

type of programming is normally 

complicated and results in large volumes 

of data and long programs for numerical 

control. 

Exchange of production 


Fig. 7 

System structure 

In practice this simulation facility has 

proved extremely valuable in the production 

of complicated pieces, since the 

trimming of the moulding tool is minimized 

and in certain cases even eliminated. 

The program has been developed 

at the Kristianstad factory. 

Preparation for manufacture 

With the CAD/CAM method the preparation 

for manufacture at the Kristianstad 

factory consists mainly of preparing the 

basic data for numerical control of different 

tooling machines. The machines 

available for numerical control are milling 

machines, spark tooling machines 

and a measuring machine. 

An important part of the preparation for 

manufacture is the production of models 

for verifying the mould. The models 

are usually made in an easily worked 

material, fig.3. Another stage of the 

preparation process might be to make 

prototype tools in, for example, aluminium 

before the mould for the production 

tool is completed. The programs for 

When the production information has 

been generated it is transferred to a local 

computer for temporary storage, 

fig. 7. The computer system is built up 

around the Digital Computer PDP-11 

and the user software CAL11501/1 for 

numerical control. The software comprises 

functions for the administration 

of archives and for the output of production 

information as required. Before the 

information is transferred to a machine 

with numerical control it may have to go 

through a process that adapts it to the 

relevant type of machine. Any drawings 

that are required are produced with the 

aid of a plotter. Programs developed by 

Ericsson are used for the transfer. 

The information received from the product 

designers is also stored in the local 

computer. As regards the measuring 

machine the local computer is only used 

for data storage at present. Development 

work is in progress to enable the 

local computer to be used to store the 

process data for the injection moulding 

machines. Plans have been made to use 

the local computer also for supervision 

and reporting, but this will require further 

development work.

Rectifier for Mobile Telephone Systems 

Folke Ekelund and Per-Uno Sandström 

Ericsson Power Systems has developed a power supply system, BZA20106, for 

base radio stations in the mobile telephone networks marketed by Ericsson 

Radio Systems AB. The main part of the system is a rectifier. BMJ401001. based 

on high-frequency rectification without any previous transformation of the mains 

voltage. 

The authors describe the basic principle, structure and performance of the 

rectifier. 

UDC 621.396 

cellular radio 

rectifiers 

power supplies to apparatus 

The mobile telephone systems 12 from 

Ericsson Radio Systems AB contain 

base radio stations that form part of cellular 

radio networks. Each cell contains 

a base station. Ericsson Power Systems 

has developed a system, BZA20108, 

which ensures uninterrupted power 

supply for such stations. The system 

comprises rectifiers, batteries, fuse unit 

for batteries, distribution units with automatic 

circuit breakers and supervision 

circuits. 

BZA201 08 can be extended in modules 

as required, by connecting units in parallel. 

In the standard version a system 

rack is equipped with two rectifiers 

BMJ401 001, a battery connection unit 

and a distribution unit, fig. 1. Rectifier 

BMJ401001 has a nominal current of 

100 A and a continuous output power of 

2.7 kW. If the battery requires a higher 

charging voltage after having been discharged, 

the regulation level in the rectifier 

can be changed over, either by 

means of an electric signal or manually, 

to the required charging voltage, e.g. 

30 V (for a battery of 12 cells with the 

rectifier providing a float charge of 

27 V). Only one voltage level is used, for 

Fig. 1 

A power supply system BZA201 08 consisting of 

three racks. The left-hand rack, with the doors 

open, is equipped with, from below, a battery 

connection unit, two rectifiers BMJ 401 001 and a 

distribution unit

185 

FOLKE EKELUND 

PER-UNO SANDSTRÖM 

Ericsson Power Systems 

RIFA AB 

Fig. 2 

Block diagram of the high-frequency rectifier with 

sinusoidal preregulator 

SMPS 

MDB 

PR 

DC 

PhaseQ. 

Switch Mode Power Supply 

Mains fed diode bridge 

Preregulator 

DC/DC converter 

SMPS 

example 27 V for a battery of 12 cells, if 

the system has battery cells with equalization 

charge units, BMP 160001, from 

Ericsson Power Systems. 

Rectifier BMJ 401 001, the central unit of 

power supply system BZA20108, has 

been designed so as to: 

- provide stable d.c. voltage, which can 

easily be adjusted within the range 

22-30 V (25-31 V in charging mode) 

- allow rectifiers to be connected in 

parallel as required, with automatic 

current distribution between the rectifiers, 

regardless of their number 

- take up little floor space, because of 

its compact construction 

- have easy-to-handle modular units 

ensuring simple installation 

- be tested and ready for operation 

when delivered 

- have a low noise level on the d.c. voltage 

side, in accordance with the demands 

made by the powered system 

- provide filtering of conducted radio 

interference and screening against 

radiated interference, in accordance 

with set requirements, e.g. FCC. and 

C.I.S.P.R. 

- have a higher power factor for the rectifier 

input power (cos cp= 1) and draw 

sine wave current from the public a.c. 

mains (rectifiers with heavily distorted, 

non-sinusoidal mains current 

cause extra losses in the power network) 

- constitute a resistive load on the a.c. 

voltage supply. When the supply is a 

diesel-electric plant it can be dimensioned 

for the real power used by the 

rectifier. (Rectifiers that take heavily 

distorted input current must be fed 

from an overdimensioned plant in 

order to ensure stable operation.) 

-O + 

Rectifier design, a new 

principle used 

The rectifier design is based on highfrequency 

rectification. The mains supply 

is rectified without any previous 

transformation. This method has the following 

advantages: 

- small dimensions 

- low weight 

- noiseless operation 

- fast, stable regulation of the output 

voltage. 

The mains fed power units with highfrequency 

rectification that have hitherto 

been produced in any quantities first 

rectify the mains voltage using a diode 

bridge that charges an electrolyte capacitor. 

This circuit design means that 

the unit draws current from the mains 

during a very small part of each mains 

period (typically about 3 ms per half 

period when fed from a 50 Hz mains network 

having a period of 20 ms). This 

method, called peak rectification, gives 

an unfavourable ratio between the peak 

and mean values. The current has a 

large harmonics content, and this leads 

to a low power factor (typically 0.6) The 

rectified voltage varies in direct proportion 

to the voltage of the mains supply. 

This variation makes it more difficult to 

dimension the following circuit for highfrequency 

conversion and transformation 

correctly. 

A new basic principle using a rectifier 

withasinusoidal preregulator,fig. 2, has 

been introduced. The preregulator, a 

chopper circuit that increases the voltage, 

is controlled in such a way that the 

rectifier receives a sinusoidal input current 

(when fed with a sinusoidal voltage). 

At the same time the output voltage 

from the preregulator is regulated to 

a constant value. This value is set to suit 

the voltage requirements of the transistors 

in the following inverter circuit. An 

advantage of this method, in addition to 

providing a sinusoidal input current, is 

that the rectifier output voltage is not 

affected by variations of the input voltage 

variations within the operating 

range (184-264 V). 

Zero 

Fig. 3 shows a simplified rectifier diagram, 

giving the constituent function 

blocks.

I OR | OFU OS 

I 

CMUP 

HlsHi 

W±±±h 

Ref 

VII 

I 

CMUB 

^=n.lLJ3UL 

u gi3 

lUg12 a "Ug-| J gi4 

U0 Output 

24 V 

! L 

IU 

1 27,0 | 

V S A 

1 

Fig. 3 

Circuit diagram of the rectifier 

IFU 

RFI 

DB 

PR 

IB 

TR 

ORB 

OFI 

IU 

OFU 

OS 

SH1 

SH2 

CMUP 

CMUB 

CUTH 

V 

Input fuse - automatic circuit breaker with 

operating handle for breaking and restoring the 

supply 

Filter that attenuates conducted radio interference 

Diode bridge 

Preregulator (controlled by pulse width modulation) 

Inverter bridge (controlled by pulse width modulation) 

Transformer (fed with 40 kHz voltage) 

Output rectifier 

Output filter 

Instrument unit 

Output fuse 

Output switch 

Shunt for measuring the current In the preregulator 

Shunt tor measuring the output current 

Control and monitor unit for the preregulator 

Control and monitor unit for the Inverter bridge 

Unit for triggering the thyristors in the mains 

bridge 

Varlstor 

The preregulator (PR) works with a 

25 kHzchopperfrequency and the pulse 

width (the time intervals when the transistor 

is conducting) is modulated by 

100 Hz synchronously with the rectified 

pulsating voltage at the output of the 

diode bridge (DB). 

The pulse width is also affected by the 

d.c. voltage value (U,) across the preregulator 

output. Control unit CMUP 

sets the mean pulse width that gives the 

desired d.c. voltage (U,). 

The d.c. voltage (LI,) generated by the 

preregulator is chopped, inverted, by 

the transistor bridge (IB), which is controlled 

by means of pulse width modulation. 

The operating frequency is 40 kHz. 

Control unit CMUB measures and regulates 

the rectifier output voltage and 

also the current to the output circuit (l 2 ). 

The current regulation is carried out by a 

fast regulating circuit, i.e. without any 

appreciable delay. The output voltage is 

regulated by a slightly slower circuit (response 

time in the order of a millisecond). 

The voltage regulator provides a 

reference level for the fast current regulator 

that is dependent on the output 

voltage. This method of operation of the 

regulation circuits makes for fast and 

stable regulation. The method ensures 

high accuracy in static and dynamic regulation 

with current limiting, and is also 

short-circuit proof. 

Components 

The power transistors in both the preregulator 

and the inverter bridge are of 

the FET type. The transistors have very 

short turn-on and turn-off times (in the 

order of 100 ns) and the delay between 

the control pulse and the main current is 

negligible. FETs are also very suitable 

for parallel connection. 

The diodes in the main circuit have been 

chosen to ensure the least possible loss 

and interference, i.e. they have short recovery 

time and smooth recovery. 

The electrolyte capacitors are RIFA's 

type PEH 169, with long life and the ability 

to withstand high temperatures. 

