RETI DI TELECOMUNICAZIONI - TLCNETGROUP Home Page

© 

Corso di Studi in Ing. delle Telecomunicazioni 

LAUREA in INGEGNERIA DELLE 

TELECOMUNICAZIONI 

RETI DI TELECOMUNICAZIONI 

Stefano Giordano 

Lezione n.9 

“Accoppiatore Ibrido Direzionale; cancellatore d’eco; 

segnalazione; traffico e routing nelle reti telefoniche 

Gruppo di Ricerca in Reti di Telecomunicazioni 

Dipartimento di Ingegneria della Informazione: 

Elettronica, Informatica, Telecomunicazioni 

1

© 

L’accoppiatore ibrido direzionale 

Hybrid 

transformer 

Il libro mostra questa figura 4.46 che risulta forviante! 

2

Telefono analogico 

Segnale ricevuto 

4 Fili 

Segnale trasmesso 


4 Fili 


© 

Hybrid 

transformer 

Hybrid 

transformer 

Modem fonico 


2 Fili 

(bidirezionale) 

2 Fili 

(bidirezionale) 

Hybrid 

transformer 

4 Fili 

Hybrid 

transformer 

Matrice di 

commutazione 

Autocommutatore 

3

© 

in 

out 

Schema elettrico equivalente dell’accoppiatore 

ibrido direzionale “forchetta telefonica” 

Z 

Z 

Z/2 

N 

N 

a 

b 

c 

d 

e 

2N 

2Z 

linea 

Accoppiatore ibrido 

direzionale 

4

© 

in 

out 

Trasferimento del segnale in linea 

Z 

- 

Z 

Z/2 

Z/2 

Z/2 

+ 

a 

-b 

+ 

c 

- 

d 

e 

2N 

2Z 

linea 


direzionale 

5

© 

out 

Trasferimento del segnale dalla linea 

Z 

Z 

Z/2 

a 

b 

c 

d 

e 

2N 

2Z 

linea 

in 


direzionale 

6

0 dB 

© 



30-40dB 


-3.5 dB 

Hybrid 

transformer 

-L dB 

3-4 dB 

3-4 dB 

-3.5 dB 

Se si desidera un rapporto segnale 

disturbo di almeno 20 dB la lunghezza 

della linea non può superare 8 Km 

(assumendo un’attenuazione di 1 

dB/Km) 

35 dB 

0 dB 

7


© 

Eco 

Eco generata dal non perfetto 

adattamento della linea 

Si produce una replica ritardata e distorta del segnale trasmesso 

all’altro capo della linea 

8

© 

+ + 

- Stima dell’eco 

Cancellatore 

d’eco 


Cancellatore d’eco 

Eco generata dal non perfetto 

adattamento della linea 

Si produce una replica ritardata e distorta del segnale trasmesso 

all’altro capo della linea 

9

© 

La rete di Segnalazione 

Segnalazione 

d’utente 

Segnalazione 

intercentrale 

RETE A COMMUTAZIONE DI PACCHETTO 

CON MODALITA’ DATAGRAMMA 

PIANO DATI 

PIANO DI CONTROLLO 

10

Signaling Systems No. 7 (SS7) 

• Out-of-band Signaling System for the exchange of 

call control information between network switching offices 

in support of voice and non voice services 

• Packet switching is the method used for transferring messages 

throughout the network 

• Functions 

• setup and tear down of network trunks 

• database access (initial purpose) 

• Main features 

• Higher utilization of network trunks 

• Additional services (800 numbers, card validation, caller 

line identification, ISDN supplementary services) 

© 

• basis for the all-digital intelligent network 

11

© 

Network Signaling Systems 

• Almost totally hidden from the user 

• Essential components that provides a mechanism for 

network switches to exchange routing, link status and 

connection control information 

• Signaling schemes have evolved significantly over time 

• DC Signaling 

• In-Band Signaling 

• Out-of-Band Signaling 

• Common Channel Signaling 

• Signaling Systems No. 7 (SS7) 

12

• Prior to the 70s 

© 

In-Band Signaling 

• Network signals shared the same physical channel as the call 

that was being set-up 

• Carried within the user’s 300-to-3400 Hz voiceband 

• Network signals and user data did not interfere with each 

other since they usually occurred at different times 

• Three major functions 

• supervisory: monitor circuit status (on- and off-hook signals) 

