Figure 7.1 General architecture of a national circuit-switched...

Figure 7.1 General architecture of a national circuit-switchednetwork.

© Pearson Education Limited 2001

Terrestrial, submarine, and satellite lines

International gatewayexchange/office

National tandemexchange/office

International network

Regional tandemexchange/office

Local tandemexchange/office

Local exchange/end office

Interexchangetrunk/carrier

network

Local access andswitching networks

Subscribers within a localized area

twisted pair subscriber lines(one subscriber per line)

high bandwidth trunk lines(multiple calls per line)

Figure 7.2 Analog subscriber line principles: (a) telephonecomponents; (b) 4-wire-to-2-wire hybrid; (c) dual-tonemultifrequency keypad.


Ringer Dialer

from microphone

to speaker

Handset

Cradle switchunit

Telephone base unit

Customer premises

Network termination(RJ11 plug/socket)

Two-way analog signals

A-LTU


Subscriber line

A-LTU = analog (subscriber) line termination unit

4-wire-to-2-wirehybridA + B

B

A

A = analog speech signal from this handsetB = analog speech signal from other handset

to speaker

from microphone

to/fromLE/EO

1209 Hz

0 #

1336 Hz 1477 Hz 1633 Hz

941 Hz

7 8 9852 Hz

4 5 6770 Hz

1 2 3697 Hz

(c)

(b)

(a)

4-wire-to-

2-wirehybrid

Figure 7.3 Access network structure with remote concentrator/switching units.


Switchingsystem

Controlprocessor

DLTU

DLTU

DLTU

DLTU

DLTU

Local tandemexchange/office


Controlprocessor

DLTU

ALTU

ALTU

RCU/RSURCU/RSU

subscribers

N subscribers

N analog subscriber lines

M × 64kbps duplex digitalchannels + associated signaling

channelsRCU

(M < N and concentration = M : N )

RCU = remote concentrator unit RSU = remote switching unitD/A LTU = digital/analog line termination unit

Figure 7.4 Modem operating alternatives: (a) 2-wire switchedconnections; (b) 4-wire leased circuits.


Demod.

Mod.

Modem

DTE

V.24/EIA-232D

Mod.

Demod.

Modem

DTE

V.24/EIA-232D

PSTN

LT

LT

LT

LT

4-wire analogleased lines

(b)

Demod.

Mod.

Modem

DTE

V.24/EIA-232Dinterface

Mod.

Demod.

Modem

DTE

V.24/EIA-232Dinterface

PSTNHT HT

2-wire analogsubscriber lines

(a)

Mod. = modulator sectionDemod. = demodulator sectionDTE = data/digital terminal equipment

HT = hybrid transformerLT = line transformer

Figure 7.5 Modem principles: (a) modulator/demodulatorschematic; (b) waveforms of basic modulation methods.


PSTN

Carrier signal

Bitstream outDemodulator

Carrier signal

ModulatorBitstream in

0

–V

0

+V

FSK

Bitstream in

t

t

t

Time, t

10 01101

(b)

Phase-coherent PSK

180° 180° 0°

90° 270° 270°

180° 0° 180°

90° 90° 270°

0

0Differential PSK

0ASK t

(a)

Figure 7.6 Multilevel modulation: (a) 4-PSK using a singlecarrier; (b) 4-QAM and (c) 16-QAM using two carriers, one at 90°(Q) out of phase with the other (I).


(a)

0°+270°+180°+90°0°

Time

11100100

0010 1

0111 –1

1–1

Q

I

1111 1

1111 –1

1–1

Q

I

1010 3

1010 –3

01

01

3

00

00

01

01

–3

00

00

ms = 00

ms = 01

ms = 10

ms = 11

I = in-phase carrierQ = quadrature carrier

ms = most significant (bits)

(b)

(c)

Figure 7.7 A selection of ITU-T V-series modem standards.


ITU-TV-series modem standards

for use with analog subscriber lines

V.24/EIA-232D(interface definition)

2-wireswitched circuits

V.21 300/300 bps duplexV.22 1200/1200 bps duplex

V.22bis 2400/2400 bps duplexV.23A 75/1200 bps duplex

V.29 4800 or 9600 half-duplexV.32 4800 or 9600 bps duplex

V.32bis 14400 bps duplexV.34 19200 or 24000 or 28800 or 33600 duplex

V.90 up to 56kbps duplex

2/4-wireleased circuits

V.23 600 or 1200 bpsV.26 1200 or 2400 bpsV.27 2400 or 4800 bpsV.29 4800 or 9600 bps

V.33 14400 bps(half-duplex 2-wire

duplex 4-wire)

Figure 7.8 V.24/EIA-232D interface standards: (a) interfacefunction; (b) connector, pin, and signal definitions.