Protective circuits 

When the rectifier is started up, by the 

operation of the input breaker, the ca- 

CMUP 

Fig. 4 

Block diagram of CMUP, the control and monitor 

unit for the preregulator, PR 

U, Output voltage from the preregulator. fig. 2 

U0 Reference waveform 

li Input current to the preregulator 

MAC Measurement amplifier, current 

MAV Measurement amplifier, voltage 

M Multiplier 

EA Error amplifier 

COMP Comparator 

AND AND gate 

OSC Oscillator. 25 kHz 

PA Pulse amplifier

CMUB 

PA 

— u q11 

PA 

~ U Q13 

PA 

— U g12 

Oi 

V^ 

> position 3 

04I • position 2 

^» position 1 

SWI 

SSC 

i 

MC 

AL4 

AL3 

AL2 

AL1 

PA — Ug 14 

Fig. 5 

Block diagram of CMUB, control and monitor unit 

for the inverter bridge, IB 

SWI 

SSC 

UBEF 

MAV 

MAC 

CSM 

EA 

OSC 

COMP 

AND 

BM 

PA 

MC 

u 2 

CSPR 

U gii-Ug,4 

AL1 

AL2 

AL3 

AL4 

Switch pos. 1: OFF 

pos. 2: ON 

pos. 3: CHARGING 

Start circuit for slow start 

Stable reference voltage 

Measurement amplifier, voltage 

Measurement amplifier, current 

Current distributor (distributes the load cur 

rent equally between rectifiers working in 

parallel) 

Error amplifier 

Oscillator, 40 kHz 

Comparator 

AND gate 

Bistable flip-flop 

Pulse amplifier 

Supervision and alarm circuits 

Rectifier output voltage 

Signal proportional to the current fed into 

the output filter 

Current signal from rectifiers working in 

parallel 

Control pulses to transistors in the inverter 

bridge 

Alarm signal: Rectifier failure 

Alarm signal: Overvoltage 

Alarm signal: No load 

Alarm signal: Mains voltage too low 

pacitors (C,) are charged via a resistor, 

which determines the magnitude of the 

start-up current. It is a maximum of 10 A, 

i.e. less than the rated current (25 A) for 

the input fuse, IFU. The voltage across 

the capacitors is monitored by a guard 

circuit. The guard lets the preregulator 

start working when the voltage has 

reached a certain value. The voltage is 

then further increased and regulated to 

a constant level. The control unit for the 

inverter bridge is then allowed to start 

up, i.e. to supply control pulses to the 

transistors in the bridge. During thestarting-up 

process and restarts (after control 

pulses have been blocked) the pulse 

width of the control pulses is slowly increased 

from zero to the value required 

in orderto obtain the normal output voltage. 

Voltage pulses are obtained across 

the windings in the main transformer 

(TR) when the inverter bridge starts operating. 

A turn on the transformer core 

feeds a circuit (CUTH) that triggers the 

thyristors in the mains rectifier bridge 

(DB). From then on the preregulator 

(PRU) is fed via this bridge. 

The supply voltage is supervised by a 

mains monitor which blocks the control 

unit in the preregulator (CMUP) in the 

case of unacceptably low mains voltage. 

After a mains break or an abnormal reduction 

of the supply voltage the mains 

monitor allows restart of control unit 

CMUP when the voltage has again risen 

to an acceptable value. The restart is 

carried out with slowly increasing pulse 

width. 

Control unit CMUB protects the inverter 

bridge against overload. If the current 

drawn tends to exceed the rated current 

(100 A), the control unit reduces the 

pulse width sufficiently to keep the rectifier 

output current constant (at approximately 

102 A). If the output is shortcircuited 

the current is also effectively 

limited to the same value. 

Alarm and supervision 

circuits 

A circuit supervises the function of the 

rectifier by checking that it generates 

voltage pulses in the main circuit. If no 

pulses are obtained in spite of the input 

fuse being operated an alarm signal, 

"Rectifier fault", is initiated. 

A load indicator circuit, which interacts 

with the above circuit, checks that the 

rectifier supplies current (takes load). If 

the rectifier does not take any load the 

load indicator, after a delay of about a 

minute, will give an alarm indication, 

"No load", by lighting an LED on the 

front of the unit. 

An overvoltage monitor checks the output 

voltage of the rectifier and prevents 

it from generating an unacceptably high 

voltage. The monitor is selective. This 

means that it blocks the control pulses 

to the transistors in the inverter bridge 

only if the rectifier has remained active 

in spite of the voltage across its output 

having risen above the operating threshold 

of the monitor.

188 

Fig. 6 

The rectifier with the outer steel cover removed. It 

contains, from the left, the printed board with 

control and supervision circuits, main transformer, 

the electrolyte capacitor in the input 

circuit, the inductor in the input circuit and the 

printed board with the preregulator control and 

supervision circuits. Behind these components is 

the large, wiring unit printed board with power 

transistors, their drive circuits and current supply 

circuits for the electronics. The angle brackets on 

which the power semiconductors are mounted 

can be seen behind the board. The brackets are 

screwed to the large heat sink, whose flanges can 

be seen at the top of the rear of the unit 

Theovervoltage monitor allows the control 

unit to make one restart attempt. If 

the rectifier gives too high a voltage 

once again within a certain time the control 

unit is blocked. Such blocking is 

followed by the alarm indications "Overvoltage" 

and "Rectifier failure". 

Indications on the front 

panel of the rectifier 

Light emitting diodes in different colours 

and with clear labelling indicate the 

following operating modes and alarm 

signals: 

Colour 

Red 

Red 

Red 

Red 

Yellow 

Yellow 

Green 

Label 

Rect. Failure 

Overvoltage 

No Load 

Mains Failure 

Current Limit 

Charge 

On 

The operation of the rectifier can be 

controlled manually by means of a toggle 

switch on the front panel: 

Position 

On 

Significance 

The control unit circuits are 

working if the input fuse is 

operated 

Off/Reset The control unit curcuits are 

blocked, i.e. the rectifier does 

not generate any voltage 

across its output (even if the 

input and output fuses are 

operated). In the case of 

blocking by the overvoltage 

monitor the switch must be 

briefly switched from "On" to 

"Off/Reset" in order to reset 

the monitor (break the blocking 

state). 

Charge 

Instrument 

In this position the rectifier 

voltage level is raised. (In certain 

cases this function is removed, 

namely in systems 

containing battery cells 

equipped with circuits for 

equalization charging.) 

The front panel contains a liquid crystal 

digit indicator. It can be switched to 

show either the output voltage or the 

output current of the rectifier. 

Special dimensioning 

problem 

The demands for compact structure, 

large variation range for the supply voltage, 

little interference and the ability to 

withstand transients, both on the input 

and the output, have greatly influenced 

the dimensioning and the mechanical 

construction. 

Great attention has been paid to the mechanical 

construction in ordertoensure 

efficient cooling of the semiconductors 

in the main circuit. This has resulted in a 

rectifier with a high heat exchange capacity 

at a low temperature difference, 

and good electrical insulation between 

live parts and the large heat sink that 

transfers the heat to the cooling air. 

The cooling flanges have been designed 

to give the best possible heat transfer 

with self-circulation of the cooling air 

through the rack. 

The ability to withstand transients from 

the mains supply is obtained by means 

of a combination of interference suppression 

filters, metal oxide varistors 

and electrolyte capacitors that can store 

a large amount of energy. 

The requirements specification defines 

what type of overvoltage transients the 

rectifier must be able to withstand. This 

property of the rectifier was tested using 

a transient generator with carefully defined 

characteristics (Schaffner type 

NSG223). 

The output filter protects the rectifier 

against transients on the output side. 

Polypropylene capacitors, placed adjoining 

the connector, have low impedance 

in order to be able to cope with 

any rapid voltage changes. The electrolyte 

capacitor in the output filter can 

absorb the energy from any transients. 

Electrical safety 

The rectifier has been designed to meet 

the electrical safety standardsof IEC435 

and BT, Technical Guide No. 25. This 

means, for example, that an insulating 

barrier has been arranged between 

components connected to the mams 

and components in the output circuit on 

the low voltage side.

189 

The remainder of the components and 

materials have been chosen in accordance 

with the requirements set by Underwriters 

Laboratories in the US. 

Mechanical construction 

System 

A 19" rack has been designed for system 

BZA201 08. The rack is 1800 mm high, 

585 mm wide and 365 mm deep. The 

rack has a built-in rail system for the 

connection of a battery fuse unit, distribution 

unit and rectifier. If the plant 

comprises several racks they are interconnected 

by means of horizontal copper 

bars. 

Rectifier 

The rectifier is constructed for installation 

in a 19" rack and designed for natural 

cooling (self-convection), fig. 6. The 

rear plate of the unit is a large heat sink. 

When the rectifier is mounted in the rack 

the cooling flanges are automatically inserted 

in the cooling channel of the 

rack. In order to obtain sufficient creepage 

distance between the power semiconductors 

and the heat sink, the semiconductors 

are mounted on aluminium 

brackets that are pressed against the 

heat sink but insulated from it by a sheet 

of capton foil, fig. 7. This means that the 

heat sink does not have to be protected 

against human contact. The semiconductors 

are connected to two printed 

board assemblies, which are mounted in 

parallel with the heat sink. The transistors 

are mounted on the foil side of the 

upper board with their drive circuits 

close by on the same board. The two 

control units are plugged into this 

printed board, as are the capacitors and 

the primary side of the transformer. The 

primary and secondary bridges and the 

output filter are connected to the lower 

board. 

The rectifier is connected to the mains 

by means of plugs and to the current 

bus-bars of the system via the output 

switch. This means that there is not need 

to call in an authorized electrical contractor 

if a rectifier has to be changed. 

The rectifier cover is made of perforated 

steel and screens radiated interference. 

Fig. 7 

An angle bracket with power transistors fixed to 

the large heat sink (see also fig. 6). An insulating 

foil with good heat transfer properties is placed 

between the bracket and the heat sink

Fig. 9 

Rectifier BMJ A01 001 with the front panel open, 

showing the inner front panel, which provides 

screening. A simplified circuit diagram is printed 

on this panel, and the measuring instrument and 

LEDs are mounted in the top left-hand corner. The 

operating handle for the output switch is placed 

in the bottom left-hand corner. 

Fig. 8 

Rectifier BMJ 401 001 with the outer front panel 

closed. The cut-out in the panel gives access to 

the electronic measuring instrument for voltage 

and current, the light emitting diodes that indicate 

operating states and the control switch (for 

selecting the operating mades ON, OFF/RESET or 

CHARGE). The output switch is placed on the left 

side of the unit. It connects the rectifier output 

circuit to bars in the rack 

The components on the mains side are 

installed in screened compartments in 

order to minimize the conducted interference 

to the mains. Heavy components, 

such as chokes, transformers and 

the large electrolyte capacitors, are 

mounted in the perforations in the unit 

frame. The plug-in printed board assemblies 

are held by guide rails in the case. 