• addressing: provide routing information 

• call information: provide call status and progress information 

13

• SF (Single-Frequency) signaling: 2600 Hz tone ↔ idle trunk 

• MF (Multi Frequency) signaling: within and between networks or 

by PBXs accessing the public network 

• DTMF (Dual Tone MF) signaling: from user equipment (telephone 

or modem) to the network 

• Trunks are allocated sequentially (bandwidth wasting) 

• Narrow range of functionality 

• limited codesets (typically 16 tones) 

• relatively long call set-up times 

• Susceptible to fraud (underground hacker literature) 

© 

In-Band Analog Signaling 

14

697 

770 

852 

941 

© 

DTMF (Dual Tone MF) signaling 

1 2 

4 

7 8 

* 

5 6 

0 

3 

9 

# 

1209 1336 1477 

fi-i fi = 

f i 

f i+i 

15

• Signaling messages are carried outside the user’s voiceband 

• frequency channel outside of the voiceband (3700 Hz signaling) 

• separate pair of wires (E&M signaling: “Ear” and “Mouth”) 

• No false signaling 

• Drawbacks: 

• one signaling circuit per voice channel 

• old analog technology 

© 

Out-of-Band Signaling 

⇒ Common Channel Signaling 

16

Common Channel Signaling 

• A CCS network is one type of out-of-band signaling network 

designed to exchange signaling information between processorequipped 

switching offices using signaling channels which are 

separate from the user’s voice channel 

• Main features 

• fast call set-up (3-5 s) 

• efficient routing 

• long-distance efficiency 

• low costs 

• signaling for several trunks multiplexed on a single signaling 

channel 

• additional user services (800 services, credit card verification, 

calling party identification) 

© 

17

© 

CCS (SS7) Network Components 

• The network comprises nodes called Signaling Points (SPs), which 

are interconnected by specialized transmission facilities called 

signaling links; 

• Three types of SPs: 

• SSP: Service Switching Point (or AP: Action Point) 

• STP: Signal Transfer Point 

• SCP: Service Control Point (NCP: Network Control Point 

in the Bell System) 

18

© 

SCP 

La rete SS#7 

SSP SSP SSP SSP SSP=Service Switching Point 

STP STP 

STP STP 

SCP 

SSP SSP SSP SSP 

STP=Signal Transfer Point 

SCP=Service Control Point 

19

SCP 

OSS 

© 

Operations Support Systems 


STP STP 

STP STP 

(OSS) 

SCP 


• Remote maintenance centers for the 

monitoring and management of SS7 

& voice networks 

• Use of a common command set for 

all the network equipment 

• Using TCAP and SCCP, the OSS is 

capable of sending SS7 messages to 

any entity within its own network 

• Service Management System (SMS) 

• management of SCP 

• monitoring 

20

SS7 Protocol Stack 

• ITU-T Recommendation Q.700 provides an overview of CCS and SS7 

• detailed descriptions of SS7 protocols and procedures are contained 

in the remaining Q.700 series recommendations 

• highly reliable packet-switching protocol, providing all of the services 

and functions required by the telephone service providers 

• comparison with the OSI model 

• layered structure 

• four “levels” vs. seven “layers” 

• SS7 functions have been refined over the years and tailored for its 

specific requirements 

• some of OSI functions have no purpose in the SS7 network 

and are therefore undefined 

• SS7 was developed before the OSI model 

© 

21

© 

Application 

Presentation 

Session 

Transport 

Network 

Data Link 

Physical 

SS7 vs. OSI 

TCAP 

ASP 

SCCP 

Application Entity 

TUP ISUP BISUP 

MTP Level 3 

MTP Level 2 

MTP Level 1 

OSI Layer CCS7 Level 

22

© 

SSP functions 

• Main SSP function: to use the information provided by the calling party (such as 

dialed digits) and determine how to connect the call 

•A routing table identifies which trunk circuit to use to connect the call and which 

exchange this trunk terminates at 

• An SS7 set up message is sent to this adjacent exchange requesting a circuit 

connection on the specific trunk 

• The adjacent exchange grants permission to connect this trunk by sending back an 

ACK to the originating exchange 

23

Signal Transfer Points (STPs) 

•ASignal Transfer Point is a packet switch: it concentrates signaling information 

from SSPs and switches messages in the CCS network 

• The STP is also typically an adjunct to a voice switch 

• STPs are paired throughout the CCS network (mate STPs) and can handle several 

SSPs in the immediate area 

•Traffic and maintenance measurements (network monitoring) 