Cable shield1

2

Received Data (RxD)3

Request to Send (RTS)4

Clear to Send (CTS)5

DCE Ready (DSR)6

Signal Ground/Common Return7

Received Line Signal (Carrier) Detector (RLSD/CD)8

Reserved for testing9

Reserved for testing10

Unassigned11

Secondary Received Line Signal (Carrier) Detector (S-CD)12

Secondary Clear to Send (S-CTS)13

Secondary Transmitted Data (S-TxD)14

Transmitter Signal Element Timing – DCE Source (TxClk)15

SxRD

Name

DTE(terminal or computer)

DCE(modem)

23222120191817

4 5 6 7 8 9 10 11DB25 connector

24

12

25

13

16

3

15

2

14

1

16

17

18

19

20

21

22

23

24

25

Secondary Received Data (S-RxD)

Receiver Signal Element Timing (RxClk)

Local Loopback (LL)

Secondary Request to Send (S-RTS)

DTE Ready (DTR)

Remote Loopback (RL)/Signal Quality Detector (SQD)

Ring Indicator (RI)

Data Signal Rate Detector (DTE or DCE)

Transmitter Signal Element Timing – DTE Source (TxClk)

Test Mode (TM)

RxClk

LL

S-RTS

DTR

RL/SQD

RI

DSRD

TxClk

TM

TxClk

S-TxD

–

–

–

CD

–

DSR

CTS

RTS

RxD

TxD

–

Transmitted Data (TxD)

S-CTS

S-CD

SBB

EIA label

D

LL

SCA

CD

RL/CG

CE

CH/CI

CI

DA

DB

SBA

–

–

–

CF

AB

CC

CB

CA

BB

BA

AA

SCB

SCF

119

ITU-T No.

115

141

120

108

140/110

125

111/112

113

142

114

118

–

–

–

109

102

107

106

105

104

103

–

121

122

(Female)(Male)

(b)

CalledDTE

V.24/EIA-232DV.24/EIA-232D

PSTNModem

(a)

CallingDTE

Modem

DCE DCE

Figure 7.9 V.24/EIA-232D connection setup, two-way alternatedata transfer and connection clearing sequences.


(b)

Called DTE(server computer)

EIA-232D/V.24EIA-232D/V.24

PSTN

(a)

DCE (modem)with

autoanswer

DCE (modem)with

autodial

DTR on

DSR on

CD on

RxD on

CD off

RTS on

CTS on

TxD

RTS off

CTS off

CD off

DTR off

DSR off

DTR on

DSR on

RI on

RTS on

CTS on

TxD

RTS off

CTS off

CD on

RxD

RTS off

CTS off

CD off

DTR off

DSR off

DTR on

DSR on

Shortdelay

Shortdelay

Shortdelay

Number ofcalled modemsent to modem

Connection setup

Carrier on

Carrier on

Data tones

Carrier off

Data tones

Carrier off

Carrier off

Time

Connectioncleared

Connectionsetup

Datatransfer

Called DTE(PC)

Figure 7.10 V.24/EIA-232D interface: (a) local and remoteloopback tests; (b) V.28/RS.232A signal levels.


Figure 7.11 ISDN network termination alternatives: (a) 4-portNTU; (b) S-bus NTU.


S-bus

DigitalTE

DLTU

DLTU

ALTU

ALTU

DigitalTE

NT

NTU

S-bus

User (U)-interface

2B + D

2B + D

to/from

LE/EO

Subscriber/terminal (S/T)-interface

AnalogTE

AnalogTE

(a)

A/D LTU = analog/digital line termination unit

DigitalTE

DigitalTE

NT

U-interface

2B + D

2B + D

to/from

LE/EO

DigitalTE

TA

(b)

NT = network terminationNTU = network termination unit

NTU

S/T-interface

TE = terminal equipmentTA = terminal adaptor

AnalogTE

Figure 7.12 ISDN subscriber/terminal (S/T)-interface: (a) interface location; (b) socket, pin, and signal definitions; (c) signal levels.


Figure 7.13 ISDN basic rate access S-bus line code principles:(a) alternate space inversion (ASI) line code; (b) example ofcontention resolution.


Start of framesequence

1 0 1 1 0 0 1Example frame

contents0

Time, t

1 1 0 0 1 1 0

TEAt

ExampleDC balance bit

1

1

0

1 1 0 1

TEBt

D-bitsequence

(b)

(a)

TEB stops transmitting (TEA continues)when reflected E-bit is not the0 level of a binary 1 which ittransmitted

Figure 7.14 ISDN subscriber line principles: (a) NTU schematic;(b) 2B1Q line signal example; (c) frame and superframe format.