The unit has an inner and an outerfront 

panel. All control devices and instruments 

are mounted on the inner panel. 

Only items needed for normal operation 

are accessible through the outer panel, 

fig. 8. 

HMIMMI 

Installation 

The system construction ensures easy 

installation and rapid putting into operation. 

Each rack in BZA20108 is delivered 

on site as a fully tested unit equipped 

with a distribution unit, battery fuse 

unit and rectifier. When the equipment 

in installed it is only necessary to check 

that there is no transport damage. The 

installation work consists merely of setting 

up the racks and connecting mams 

cables and distribution cables. 

The rectifier is designed for high personal 

safety. All live parts are inaccessible 

even when the outer front panel is

Technical data for BMJ401 001 

191 

Input data 

Mains voltage, single phase 

nominally 

input voltage range 

permissible variations 

Permissible frequency variation 

Input impedance 

(with an output current of 

10-100 A) 

Input current waveform 

(with sine-wave voltage) 

Power factor 

with full load 

with >20% load 

Efficiency with nominal input voltage 

with full load 

with 25-100% load 

Radio interference 

meeting CISPR standards 

mains side (0.15-30 MHz) 

load side (0.5-30 MHz) 

Output data 

System voltage 

Voltage at the rated power, 2.7 kW 

Adjustment range 

Adjustment range in 

charging mode 

Output current, rated value 

Adjustable value, when working 

with current limiting 

Active current division, with 

rectifiers working in parallel 

Static regulation 

Static (0-100A) 

Temperature dependence 

(-20° C to +35° C) 

Dynamic regulation 

voltage deviation with a 25 A 

change in the load 

response time, with resistive 

load change, 25 A 

Noise voltage 

psophometric value 

r.m.s.value (10Hz- 450kHz) 

V 

V 

V 

Hz 

V 

V 

V 

V 

A 

General data 

Permissible ambient temperature °C 

(with one or two rectifiers in the same 

rack) 

Sound level, measured at a distance 

of 1 m 

Reliability 

MTBF 

Dimensions 

width 

depth 

height 

Weight 

A 

mV 

mV 

V 

ms 

mV 

mV 

dBA 

years 

mm 

mm 

mm 

kg 

230 

208-240 

184-264 

47-63 

resistive 

sinusoidal 

0.99 

>0.95 

>0.87 

>0.85 

The Renewal of the London Underground 

Telecommunications Network 

Roger Linton 

The London Underground railway covers 668 miles of surface and tube track, 

served by 272 stations, and carries over 2 million passengers every working day. 

In 1979 this busy undertaking decided to replace its telephone system, which is 

vital to the efficient operation of the railway and administrative organisation. The 

old one was a Strowger system of the 40s. 

The author gives the background of the project and describes the various parts 

of the provision of MD110 exchanges, optical fibre transmission systems, the 

cut-over and the organisation required to complete the task. 

office wiring changes was also a new 

feature. However, there were a number 

of disadvantages when applied to railway 

use since: 

- it was not a fully solid state switch, 

relays were used as an interface on 

exchange and tie line boards. 

- it consumed a large amount of power: 

10 kVA for a 248-line exchange. 

- it had no battery backup. 

UDC 621.395.2 

535.394 

private telephone exchanges 

telephone networks 

optical links 

railways 

installation 

In 1974 the major part of London Transport's 

Strowger PABX had been in service 

for 35 years, and preliminary planning 

commenced to replace it, fig. 1. 

Generally, Strowger equipment was expected 

to have a service life of between 

25 and 30 years but with the development 

of crossbar, followed by reed electronic 

exchanges in the late 1960s, it 

seemed prudent to wait for a fully electronic 

exchange to appear. 

IBM manufactured their 3750 Stored 

Programme Controlled (SPC) analogue 

exchange, and London Transport purchased 

one for evaluation in 1975. It 

proved very popular with users, especially 

the facilities; and the ability to 

change numbers from the maintenance 

terminal which did not normally involve 

The IBM 3750 was marketed at a time 

which caught the UK Telecommunication 

Industry without an equivalent 

product, but by the late 1970s other 

SPC exchanges became available and 

with battery backup. 

In 1975 we commissioned our first 30- 

channel PCM system to Hounslow West 

exchange for the extension of the Piccadilly 

Line to Heathrow Central. Other 

PCM systems were quickly incorporated 

into the network to improve overall 

transmission quality, and having 

adopted time division multiplexing for 

junctions it became clearthere would be 

considerable advantages to do likewise 

with the new exchanges. 

Traffic studies of current and future 

growth led to a requirement for 18 telephone 

exchanges to cover the London 

East Finchley 

41XX 

Finsbury Park 

6XXX 

Barking 

29XX 

Fig. 1 

The old Strowger telephone system 

Tandem exchange 

Hounslow 

West 

22XX 

^P 

Main exchange 

Stockwell 

42XX

193 

ROGER LINTON 

Communications Engineer (Signalling) 

London Underground Limited 

Transport area of operation, fig. 2. To 

provide the desired degree of security 

against route and central exchange 

failure two transit switching centres 

(TSC) were planned, interconnected by 

high capacity links and each having its 

own independent junction route to each 

of the 18 exchanges. 

A project team of 10 people was established 

and specifications were prepared 

and issued in 1978 for replacement exchanges 

and transmission systems, all 

to be of solid state equipment. After detailed 

assessment of competing combinations 

of exchange and transmission 

networks, an integrated digital exchange 

and PCM transmission system 

was selected which complied with the 

specification and gave the best value for 

money. A contract was placed with 

Thorn Ericsson Telecommunications in 

September 1979 to supply, install and 

commission a 10000-line digital PABX 

(later known as MD110) configured to 

London Transport network requirements, 

with a Ready for Service date of 

September 1984. 2 " 4 

This project involved the work of a number 

of London Transport internal departments, 

as well as contractors. Sites 

had to be found for new exchange buildings 

and it was imperative that critical 

path analysis techniques were applied 

to this project. This was done by the 

projectteam and it proved invaluable for 

the coordination of work and in making 

certain changes as the project progressed. 

Exchange buildings and 

equipment 

Eleven of the new exchanges are flatroofed 

single-storey brick buildings, 

fig. 3, and of three standard sizes, the 

remaining eight exchanges are accommodated 

in existing office buildings or 

stations. Buildings are divided into separate 

rooms for switching equipment, 

battery and rectifier, MDF and Technicians 

store, fig. 4. The equipment room 

has a computer type floor and is air conditioned. 

The battery room has a solid 

floor, and an extraction unit is provided 

for the two batteries. 

The old network consisted of 15 exchanges, 

and as some exchange areas 

had been stretched beyond normal line 

limits the opportunity was taken to correct 

this situation by providing three additional 

exchanges. Harrow-on-the-Hill 

has been replaced by the three exchanges 

Neasden, Ruislip and 

Loughton 

Rickmansworth 

nault 

Fig. 2 

The new MD110 exchange network with transit 

exchanges at Baker Street and Embankment has 

an initial capacity for 10000 extension lines 

A 

MD 110 transit exchange, TSC 

MD110 

Hounslc 

West 

Acton 

Lillie 

Br 'dge 

Head 

office 

Stockwell 

Becontree 

stepney 

breen

194 

Rickmansworth, and the Loughton area 

has been cut in half by the provision of 

an exchange at Hainauit. Also Leicester 

Square area has been divided between 

Baker Street and a new exchange at Embankment. 

The switching equipment consists of 

circuit boards which plug in a shelf 

called a magazine; the magazines are 

housed in a lockable cabinet. Each 

equipped cabinet is known as a Line Interface 

Module (LIM) and contains control 

equipment, digital switching equipment, 

processor and telephone line interface 

boards. 

If an exchange has more than two LIMs 

(i.e. approx. 400 analogue telephones) 

then a Group Switch Module (GSM) is 

provided to route calls between LIMs. 

The common equipment, such as Group 

Junction Units (GJUs), Tone Receivers, 

Tone Senders and Multi-Party Conference 

Units have assigned positions in 

the magazines due to the large number 

of time slots that these boards require. 

All remaining board positions are interchangeable 

as each of these units only 

requires a maximum of eight time slots. 

Extra GJUs may be added to cope with 

high levels of LIM-to-LIM traffic. The 

control equipment in a LIM has the capacity 

to handle all traffic originating 

and terminating within the LIM. In the 

event of a fault on a PCM link, the LIM 

will recognise the problem and continue 

to process internal callsand routeexternal 

calls via Baker Street/Embankmeni 

TSC as appropriate. 

A main distribution frame (MDF) is 

provided with fused connections and 

test jacks on the line side for the termination 

of trackside cables, and with gas 

discharge tubes for high voltage protection 

on the exchange side. All cable terminations 

are wire wrapped. Flexiblecables 

with connectors are run from the 

exchange side of the MDF to the LIM and 

plug directly into telephone line interface 

boards. 

Two lead acid batteries to give a six busy 

hour reserve capacity are provided in 

each exchange and are arranged in a 

divided floating system. Normally one 

battery is charged from a Transformer/ 

Rectifier set connected to the local authority 

supply and the other is connected 

to the London Transport generated 

supply. This power supply and battery 

arrangement provides maximum 

flexibility so that either battery can be 

easily isolated from the exchange for 

maintenance and, under mains failure, 

either rectifier is available to charge 

both batteries. 

Transmission system 

Each exchange has a number of PCM 

routes to both Baker Street and 

Embankment TSCs. 2Mbit/s primary 

PCM multiplexers are not required as a 

Fig. 3 

One of the eleven brick buildings that were 

erected for the new exchanges

195 

Fig. 4 

Layout of a typical MD110 exchange 

MDF 

LEB 

LRT 

Main Distribution Frame 

London Electricity Board 

London Regional Transport 

LIM directly interfaces with a 2Mbit/s 

PCM digital stream. Originally it was intended 

to use 2Mbit/s PCM routes 

throughout the network, but this would 

have required many regenerators for a 

reliable system and would not be cost 

effective. It was therefore proposed to 

work at 8 Mbit/s between the TSCs and 

the major exchanges, and then revert 

back to 2 Mbit/s to serve outlying sites. 

The 8 Mbit/s PCM systems required a 

specially designed screened group cable 

in order to achieve the necessary 

attenuation and crosstalk performance. 

On investigating the market in 1980 no 

such cable was available in the UK, and 

we found that British Telecom were interested 

to develop such a cable for their 

use. We commenced development of a 

screened group cable, and sample 

lengths were manufactured and installed 

on the railway. It soon became 

evident that the size of the cable was 

becoming very large in order to achieve 

the required crosstalk characteristics, 

and the jointing of this cable was very 

specialised, particularly in maintaining 

the structure of the group screen within 

the joint. From this situation consideration 

was given to an alternative method 

of transmission. 