• Usage measurements: billing for long distance service providers and 

independent telephone companies 

• Three levels of STPs: 

• National STP (national plane of the SS7 Network): capable of transferring 

messages using the same national standard of protocols 

• International STP (international plane of the SS7 Network): used in the 

international network with the same function as the national STP 

• Gateway STP: serves as an interface into another network 

• international plane (protocol conversion) 

• interactions between long distance service provider and local telephone 

© company 

• SS7 protocols are very relevant within the cellular network 

24

© 

Service Control Point (SCP) 

•A Service Control Point is an interface to telephone company databases, used to 

store information about 

• subscribers’ services 

• routing of special service numbers (such as 800 and 900 numbers) 

• calling card validation and fraud protection 

• (Advanced) Intelligent Network services 

• The SCP is usually a computer used as a front end to the database system. New 

SCP application are being implemented in STPs, providing an integrated solution 

• SCP as the interface to the mainframe or minicomputer system used for the 

actual database 

• Integrated STP/SCP: the database is resident in the SCP 

25

© 

SCP: basic models of databases 

• The type of database depends on the network 

• Each database contains information for a specific application 

• Most commonly used databases: 

• Call Management Service Database (CMSDB) 

• Local Number Portability (LNP) 

• Line Information Database (LIDB) 

• Business Services Databases (BSDB) 

• Home Location Register (HLR) 

• Visitor Location Register (VLR) 

• Call Management Service Database (CMSDB) 

• call processing 

• routing instructions for special service numbers (such as 800, 900, 976) 

• billing information (billing address and third-party billing) 

• network management (rerouting around a congested node) 

• call sampling (for traffic studies): determine if additional facilities 

are needed to handle voice traffic 

26

© 

Application 

Presentation 

Session 

Transport 

Network 

Data Link 

Physical 

SS#7 

Application Entity 

TCAP 

SCCP 

TUP ISUP BISUP 

MTP Level 3 

MTP Level 2 

MTP Level 1 

OSI Layer CCS7 Level 

TUP =Telephone User Part 

ISUP = ISDN User Part 

BISUP = B-ISDN User Part 

TCAP =Transaction Capability Part 

SCCP= Signaling Connection Control Part 

ASP = Application Service Part 

27

© 

D-Channel Signaling (ISDN) 

Segnalazione 

d’utente 

Network Layer 

Data Link Layer 

Segnalazione 

intercentrale 

28

© 

Messaggi Q.931 

• SETUP: Dà inizio alla chiamata; contiene il numero chiamato ed altre informazioni 

di set-up 

• SETACK: Richiesta di maggiori informazioni per dare inizio alla chiamata 

• CALPRC: Notifica il ricevimento di un messaggio di SETUP e che la fase di set-up 

è iniziata 

• PROG: Contiene informazioni sullo stato della chiamata 

• ALERT: L’utente chiamato è stato informato dell’arrivo di una chiamata 

• CONN: L’utente chiamato ha risposto 

• CONACK: Notifica la ricezione di un messaggio CONN 

• DISC: Richiede l’abbattimento della connessione 

• RLSE: Notifica la ricezione di un messaggio DISC e che l’interlocutore ha abbattuto 

la connessione 

• RLCOM: Notifica la ricezione di un messaggio RLSE e l’abbattimento della 

connessione da parte del mittente 

• INFO: Se inviato dal chiamante contiene una o più cifre del numero chiamato; 

se inviato dalla rete richiede l’emissione di un segnale acustico (es. occupato) 

Messaggio Globale Messaggio locale 

29

© 

Tipica sequenza di segnalazione 

User P 

Network 

User Q 

TE - P 

Local 

Exchange 

Local 

Exchange 

TE - Q 

SETUP 

CALPRC 

ALERT 

Ringing tone 

CONN 

CONACK 

DISC 

RLSE 

RLCOM 

Speech or Data 

SETUP 

ALERT 

CONN 

CONACK 

DISC 

RLSE 

RLCOM 

30

© 

Q.931 & ISUP messages 

TE LE SS7 LE TE 

SETUP 

CALL 

PROC. 

ALERTING 

CONNECT 

DISCON. 

RELEASE 

RELEASE 

COMPLETE 

Q.931 

IAM 

CIC 1 

ACM 

ANM 

REL 

RLC 

SS7 

IAM 

CALL 

SETUP 

CIC: Circuit Identification Code 

identifies the logical connection 

provided by the ISUP 

PROC. 