Optical fibre trial 

During 1978, when the new telephone 

system was being designed, it was evident 

that optical fibre transmission 

techniques offered attractive advantages 

in the environment of an electrified 

underground railway. London 

Transport therefore decided to install a 

field trial linking the existing Earls Court 

and Acton exchanges. This was a route 

where added capacity was required, and 

the practical experience of handling an 

optical cable in tube tunnels as well as 

on open sections of t he rail way would be 

invaluable. All installation of cable and 

terminal equipment was carried out by 

London Transport communications 

staff with the cable spliced by contractors. 

This 7 km link carries four 2 Mbit/s PCM 

systems which are multiplexed to give 

one 8 Mbit/s PCM stream without any

196 

Fig. 5 

Optical fibre cable routes 

4(2) 4 tlbres on route, (2) indicates number of fibres 

spare to Initial requirements 

KT» 

Copper conductor cabling 

Optical repeater 

(~\ Rickmansworth 

r\ 

\ i 

-f i Neasden 

intermediate regeneration. After a testing 

period the trial link was placed into 

full traffic service in July 1979. It is believed 

that this was the first operational 

railway optical fibre link, and certainly 

the first to be placed in live traffic service 

in UK. 

Following the trial a study was implemented 

to see if optical fibres could be 

incorporated in the new exchange 

transmission network. The principal advantages 

to London Transport would 

be: 

- Vast reduction in weight and diameter 

as compared with the proposed 

8 Mbit/s screened copper cable, leading 

to reduced installation costs (no 

cable trains reguired as optical cable 

can be run from a trolley) and saving 

much reconstruction of cable runs. 

This saving alone more than cancelled 

out the higher capital cost of 

the optical fibre system. 

- Elimination of repeaters from most 

routes which would greatly reduce 

Golders 

Green 

East 

Finchley 

fault liability. The larger toleranceson 

distance between remaining repeaters 

allowed them to be situated at stations 

with easy access. 

- The intrinsic immunity of optical 

fibres from electromagnetic interference 

allowed better error rates to 

be achieved on the PCM which was 

noticeably going to become apparent 

in the quality of transmission. 

It was found that the optimum rate for 

digital transmission for the size and geography 

of the London Transport network 

was 34 Mbit/s. It was not economic 

to extend the optical fibres to the outlying 

exchanges so a portion of these 

2 Mbit/s systems remains on copper cables. 

Two 140 Mbit/s links between Embankment 

and Baker Street TSCs were 

also reconfigured from coaxial to optical 

cable. The net result would be a total 

of 120 route km of optical fibre cables 

against 224 route km on copper cable, 

fig. 5. The study proved it would be viable 

to alter our plans at this stage of the 

Ruislip [^ 

^ ^ 

(~\ Loughton 

s "Q Hainau " 

j 

Hounslow 

West 

/ / 

/ / 

/ I - 4(2 > 

Becontree 

4(2) Lillie 6(2) Head 

Bridge 

Office 

Embankment 

-i I 

4(2) Stepney 

Green 

I 

Qj 

i 

Stockwell

197 

Fig. 6 

Splicing case with coiled up spare cable 

Table 1 

Actual measured loss for the 34 Mbit/s route 

Embankment - Stepney Green 

Fibre No. 

1 

2 

3 

4 

Route Loss 

(dB) 

19.8 

18.0 

18.2 

19.9 

Actual Safety 

Margin 

(dB) 

29.2 

31.0 

30.8 

29.1 

project to incorporate optical fibres. A 

contract was placed to supply, install 

(except trackside cables) and commission 

the optical network. 

Cable construction/splicing 

The cable construction was carefully developed 

to meet railway environmental 

needs. The cable consists of a steel strip 

laminate formed into a tube with a 

covering of polythene bonded to it. This 

is protected by a single layer of steel 

wire armouring and a PVC oversheath 

for use in the open sections of the railway. 

In tube tunnels more stringent precautions 

have been taken with the oversheath 

material to reduce toxic smoke 

emission and combustion. 

Contained within the cable are up to 8 

graded-index fibres, having a maximum 

attenuation of 3.5dB/km and a bandwidth 

of 400MHzkm. On the Stepney 

Green to Becontree route, where it was 

desirable not to have repeaters, high 

grade fibres with an attenuation of 

only 2.5dB/km and a bandwidth of 

600MHzkm were used. 

The cable was ordered in defined interstation 

lengths so that all splicing could 

be carried out in station locations. A 

stainless steel weatherproof splicing 

case was selected to allow easy access 

for maintenance compared with the normal 

cable jointing enclosures. In order 

to allow for future diversions to be made 

without cutting and jointing, up to 10 

metres of spare cable has been left 

coiled up on either side of the case, 

fig. 6. If a cable is damaged this arrangement 

should allow a repair to be done 

with only one joint where two would normally 

be required. A socket is provided 

for a point-to-point telephone connection 

between splicing sites and on to the 

exchange. One wire is connected to the 

steel wire armouring and the other to the 

inner metal tube of the fibre cable. 

Within each splicing case sufficient 

spare fibre has been coiled up in a cassette 

to allow a splice to be re-made several 

times. Splicing was carried out 

using electric arc fusion welding technique. 

Each fibre splice is enclosed in a 

heat shrink sleeve which has a stainless 

steel strengthening member. A similar 

arrangement was used in the exchange 

transmission bays to house the splices 

of the incoming optical cables to the 

flexible optical tails which plug into the 

optical transmit/receive boards. 

Optical transmitters 

Lasers were chosen as transmitting devices 

in preference to light-emitting diodes 

even though they require changing 

on a more regular basis. This choice was 

made so that, in the main, repeaters 

would not have to be provided and a 

lower grade of fibre could be used. Repeaters 

were expensive if power supply 

security commensurate with the rest of 

the telephone exchange system was 

provided. 

Typical route design data 

Each route loss value was estimated so 

that a satisfactory safety margin could 

be established in addition to the optical 

requirements. This margin is required to 

allow for possible increase in attenuation 

on a route caused by additional 

splicing, route diversion, or falling output 

of the laser. The basic data used 

was: 

Laser output 

-2dBm 

Receiver sensitivity -51 dBm 

Splicing loss 

0.3dB 

Connector loss 2dB 

For example: Embankment - Stepney 

Green 34 Mbit/s route 

Length 

7.14 km 

Fibre loss 

3.5dB/km 

Joints 5 

Calculated loss 

(7.14 x 3.5) + (5 x 0.3) + 2 = 28.5 dB 

Safety margin 

49.0-28.5 = 20.5dB 

Actual measured loss for the route is 

shown in table 1. 

The 2 Mbit/s PCM links from individual 

LIMs and destined to one TSC are multiplexed 

together in groups of four to 

provide an 8Mbit/s tributary. Sixteen 

2 Mbit/s links multiplexed in this way will 

produce four 8 Mbit/s tributaries which, 

when multiplexed together, produce a 

34 Mbit/s signal. This is connected to the 

optical transmit board and is launched 

into the fibre. At the receive end the reverse 

process is carried out to retrieve 

the 16 original 2 Mbit/s PCM links.

198 

In planning the optical fibre network, 

spare fibres have been allowed to cater 

for future growth of the telephone and 

data transmission systems or for other 

communication systems such as the 

new London Underground Ticketing 

Communications network. 

Preparation and testing for 

cut-over 

The new digital system could not work 

directly into the old Strowger exchanges 

unless expensive interface 

equipment was designed. With such 

equipment it would have allowed the 

new exchanges to be commissioned individually, 

but with a period of a mixture 

of 4- and 5-digit telephone numbers 

which was considered unacceptable. It 

was therefore decided to cut over in one 

operation, and careful planning was 

needed to ensure a successful introduction 

of the new system. Each of the 6000 

telephones was wired to its new exchange 

and temporarily back to the old, 

fig. 7. This involved a number of miles of 

extra tie cabling being added to some 

telephone lines, thus causing an increased 

fault liability and reducing 

speech quality in the months prior to 

cut-over. In the old exchanges cross 

connections were made in order to route 

telephones to the new exchange, and 

connections were then arranged to be 

isolated in both old and new by means of 

break jacks on the MDFs. The cut-over 

wiring where an exchange area was not 

to be altered was fairly straight forward 

but where a large existing area was 

being divided up, complex teeing had to 

be made at trackside location boxes. 

In February 1984 exchange co-ordinators 

were appointed for every exchange 

to ensure that the cut-over connections 

were correctly installed and to coordinate 

all other work within their exchange 

prior to cut-over. This included 

both exchange and transmission equipment 

and alarm panel. Regular meetings 

were held with all exchange co-ordinators 

and the project team to enable 

overall progress of the project to be 

monitored. 

Once the cut-over MDF connections 

were completed, each line in turn was 

disconnected from the old exchange 

equipment by placing wedges in the 

break jacks. The associated connection 

was then made in the new exchange and 

the line was tested. The user was requested 

to dial a test number and finally 

the line was restored back to the old 

exchange. 

During one Sunday prior to the main 

cut-over all telephone users connected 

to Loughton old exchange were temporarily 

transferred to Loughton new 

exchange enabling the users at the east 

end of the Central Line to try out the new 

system. This provided the project team 

with valuable feedback information 

both technically and operationally. 

A flood test of telephone calls was also 

undertaken in January 1985 between 

Acton Head Office, Lillie Bridge and 

MDF for 

new 

exchange 

F 

i IT 

MD110 

Fig. 7 

Exchange cutover jumpering 

F 

G 

A 

B 

E 

P 

T 

Fuse mounting 

Gas discharge tube 

Arrester 

Tag block 

Existing jumpering 

Permanent jumpering 

Temporary jumpering

199 

Telstar House new exchanges. Twenty 

lines were connected at each exchange. 

Then between 10.30 hours and 12.30 

hours the twenty users at each exchange 

had a schedule of numbers they 

were required to call on other exchanges. 

The results indicated that exchanges 

performed reasonably satisfactorily 

under load, and the excellent 

speech quality was noted for the first 

time. 

The cut-over night 

A control centre was established in the 

Communications Design office at Paddington 

from where the cut-over was coordinated. 

The network was split into three areas, 

each consisting of approximately five 

old and five new exchanges, and each 

area was allocated a telephone number 

to report to when the cut-over of exchanges 

were completed. 

The cut-over took place during the night 

of Friday/Saturday 8/9 February 1985. 

Work commenced at 21.30 hours to 

transfer telephones at four large office 

buildings with the exception of certain 

operational lines such as the Police and 

Head Controller which could not be 

changed until 02.30 hours the following 

morning. 

The main cut-over was successfully 

completed between 02.30 hours and 

02.45 hours on Saturday 9 February 

1985. After lines were transferred, an exchange 

co-ordinator first reported completion 

to the control centre and then 

rang the Communications Maintenance 

Centre to officially hand the new exchange 

over to the maintenance staff at 

Baker Street. Between the hours 03.00 

and 07.00, telephones were tested until 

the second shift reported for duty to 

continue the process alongside contractor's 

staff. During the rest of the 

weekend installation staff commenced 

replacing rotary dial telephones with 

new multifrequency instruments. 

The weather during the cut-over night 

was terrible as heavy snow fell throughout 

the night. At the start of traffic on 

the Saturday morning trains were found 

frozen to the depot rails. The new telephone 

system was quickly put through 

its paces as Railway Operating staff telephoned 

throughout the railway in order 

to get a train service running. The only 

minor error with the cut-over was that 

three police lines were found to be left 

connected to the old telephone system. 

This was noticed by staff in an old exchange 

who heard equipment functioning. 

The matter was then quickly rectified. 

Post cut-over problems 

Below are some of the problems that 

have been experienced with the new 

system: 

- At Ruislip the exchange was repeatedly 

failing and then restarted itself 

automatically; the failures were due 

to some telephone lines indicating 

lost signalling. These lines were subsequently 

blocked on the terminal 

and manuallywedgedoutonthemain 

distribution frame, but no significant 

line fault appeared to exist. 

Stepney Green exchange also experienced 

troubles similar to those at 

Ruislip. Upon investigation it was 

found that only one line was indicating 

lost signalling due to an earth 

fault. 

- Lost signalling was also causing 

other exchange LIMs to crash, and 

the offending line faults ranged from 

unbalanced cable pairs to disturbances 

through small induced voltages. 

The MD110 system was highly sensitive 

to line characteristics; lost signalling 

indications were stored in a 

counter file (maximum 100) and, 

when an unacceptable level was 

reached, the respective LIM crashed. 

Once the LIM failed, the counter file 

was wiped clean, the LIM restarted 

and the whole sequence was repeated. 

The London Underground system 

was the first ti me the MD110 had been 

subjected to the majority of telephone 

lines being routed in external cabling 

running alongside an electrified railway. 

These cables obviously have 

other characteristics than the normal 

short internal cabling of a PABX in an 

office building. It was interesting to

Fig. 9 

The digital telephone DIAVOX Courier 700 with 36 

or 12 burtons 

note that these lines had worked fairly 

satisfactorily on the old electromechanical 

exchanges. To eradicate 

this problem, the contractor's 

engineers produced a successful 

software system patch, to only log lost 

signalling and not to crash the LIM. 

- Attheend of July acompletefailureof 

Embankment transit and local exchanges 

was experienced. Due to a 

defective cooling unit in the apparatus 

room, the temperature rose to 

over 45°C for a prolonged period. A 

temporary cooling unit was installed 

and both exchanges were reloaded, 

but only the local exchange came 

back successfully. The transit exchange 

was isolated from the rest of 

the system, so all inter-exchange calls 

were routed via Baker Street transit 

exchange without any loss of service. 

On investigation it was found that 

most of the controlling boards on 

LIMs 1 and 10, the Group Switch, 

Transmission Bay and both tapes 

from the tape unit were defective due 

to thermal breakdown. These boards 

were replaced and the transit exchange 

was finally tested and put 

back into service; all this took place 

over a weekend, and it was reassuring 

that it was not necessary to call out 

contractors for assistance asourown 

maintenance staff managed adequately. 

- Every week many lines are initiated or 

facilities changed in the customer 

data; this information is inserted directly 

to the memory boards from the 

exchange terminal and is stored 

"soft". If a data reload takes place, 

then soft information is lost; therefore 

the "soft" information is regularly 

dumped onto the tapes to provide a 

"hard" copy. For security reasons 

each exchange has three copies of 

the tape: one copy remains on site for 

backup purposes, and an identical 

copy and a reference copy of the previous 

issue are retained at Baker 

Street Maintenance Centre. 

Overall the new system has settled down 

very well, with only minor problems, 

which the maintenance staff are becoming 

competent to handle. 

Telephone instruments 

As the MD110 supports either rotary 

dial, Dual Tone Multi Frequency (DTMF) 

key phones or digital instruments this 

enabled the cut-over to be carried out 

without the need to change the existing 

loop disconnect telephones. Standard 

telephones with DTMF signalling and 

the digital telephone DIAVOX Courier/00 

have been selected to cover the 

needs of London Transport telephone 

users. 

Digital telephone 

The digital telephone known as the 

DIAVOX Courier700, fig.8, is currently 

being installed as a replacement for 

existing Manager/Secretary arrangements, 

which were dial type instruments, 

and required multiple wiring between 

the manager's and secretary s instruments. 

A mains power supply was 

required to run this arrangement. In addition 

a further unit was needed to 

provide facilities like "stored numbers 

and "loudspeaker".

201 

The Courier telephone replaces all this 

complex wiring and still retains the manager's 

engaged lamp on the secretary's 

instrument which has been lost on most 

modern telephones. It is designed to allow 

the user easy operation of the wide 

variety of facilities available by pushing 

a button which is programmed to each 

user's requirements. A liquid crystal display 

indicates the telephone number of 

an incoming call and on outgoing calls 

will change to indicate whether the 

called number is on diversion. With the 

loud speaking function of the Courier 

there is no need for a separate intercom 

system. 

In the future, by the addition of a "Terminal 

Access Unit" plugged to the rear of 

the Courier, the telephone can function 

as a link between a data terminal and the 

telephone exchange. This will enable simultaneous 

voice and data transmission 

over the same extension line. The 

signalling between the exchange and 

telephone is in digital bursts. A digital 

extension line unit sends a 12-bit burst 

at a transmission rate of 256 kbit/s every 

125 us. The telephone is synchronised to 

this rate so that it sends its "burst" after 

receiving a "burst". The Courier is, 

however, restricted to approximately 

1000 metres from an exchange mainly 

due to signal attenuation and disturbances. 

Programming 

There are two types of programming involved 

when installing a digital telephone. 

The exchange programming involves 

the "Customer Data" which is 

done to suit individual requirements, although 

with most manager/secretary 

needs being similar, some form of standard 

has been adopted. The customer 

data is entered via a data terminal connected 

to the input/output interface to 

the exchange. The customer data includes 

the initiation of function and facilities 

required for a particular Courier 

telephone and the keys that will control 

those facilities. This information is finally 

"dumped" to achieve a "hard 

copy". 

The other programming involved is entered 

by the user directly into the Courier 

and includes the programming of 

individual abbreviated numbers and 

ringing characteristics. 

Maintenance organisation 

Prior to the introduction of the new system 

the maintenance of the old 

Strowger system was carried out by a 

team of 23 technicians under one supervisor. 

These staff carried out fault rectification 

and preventive maintenance 

on all exchange, transmission and telephone 

instruments and similar equipment. 

Other staff dealt with the maintenance 

of the cable network. The maintenance 

of telephone instruments and associated 

equipment was the responsibility 

of a Technician 2. There were five 

staff in this grade working on a shift 

basis and operating from two depots. 

The shift roster provided for two men to 

be on duty between 07.00 and 23.00 

hours Monday to Saturday and between 

07.00 and 19.00 hours on Sunday. Of the 

staff at Technician 1 level, 15 operated 

on a shift basis and 3 were primarily on 

day duty covering special investigations. 

The shift staff operated from 3 depots 

and cover was provided 24 hours a 

day, all days of the year. During night 

shifts these staff undertook preventive 

maintenance or followed up fault investigations 

in a particular, designated exchange. 

Overall the Strowger system required a 

staffing level of one man per 220 exchange 

lines to undertake the necessary 

maintenance of the system and provide 

the quick response to faults required by 

the needs of the railway. 

With the introduction of the new telephone 

system a major reorganisation of 

the maintenance section was undertaken 

with full co-operation from the 

trade unions concerned. The existing 

day supervisor's post was abolished and 

replaced by five supervisors operating 

on a full 24-hour shift basis. The number 

of technicians was reduced from 23 to 

17. This gives an initial manning level of 

one man per 270 lines with potential for 

considerable growth in installed exchange 

connections with only marginal 

increase of staff numbers. 

The reorganisation had little impact on 

the Technician 2 but major changes 

were required in the Technician 1. The 

shift roster cover for these Technicians 

was considerably reduced and the staff 

based at new locations better suited to 

the new system. In addition the new ex-

202 

Fig. 11 

Transmission equipment for 34 Mbits/ at Embankment 

changes require only a minimum of routine 

and preventive maintenance. A major 

change was to provide more staff on 

day duties and less on shift work. A new 

grade entitled Communications Equipment 

Technician has also been introduced 

to cater for the increased sophistication 

of the new network. The staff 

should provide better flexibility as they 

will be expected to deal with the more 

complex faults on all types of communication 

systems including train radio, 

closed circuit television, public address 

as well as the new telephone system. 

Training 

To achieve this reorganisation it has required 

extensive retraining of existing 

staff. Formal training was supplied by 

the contractor responsible for the new 

system supported by field training by attaching 

staff to his installation personnel. 

In practice, due to the diversion of 

resources to the installation of the new 

system, it was not possible to train the 

staff to the degree of competence necessary 

to maintain the new system with 

100% confidence from day one. Assistance 

was therefore obtained from the 

contractor for the first six months of service 

in the form of one man on permanent 

day attachment to London Underground. 

As previously mentioned our 

technicians quickly became proficient 

in dealing with defects, and the services 

of the contractor personnel on a regular 

basis have now been terminated. 

An important aspect of the new maintenance 

discipline has been the need to 

closely control spare boards and equipment. 

With the St rowger equipment only 

a small number of items were essential 

to the operation of the whole exchange 

and these could easily be controlled. 

Now it is vital to ensure that spare 

boards and parts are available at all 

times as the lack of a board could result 

in extensive downtimes with little room 

for alternative temporary repairs. To this 

end a computerised inventory system 

will soon be introduced to record and 

control all spare boards. 