ACM 

ALERTING 

ANM 

CONNECT 

ACM: Address Complete Message 

ANM: Answer Message 

IAM: Initial Address Message 

REL: Release 

REL 

RLC: Release Complete 

DISCONNECT 

CIC 2 

RLC 

Q.931 

RELEASE 

RELEASE 

COMPLETE 

31

© 

Concentration 

Provide a certain blocking prob. 

Traffic and overload control 

in telephone networks 

Maximize trunks utilization 

32

© 

N(t) 

Trunk Number 

1 

2 

3 

4 

5 

6 

7 

Number of trunks in use 

All trunks busy 

33

© 

Trunk Number 

1 

2 

3 

4 

5 

6 

7 

Processo di arrivo delle chiamate 

Processo di Poisson di intensità λ 

34

© 

Processo di Poisson 

I tempi di interarrivo sono indipendenti e distribuiti 

esponenzialmente. 

In un intervallo di tempo sufficientemente piccolo 

∆t possono avvenire solo due cose: 

• si presenta una chiamata con probabilità λ ∆t 

• non si presenta nessuna chiamata con prob. 1- λ ∆t 

35

© 

Trunk Number 

1 

2 

3 

4 

5 

6 

7 

Processo di durata delle chiamate 

E[X]=Valor medio della durata di una chiamata 

36

© 

Carico offerto 

(Intensità media di traffico) 

A=λ E[X] 

Chiam./sec sec. 

Formula B di Erlang 

Prob. di rifiuto della chiamata 

P 

B 

= N 

∑ 

k = 0 

Erlang 

N 

A 

N! 

k 

A 

k! 

N è il numero di trunk disponibili, A è il carico offerto 

37

© 

Utilizzazione 

ρ=num . medio di linee occupate/N 

Num. medio linee occupate 

ρ=λ(1-P B)E(X)/N 

Freq. media di chiamate accettate 

38

© 

Num. di trunk necessari a garantire una prob. 

di rifiuto media pari a 1% 

Load Trunks Utilization 

1 5 0.20 

2 7 0.29 

3 8 0.38 

4 10 0.40 

5 11 0.45 

6 13 0.46 

7 14 0.50 

8 15 0.53 

9 17 0.53 

10 18 0.56 

30 42 0.71 

50 64 0.78 

60 75 0.80 

90 106 0.85 

100 117 0.85 

39

© 

A 

B 

C 

Routing Control 

D 

E 

F 

Se ciascuna centrale offrisse 10 

erlang di traffico alle tre centrali 

remote sarebbero necessari 18 

canali al fine di garantire una Prob. 

di rifiuto pari all’1%. In totale 

sarebbero quindi necessari 

9x18=162 Trunks 

40

© 

A 

B 

C 

Tandem 

switch 1 

Routing Control 

Tandem 

switch 2 

Le centrali di sinistra offrono complessivamente 

90 erlang di carico a quelle di destra: 

per garantire la stessa prob.di rifiuto sono necessari 

solo 106 trunk aumentando inoltre l’utilizzazione delle linee 

D 

E 

F 

41

© 

Sensibilità alla variazione del carico 

Se il carico aumentasse del 10% nel primo caso si 

passerebbe a 11 Erlang che a parità di numero di linee 

(18) produrrebbe P B =2.45% 

Nel secondo caso invece si passerebbe a 99 Erlang ed in 

tale condizione a parità del numero di canali (106) si 

passerebbe ad un incremento eccessivo della probabilità 

di blocco P B =9.5% 

42

© 

Alternative Routing 

Alternative Path 

P B’ =10% ⇒ P B =1% 

Direct Path 

High usage ⇒High P B 

P B =10% 

43

© 

A 

Hierarchical Network 

Tandem Switch 1 Tandem Switch 2 

C 

B 

Alternative routes 

for B-E, C-F 

High-usage route B-E 

High-usage route C-F 

A-D requires 1% blocking probability (priority) 

E 

F 

D 

44

© 

Dynamic nonhierarchical routing 

(DNHR) 

Tandem Switch 2 

Tandem Switch 1 Tandem Switch 3 

A B 

The order in which tandem switches are attempted 

as alternative routes is determined dynamically 

according to the state of the network 

45

© 

Carried load 

Traffic Overload 

Network capacity 

Offered load 

46

RETI DI TELECOMUNICAZIONI - TLCNETGROUP Home Page

Create successful ePaper yourself

Delete template?

Save as template?