The transition from electromechanical 

to an electronic system has caused 

many long established organisation 

procedures and methods to be reviewed, 

and many changes have now 

been implemented to create an 

organisation more suited to the electronic 

age. 

References 

1. Knipe.V.T.A.: The Use of Optical Fibre 

Cables in the Modernisation of the 

London Transport Automatic Telephone 

System. IRSE Conference 198* 

2. Mörlinger, R.: MD110 - a Digital SPC 

PABX. Ericsson Rev. 59 (1982):1,PP 

2 — 13. 

3. Reinius, J., Svensson, B. and Åkerlund. 

S.-O.: Digital Signal Processing in System 

MD 110. Ericsson Rev. 59(1982) :i, 

4. Reinius, J. and Sandström, 0.: DIAVOX 

Courier 700. Digital System Telephone 

for MD 110. Ericsson Rev. 59 (1984'• 

pp. 58-66. 

5. Barnicoat, G., Boman, L. and Ulander 

O.: Dafa Communications m MO""- 

Ericsson Rev. 59 (1982):2, pp. 67-'=. 

6. Hedman, J.-O.: MD110 in the Auto^ 

mated Office. Ericsson Rev. 

(1983):2, pp. 88-93.

Reliability of 

Equipment 

Branko Tigerman 

Calculations of the reliability and availability performances of transmission 

systems and networks are based on data concerning the reliability of the 

equipment used. Ericsson s transmission equipment has high built-in reliability 

which affects system and network planning, for example by reducing the need 

for redundancy in the equipment. 

The author describes the prediction method used and its trustworthiness, 

together with the way of presenting reliability data for transmission equipment 

and systems. Ericsson's views on redundancy in connection with demands for 

duplication of multiplexing and line equipment are also given. Finally some 

typical MTBF values for some of the most frequent function blocks are shown in 

order to give an idea of the reliability of Ericsson's transmission equipment. 

UDC 621.395.45 

telecommunication 

reliability 

transmission lines 

redundancy 

Reliability, which originated in the fields 

of defence and space research, is today 

an important parameter also for transmission 

equipment. It should therefore 

be specified together with other technical 

data of the system. 

Quantitative reliability data are required, 

together with other transmission 

engineering and economic factors, 

BRANKO TIGERMAN 



in order to be able to assess the efficiency 

of equipments and systems. Such 

quantitative data consist of reliability 

and availability values, which have 

therefore become important basic parameters 

for transmission equipment. 

In order to be able to calculate the real 

value of different reliability parameters 

the equipment must have been in operation 

for a certain length of time and a 

number of failures must have occurred. 

The more reliable the equipment, the 

longer the time required for a relevant 

number of failures and the smaller the 

practical possibilities of making an assessment 

based on observations. 

Predictions are therefore used during 

the development and design of new 

equipment; they are based either on experimental 

values or observations of 

other, similar units. 

Fig. 1 

All integrated circuits are fully tested before 

assembly

204 

Fig. 2 

Automatic testing of multiplexing equipment after 

manufacture 

Reliability predictions of failure occurrence 

as a function of time provide information 

regarding the future behaviour 

of the equipment and the system. The 

result is also used as a basis for the planning 

of maintenance and for calculating 

the operating costs during the life of the 

equipment, life cycle costs (LCC). 

The reliability of complex systems and 

networks depends on their design and 

construction. The calculations are 

based on the predicted reliability of the 

equipment used. 

The purpose of the telecommunication 

network is to offer the customer good 

services. The service quality in the network 

is a function of such factors as the 

availability of the network. 

In order to meet technically and economically 

feasible availability requirements 

and to increase the probability of 

transmission paths surviving catastrophes 

it is sometimes necessary to introduce 

redundancy at relevant points in 

the network. The views on the use of 

redundancy expressed here are based 

on Ericsson's experience of the operation 

of equipment with high built-in reliability. 

They may be of assistance when 

deciding on the requirements and design 

of redundancy in systems, links or 

networks. 

Reliability is one of the basic design parameters 

in Ericsson's transmission 

equipments. High built-in reliability has 

been achieved by 

- using selected components of high 

quality 1 

- limiting the stress by reducing the 

power (derating) 

- reducing the power consumption in 

order to keep the temperature low 

- careful checking of components and 

manufacture 

- improving the design on the basis of 

operational experience (follow-up). 

It is certainly economically justifiable to 

have high reliability built into the equipment 

right from the start. Loss of income 

and the cost of maintenance work and 

spares in connection with failures can 

warrant the expenditure of up to 40% 

more for equipment with higher built-in 

reliability 6 . 

If a decision-maker is to be able to assess 

and trust the data supplied by the 

manufacturer the prediction method 

must be known and its trustworthiness 

verified. Prediction results must be presented 

in a suitable form and a clear 

manner and should be easily understandable 

even by non-specialists, i.e. 

anybody with a normal technical training. 

The panel "Terminology and Definitions" 

explains concepts and termsina 

somewhat simplified form. 

It is a difficult task, and one of considerable 

responsibility, to express oneself 

in a simple and easily understandable 

way in a discipline that uses such complicated 

mathematical tools as those 

used for reliability calculations. The 

price that has to be paid for simplicity is 

reduced mathematical accuracy, limited 

terminology, simplified models and 

last but not least, some loss of the author's 

technical prestige. 

Reliability prediction and its 

trustworthiness 

Prediction method 

The prediction is carried out by mean 

of special computer programs, using 

the parts count method, i.e. the fa""

Terminology and Definitions 

The terminology and definitions used in the 

article are explained here. Some ot the terms 

are in general use, others are specific to the 

article. The definitions of reliability concepts 

are based on the terminology in the report of the 

Nordic Working Group 3 with certain 

simplifications made for the relevant 

application. 

Availability 

Instantaneous availability is the probability that 

a unit is functioning at a certain moment of 

time. 

Complete failure 

A failure that means total loss of the required 

function. 

Constant failure rate period 

A period of time, between the early failure 

period and the wear-out failure period, when 

failures occur at an approximately uniform rate. 

Degradation fa/lure 

A characteristic exceeds the given tolerance 

limits without constituting complete failure, but 

the process is so slow that the failure could 

have been anticipated by prior examination. 

Down time 

The time during which a unit should be working 

but is faulty or being repaired. The down time 

comprises fault detection time, administrative 

time, waiting time and repair time. Net down 

time includes only fault detection time and 

repair time. 

Early failure period 

Early period with a noticeable decrease in the 

failure rate. 

Failure 

Undesirable deviation of a certain characteristic. 

In practical applications failures should be 

defined in detail with respect to cause, consequences 

and extent. 

Failure rate 

Instantaneous failure rate, z(t), is the limit value 

of the ratio of the probability of failure during an 

interval of time to the length of the interval when 

the latter tends to zero. 

The mean failure rate, z, is the average value of 

the failure rate during an interval of time, i.e. the 

ratio of the integrated instantaneous failure rate 

during the interval to the length of the interval 

and is given in 1/h. 

In the US the unit FIT = 10~ 9 failures/hour is 

used. 

Failure that prevents operation 

A failure which means that the unit does not 

function. However, the failure does not 

necessarily prevent transmission. 

Function block 

A part of the equipment with a specific function, 

often in the form of a magazine or a shelf (e.g. 

60-channel multiplexer, 30-channel PCM, 565 

Mbit/s line terminal). 

Intermittent failure 

The function returns without any repair having 

been made. The failure can be recurrent. 

MTBF 

Mean time between failures, which is the mean 

value of the time intervals between failures. 

During the constant failure rate period MTBF = 

1/z. 

MTTR 

Mean repair time. In practical applications it 

should be specified whether the Mean Time To 

Repair is meant, i.e. net down time (fault 

location, net repair and function test time), or 

Mean Time To Restore, i.e. total down time 

(fault location, administrative, waiting and 

repair time). The mean time observed by 

different administrations is usually given as a 

ratio of the total duration of the failures to the 

number of times failures occur during a given 

period. In these cases the MTTR is the mean 

time to restoration of function after a failure and 

can be used direct in the calculation of the 

mean availability. 

Observed failure rate 

A quantitative observation result, which during 

the constant failure rate period (with an 

approximately constant failure rate) is obtained 

as the ratio of the number of occurring relevant 

failures to the observation time. 

Observed MTBF 

A quantitative observation result, which during 

the constant failure rate period is obtained as 

the ratio of the observation time to the number 

of relevant failures occurred. 

Operating time 

The time during which a unit is in operation. 

Permanent failure 

A failure that remains until it has been repaired. 

Predicted failure rate or MTBF 

Calculated failure rate or MTBF based on 

Component failures can affect the function 

of a unit in different ways. Conestimated 

values from experiments or 

observation of other or similar units. 

Primary failure 

Failure that is not caused by the failure of 

another item. 

Probability of failure 

The probability that a failure will occur during a 

given interval of time undergiven operating and 

environmental conditions. 

Redundancy 

Redundancy means that a given function mode 

is maintained by more than one means (e.g. 

several parallel routes). 

Reliability 

The probability that a unit will work properly 

during a certain interval of time under given 

operating and environmental conditions. 

Repair time 

Time for fault location, fault correction and 

functional test. 

Shortage risk 

The probability that a spare unit is not available 

when needed. 

Sudden failure 

A failure that occurs so suddenly that it could 

not have been anticipated by prior examination. 

Telecommunication network 

All lines and equipment used to set up 

communication between a number of different 

places. A network contains nodes (exchanges) 

and links (transmission systems) between the 

nodes. 

Transmission equipment 

Equipment (hardware) in the form of different 

function blocks that form part of a system for 

the transmission of information. 

Transmission network 

A network of links between nodes, i.e. 

transmission systems between exchanges. 

Transmission system 

Various equipments or transmission media 

connected together in a configuration that 

makes it possible to transmit information (e.g. 

multiplexer, lineequipment, cable, line system). 

Wear-out failure period 

Late period with a noticeable increase in the 

failure rate. 

rates for all components in the unit are 

added. This means that from the point of 

view of reliability all components in the 

unit in question are considered as being 

series connected without any internal 

redundancy. 

Practical calculations are usually made 

for an operating temperature of +40° C 

(104° F) and standardized stress models. 

The operating temperature is considered 

to be the result of a room temperature 

of +25° C (77° F) with an addition of 

15° C (27° F) caused by dissipated heat. 

However, both temperature and stress 

models can be varied within the limits of 

the operating range if necessary. 

sequently, different failure modes with 

different failure rates arise, which can 

be related to different functions in a 

complex unit. Furthermore, each unit 

can affect the function block in different 

ways depending on its own failure 

modes, the structure of the block, the 

position of the unit in the magazine and 

other units in the block. 

For practical reasons certain assumptions, 

conditions and limitations are introduced 

when making predictions, resulting 

in the following model: A failure 

is considered to be primary (not caused 

by another failure), total, sudden and 

preventing function but not necessarily 

preventing transmission. It is permanent, 

and manual action is required to 

restore function, it has occurred under

206 

Relative 

occurrence for F 

Fig. 3 

Histogram where the factor F = observed/ 

predicted failure rate for printed board 

assemblies 

• 

52% of the printed board assemblies better than 

predicted (F1) 

i 

r 

0,2 0,3 0,5 1 

normal operating conditions and affects 

the transmission function in different 

ways. 

The failure concept thus excludes the 

human factor and any external influences, 

such as mechanical and electrical 

damage or intermittent interference. 

Such factors are almost completely dependent 

on user and applications and 

are therefore beyond the control of the 

manufacturer. 

After a few months of operation (burn-in 

period) the equipment is in its "constant 

failure rate period", and the failure rate 

is considered to be constant with time. 

All data refer to this period. 

Trustworthiness of prediction 

The trustworthiness of the prediction is 

mainly dependent on the trustworthiness 

of the failure rates of the components 

used. The component failure rates 

are stored in a data base which is available 

to the whole Ericsson Group. These 

failure rates are based on operational 

experience and have been obtained by 

means of long-term follow-up and analysis 

of repair activities. 

Experience with newer components is 

insufficient, however, and the amount of 

operational data is limited. In such cases 

the new component must be compared 

with a similar, older component. If there 

is no such component available for comparison, 

the manufacturer's data are 

studied, tested and verified, and laboratory 

tests are made. 

The use of advanced technology with 

integrated circuits in modern system designs 

increases the reliability of the system. 

The follow-up of the behaviour of the 

equipment in operation has shown that 

there is good agreement between the 

predicted and the observed reliability of 

equipment in construction practices M4 

and M5. This conclusion is based on 

analysis of the predicted and the observed 

failure rates for 138 types of 

printed board assemblies from the 29 

most common function blocks. 

Fig. 3 shows the result of a comparison 

of 79 different types of printed board 

assemblies with more than four relevant 

failures. It can be seen that 51 % of the 

observed failure rates lie between "2 

times better" and "2 times worse" than 

the predicted value. 

The function blocks show even better 

correlation, fig. 4. As many as 62% of the 

analyzed function blocks have been better 

than predicted, 17% "considerably 

better", and no function block has 

proved to be "considerably worse" than 

predicted 2 . 

Table 1 shows a comparison between 

the predicted and observed reliability 

for some of the most frequent equipments. 

Presentation of reliability 

data 

Reliability parameters 

Reliability parameters describe quantitatively 

the reliability performance o 

the equipment. The parameters used a 

chosen with regard to the object ot

Table 1 

Comparison of predicted and observed MTBF for 

some function blocks 

Equipment 

Channel/SG 

SG/MG 

MG/SMG 

SMG/LG (2700) 

30-channel PCM 

2 Mbit/s line system 

(2 terminals + 2 rep.) 

8 Mbit/s line system 

(2 terminals + 4 rep.) 

Older 

Predicted 

5.2 

27 

30 

36 

10 

- 

- 

Mean time between f; tilures, IV TBI- (years 

des gn (M4) 

New desig 

Observed Predicted 

11 

7.1 

27 

33 

31 

31 

27 

36 

11 

18 

- 

- 

26 

19 

i (MS) 

Observed 

10 

71 

28 

31 

18 

39 

29 

Fig. 4 

The correlation between predicted and observed 

failure rate for function blocks (magazine = shelf) 

x 

Different function blocks 

Observed value = Predicted value 

prediction and the use of the data in 

future calculations. Telecommunications 

administrations and manufacturers 

represent different viewpoints in 

practical applications. 

Telecommunications administrations 

and users of telecommunication services 

are mainly interested in the availability 

of the service and equipment. 

However, the availability is dependent 

on external application and maintenance 

factors as well as the built-in reliability 

of the equipment. The calculation 

model becomes complex because 

of factors that vary with time and space 

and also from one case to another. 

The manufacturer of transmission 

equipment, on the other hand, is only 

responsible for the built-in reliability. 

Application factors are beyond his control. 

Under given operating conditions 

the built-in reliability is only dependent 

on components and design parameters, 

and the calculation model is therefore 

relatively simple. It is also easy to compare 

equipment from different manufacturers. 

The reliability parameters of the 

equipment, i.e. the failure rate or mean 

time between failures, constitute the 

basic data for calculations concerning 

systems, links and networks. 

The following parameters have proved 

suitable for use in most practical cases: 

the failure rate z, the mean time between 

failures MTBF, reliability R, availability 

A, and down time DT. See the panel 

"Terminology and Definitions". 

Of the many possible ways of presenting 

reliability data two extreme cases may 

be mentioned. In the first case the whole 

equipment is regarded as a single item 

and all failures are counted without their 

effects being considered. In the other 

case a failure effect analysis is made, i.e. 

the effect the failure has on the transmission 

capacity of the equipment is 

considered and the failures are structured. 

The presentation method using structured 

failures is considered most suitable 

for transmission equipment 4 . A brief 

description is given below. 

Observed 

failure rate 

(failures/10 9 h) 

10 5 

Structured presentation 

Different types of failures in equipment 

can affect the transmission capacity in 

different ways. With respect to its transmission 

capacity the equipment can be 

in different (but well defined) states depending 

on which and how many functions 

are being affected by the failure. 

10 4 

10 3 - 

102 

102 

Worse than 

predicted: 38% 

x 

X 

X * / 

/ 

X X X 

X/ 

/ 

/ 

TT1— 

103 

w / 

* 

/ 

/ 

X 

X 

X 

Fig. 5 

Field testing of a 140 Mbit/s coaxial cable system 

taken into account by presenting the 

corresponding reliability parameter 

"cumulatively upwards", i.e. for equipment 

"at or above" the given capacity 

level. 

In practice, information is sometimes required 

regarding the reliability of a certain 

channel, a frequency band or at bit 

stream. This is also provided by 

cumulatively upwards structured presentation. 

However, it is often sufficient to present 

just the values for "complete failure" 

and for "failures preventing transmission". 

A simplified presentation can, for 

example, take the following form: 

Complete failure: MTBF (45) = 61 years 

Failures preventing 

transmission: MTBF,, 5+) = 8.5 years 

The indices in brackets give the number 

of affected Mbit/s, and a plus means that 

it is cumulative upwards. MTBF n 5+, thus 

means that everything is considered 

that affects "1.5 and more Mbit/s", i.e. all 

equipment for 1.5, 6 and 45 Mbit/s. 

IfevenMTBF, 7.6 years is given, the 

value includes failures that affect "Oand 

more Mbit/s", i.e. all equipment failures. 

The information is used in the planning 

of the life cycle costs (LCC). 

This presentation method has been applied 

for more than ten years in the tendering 

for transmission equipment. The 

extra work involved in obtaining the 

data and the slightly lengthy presentation 

are more than compensated by the 

detailed information it provides regarding 

the reliability of the equipment with 

respect to its transmission capacity. 

Failure effect analysis and structured 

presentation also show the advantages 

of a well planned design with high maintainability. 

These advantages would 

otherwise remain hidden or even be interpreted 

as disadvantages. For example, 

sophisticated control and supervisory 

equipment, which does not have 

a direct effect on the transmission, can 

reduce repair times and increase the 

availability. However, in an unstructured 

presentation the supervisory equipment 

would give rise to a higher failure rate 

for the equipment as a whole than for 

similar equipment without these supervision 

facilities. 

Optical fibre equipment, for example, 

can have the following failure rates: 

2(565) = 11^3 FIT MTBF l565l =103år 

Z (0 , = 992 FIT MTBF (01 = 115 år 

Z (0+) = 2105 FIT MTBF |0+l = 54 år 

Without structuring the MTBF would be 

given as MTBF = MTBF (0 ,, = 54 years. 

The value that is relevant to the transmission 

capacity is MTBF (565) = 103 

years, however. The fault detector and 

alarm unit contribute with 992 FITbutdo 

not affect the transmission capacity of 

the equipment. An unstructured presentation 

would show a higher MTBF for 

equipment without these units, and this 

equipment would then be seen as more 

reliable. 

Reliability data in tenders 

When tendering, reliability data can be 

presented asgeneral information forth 

customer or in response to certain 

points in the specification. At the same 

time a list is supplied of units (print 

board assemblies) recommended a: 

spares for the equipment.

209 

If the customer does not have special 

requirements as regards the presentation 

of reliability data, Ericsson usually 

provides a separate report with data 

concerning: 

- failure rate and MTBF for individual 

function blocks (e.g. magazine or 

shelf) 

- failure rate, MTBF, trustworthiness 

reliability, availability and down time 

for a selected, worst case system or 

link combination 

- failure rate and MTBF for the whole 

equipment quoted for 

- maximum number of failures for 

equipment in accordance with the 

two previous items, which with a certain 

probability will not be exceeded 

during a given period of time 

- the prediction method and its trustworthiness 

These data are a best estimate, and in 

the case of a guarantee a higher confidence 

level is chosen. The amount of 

equipment, operating conditions and 

verification methods must also be carefully 

specified. 

There are different methods and models 

for verifying that the equipment meets 

the guaranteed reliability. In all cases, 

however, the test plan must define: 

- the equipment to be tested 

- the beginning and end of the test 

period 

- operating conditions and failure concepts 

- statistical evaluation method. 

The text in the technical part of the guarantee 

should be clear, unambiguous 

and easy to understand, and only contain 

data that can easily be checked. It 

could be formulated as follows: 

"An MTBF>6.7 years is guaranteed for 

the specified equipment, i.e. a failure 

rate z

210 

Fig. 7 

The system down time as a function of the down 

time for the multiplexing equipment. System = 

multiplexer + line system 

a^Hî 

Coaxial system for 140 Mbit s including 

mechanical damage to the cable 

mmmm Optical fibre system. 140 Mbil s 

•• •• 

Down time 

for the system 

(minutes/year) 

1000 - 

100 

Coaxial system, 140 Mblt/s 

Coaxial system. 12 MHz 

Optical fibre system for 140 Mbitys with line 

system redundancy 

permits up to C = 30 failures. The guarantee 

is considered not to have been 

met if more than 30 failures occur during 

the period." 

In this calculation it has been assumed 

that both the producer's and the consumer's 

risk is 10%. For the manufacturer 

this means a risk (10%) that the 

equipment is rejected (C>30 failures) in 

spite of the fact that its real MTBF meets 

the requirements (i.e. MTBF>6.7 years). 

At the same time the customer runs a 

risk (10%) of accepting the equipment 

(C

211 

Transmission system Dow n time 

(100 miles = 160 km) l System 

i ;min/year) 

Multiplexer 

(% of total) 

140 Mbit/s coaxial 

system (incl. 

mechanical damage to 

the cable) 

140 Mbit/s coaxial 

system (only 

equipment failures) 

12 MHz coaxial system 

(only equipment 

failures) 

140 Mbit/s optical fibre 

system (excl. 

mechanical cable 

damage) 

140 Mbit/s optical fibre 

system with 

redundancy (1 standby 

for every ten systems 

in operation) 

2170 

251 

49 

696 

19 

0.4 

2,5 

47 

1.3 

1.3 

Table 2 

Down time for different transmission systems 

over a line of 100 miles (160 km) 

ability of the system will not be improved 

significantly by redundancy in the multiplexing 

system if the availability of the 

line system is low. 

Instead of introducing redundancy in individual 

function blocks the availability 

requirements for the network can be 

met by means of distributed traffic, rerouting 

or standby equipment at the link 

or system level. Such measures are considered 

advantageous since they also 

increase the probability of the system 

surviving sabotage, catastrophes and 

war. 

One exception that should be mentioned 

is line systems for submarine cables. 

Long repair times and high costs 

justify redundancy for function blocks in 

the form of standby equipment in repeater 

stations at the bottom of the sea. 

Some manufacturers apply the redundancy 

principle for equipment at the 

function block level as a general measure 

to improve availability. It results in 

increased volume and higher power 

consumption (higher temperature) and 

a larger number of failures overall. 

However, failures in the duplicated 

equipment do not prevent transmission, 

and as a consequence longer repair 

times can be accepted, which can have a 

positive effect on the maintenance routines. 

Operation and maintenance 

of transmission equipment 

The availability of the equipment and the 

system is affected by the maintenance. 

Modern transmission equipment requires 

only corrective maintenance. 

However, a certain amount of preventive 

maintenance is required for the laser diodes 

in optical fibre systems, in which 

failures can start as degradation of certain 

parameters. Continuous automatic 

monitoring with alarm means that a unit 

that has started to degrade can be exchanged 

at a suitable time, before a 

complete failure occurs. 

The telecommunication networks of today 

contain electronic equipment with 

similar components for transmission 

and switching. Technically (and financially) 

it is thus possible to integrate the 

operation and maintenance of these two 

types of equipment. Most administrations 

still have separate organizations 

for the maintenance of transmission and 

switching equipment, however. 

Supervisory equipment in computercontrolled 

exchanges makes it possible 

to indicate and locate faults also in the 

transmission system. The mean time to 

repair, MTTR, is dependent on the policy, 

organization and technical facilities 

of the administration, and also the geography 

and communication facilities of 

the country. Adistinction should also be 

made between MTTR for failures in 

manned and unmanned exchanges and 

in equipment and cables. 

Thanks to a modular structure with 

plug-in units (printed board assemblies) 

the net repair time is very short and in 

practice is reduced to just replacing the 

faulty board. The down time is considerably 

longer, however, because of the 

various actions that are necessary during 

the time from when the fault occurs 

to when the equipment is back in operation, 

see "Terminology and Definitions". 

As a guide value for practical calculations 

it may be assumed that the mean 

times for restoration of function after a 

failure has occurred is two hours for a 

manned exchange, six hours for an unmanned 

exchange and repeaters in 

housings and twenty hours for cable 

damage. For the sake of simplicity an 

MTTR of four hours is often used in calculations 

for all electronic equipment. 

Repair in the form of a replacement of a 

faulty printed board assembly requires 

good administration of spare parts, with 

a sufficient number of spares held at 

strategic points. Tenders therefore include 

a recommended list of spare 

parts. 

The calculation of the number of spares 

is based on the failure rate for the unit 

under the given environmental and operating 

conditions, the number of units 

in operation and their importance to the 

transmission, and also the time it takes 

to obtain replacements. 

The number of spare units is set so that a 

given shortage risk is not exceeded during 

the period until the next replenishing 

of the stock of spares 7 .

MTBF 

(years) Equipment 

2-5 Transmultiplexer 5x24/2x60 

Transmultiplexer 2x30/60 

Multiplexer E-D4, Mode 4 (6 Mbit/s) 

5-10 Exchange terminal circuit ETC 24/96 

Optical fibre terminal with 565 Mbit/s 

muldex 

Muldex 12x45/565 Mbit/s for optical 

fibre system 

Multiplexer 1/60 

Multiplexer E/D4, Modes 2 and 3 

(3 Mbit/s and 2x1.5 Mbit/s) 

Digital multiplexer M12 (1.5/45 

Mbit/s) 

10-20 Optical fibre terminal with 45/140 

Mbit/s multiplexer 

Multiplexer E-D4, Mode 1 (3 Mbit/s, 

48 channels) 15SG/SMG multiplexer 

Digital multiplexer 34/140 Mbit/s 

30-channel PCM (with and without 

signalling) 



Line terminal in 140 Mbit/s coaxial 

line system 


20-50 SG/600G multiplexer 

140 Mbit/s optical fibre line terminal 

600G/2400G multiplexer 

Channel in loop-connected 2700- 

channel FDM terminal 

34 Mbit/s optical fibre line terminal 

140 Mbit/s optical fibre line repeater 

Basic Group in loop-connected 

2700-channel FDM terminal 

Supergroup in loop-connected 2700- 


SG/MG multiplexer 

Channel/BG multiplexer 

Carrier generation (channel, 

subgroup, group) 

MG/SMG multiplexer 

Digital multiplexer M12 (1.5/6 Mbit/s) 

50-100 34 Mbit/s optical fibre line terminal 

SMG/LG multiplexer 

Mastergroup in loop-connected 

2700-channel FDM terminal 

Line repeater in 140 Mbit/s coaxial 

line system 

ZAX 480 T line terminal with remote 

power feeding 

ZAX 120 T line terminal with remote 

power feeding 

100-200 BG/SG multiplexer 

565 Mbit/s optical fibre terminal 

repeater 

SMG in loop-connected 2700- 


Line terminal in 4 MHz coaxial line 

system 

Line terminal in 12 MHz coaxial line 

system 

200-500 Remote power supply for line 

system over coaxial cable 

Line repeater in 4 MHz coaxial line 

system 

Line repeater in 12 MHz coaxial line 

system 

LG in loop-connected 2700-channel 

FDM terminal 

Line repeater in line system 

ZAX 480 T 

Line repeater in line system 

ZAX 120 T 

Line repeater in 2 Mbit/s line system 

>70 000 Individual basic frequencies in 

centralized basic frequency 

generating equipment (duplicated) 

Table 3 

MTBF for transmission-preventing failures in 

multiplexer and line equipment. 

Reliability data for typical 

transmission equipments 

The range of transmission products 

comprises several types of equipment 

with many variants. It is impossible to 

present data for all these without also 

giving an extremely detailed specification 

of the design and construction. 

In order to give a general idea of the 

reliability of Ericsson's transmission 

equipments the order of magnitude of 

the MTBF for some of the most frequent 

products in the transmission field is 

given in table 3. 

The values given comprise all equipment 

failures that prevent traffic. The 

MTBF for complete failure in multiplexing 

equipment is considerably higher 

(1.5-200 times), however, and is dependent 

on the design and construction of 

the function block. 

Summary 

Reliability is one of the most important 

equipment characteristics and should 

be considered just as seriously as other 

parameters when evaluating transmission 

products. Most technical data can 

be assessed immediately in a deterministic 

process, but the evaluation of 

quantitative reliability parameters is a 

probabilistic process that requires time. 

The better the product, the longer the 

test period required for verification. 

With only small amounts of equipment 

in operation it is practically impossible 

to verify quantitatively the guaranteed 

reliability. In such cases the reliability 

data are given weight and trustworthiness 

through well defined prediction 

methods and a sensible guarantee. 

High built-in reliability also affects system 

and network planning in that it reduces 

the need for redundancy in the 

equipment. Distributed traffic, rerouting 

of links and redundancy at the system 

level are better solutions than duplication 

of the multiplexing equipment. 

Such measures also increase the 

chances of survival of the system and 

network in cases of sabotage, catastrophes 

or war. 

Ericsson's policy of designing and manufacturing 

products with high built-in 

reliability has proved to be economically 

advantageous to the user. Many of 

Ericsson's equipments have been in service 

for long periods and under widely 

varying operational och environmental 

conditions with different telecommunications 

administrations around 

the world. The world-wide experience 

thus obtained confirms that this is the 

right policy. 

References 

1. Harris, P. O.: No System is Stronger 

than its Weakest Component. LM 

Ericsson's Leaflet XF/YG 164219. 

2. Tigerman, B. and Ahlbom, 0.: Are Reliability 

Figures Actually Realized in 

Practice?LM Ericsson's Leaflet XF/YG 

164 218 and/or: Correlation Between 

Predicted and Observed Reliability tor 

Telecommunication Transmission 

Equipment. Proceedings, Relectronic' 

82, 5th Symposium on Reliability in 

Electronics, Budapest, Hungary, Oct. 

1982. 

3. SAKE-UHT (Nordic working group on 

reliability): (Evaluation of Reliability 

with Annex 1, Vocabulary). 

4. Tigerman, B.: Presentation of Reliability 

Data for Telecommunication 

Transmission Systems. Proceedings, 

Relectronic -77, Symposium on Hi 

liability in Electronics, Budapest, Hungary, 

Oct. 1977. 

5. Tigerman, B.: Economy of Redundancy 

in Telecommunication Systems^ 

Proceedings, Annual Reliability a 

Maintainability Symposium, Philadelphia, 

USA, Jan. 1985. 

6. Johansson, P.: Improved Reliability » 

Telecom Equipment Could Justifyj 

40% Higher Price. LM Ericsson sLeai 

let XF/YG 164 220. ,.„., 

7. Tigerman, B.: Method for Estimation ° 

Spare Parts Requirement for Irani 

mission Systems Ericsson Rev. 

(1971):4, pp. 141-152.

ERICSSON 

^ 

ISSN 0014-0171 Telefonaktiebolaget LM Ericsson 02186 yungforetagen,

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