Receivedecoder

Transmitencoder

Adaptiveecho

canceler

Hybridto/from

LE/EO

NTUTransmit signal, A

Received signal, B+ attenuated and delayed

version of A

Receivedsignal, B

only

S-businterface

andcontrolcircuits

S-bus

(a)

user data field = 12 × (8B1 + 8B2 + 2D) M-bitssync. word

216 618

1 frame = 240 bits

1 2 3 4 5 6 7 8

1 superframe = 8 frames

10 10 00 00 11 01 00 01 10 10 11 00 01 11Binary bitstream

–3

–1

+1

+3

Time, t

(b)

(c)

2B1Qsignal

Figure 7.15 ISDN 1.544Mbps primary rate interface principles:(a) line code; (b) frame and multiframe structure.


(a)

One frame = 193 bits

24

One multiframe = 24 frames

1 0 1 0 0 0 1 0 0 0 0 0 0 0Bitstream

–V

+V

(or inverse)

(b)

AMI + B8ZS

0 1 1

–V

+V

(or inverse)AMI

–V

+V

tNRZ

0 0 0 V

B

0

V

B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

125µs

Time slot 1 Time slot 2 Time slot 24

Frame bit(F-bit)

Note: Frame alignment signal (FAS) = F-bits from frames 4, 8, 12, 16, 20, 24 and these are set to 001011 respectively

0

0

F

Figure 7.16 ISDN 2.048 Mbps primary rate interface: (a) linecode; (b) frame and multiframe structure.


(a)

One frame = 256 bits

15

One multiframe = 16 frames

1 0 1 0 0 0 0 1 0 0 0 0 0 0Bitstream

–V

+V

(or inverse)

(b)

AMI + HDB3

0 0 1

–V

+V

(or inverse)AMI

–V

+V

tNRZ

0 0 0

B

V

V

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0

1 2 8 1 2 8 1 2 8 1 2 8

125µs

Time slot 0

0 00 V0 0

Time slot 1 Time slot 2 Time slot 31

x 0 0 1 1 0 1 1

x 1 x x x x x x

Frame alignment signal

Contents of time slot 0in alternate frames

0

0

0

Figure 7.17 Plesiochronous digital hierarchies: (a) 1.544Mbpsderived multiplex hierarchy (b) 2.048Mbps derived multiplexhierarchy.


21

Primarymultiplexgroup24

64kbpschannels

×4

DS1/T11.544Mbps(24 channels)

21

Primarymultiplexgroup32

64kbpschannels

×4

E12.048Mbps(32 channels)

×7×3

×6

×12

DS4/T4(E)139.264Mbps(2016 channels)





×4

×4

×4

E28.448Mbps

(120 channels)




(a)

(b)

Figure 7.18 Private network provision with a PDH transmissionnetwork: (a) existing network; (b) modified network; (c) ADMprinciples.


Figure 7.19 SDH/SONET multiplexing hierarchy andterminology.


Figure 7.20 SDH/SONET detail: (a) managed entities; (b) frameformat; (c) example VC mapping.


Figure 7.20 Continued.


VC-12PTR

VC-12PTR

VC-12PTR 1

2

3

4

5

6

7

8

9

1

2

3

9

8

TU-12 Frame N

Frame N + 1

Frame N – 1

TUG-2 = 12 columns

(c)

Figure 7.21 Service provision with SDH equipment usingnetwork management.


Figure 7.22 Time switch principles: (a) switching schematic; (b) timing.


t0 t4 t3 t2 t1 t0 t4

D' A' B' — C D A

t1 t0 t4 t3 t2 t1 t0

B" A" D' C' — B' A'

t4

D

t3

C

Input/write timing

Time slots

Inputline D C — B A FS

Write address

Read address

0 1 — 3 4 CS

4 3 2 1 0 MC

Time slots

Outputline

t = 0

t

t = ∆t

t

Time switch

Read/output timing

Time, t

0t0t1t2t3t4t0t1

Input byte

DA'B'–C'D'A"B"

FS contents

DC–BADC–BA'DC–B'A'DC–B'A'DC'–B'A'D'C'–B'A'D'C'–B'A"D'C'–B"A"

Output byte

ADC–B'A'D'C'

Delay

2ST4ST1ST3ST

(b)

(a)

MC = modulo-N counter∆t = switching delay = Twrite + Tread

FS = frame store CS = connection storeST = slot time

Figure 7.23 Space switch principle of operation: (a) basic spaceswitch; (b) PCM space switch.


t3 t2 t1 t0

1 2 0 2

L K N E

P C B A

D O J M

H G F I

t3 t2 t1 t0

Connection store withexample contents

Output lines

D C B A

H G F E

L K J I

P O N M

t3 t2 t1 t0

0

1

2

3

0 0 2 3

3 3 3 0

2 1 1 1

Inputlines

23 01(b)

2

Connection store withexample contents

Output lines

0

1

2

3

3

0

1

Inputlines

23 01(a)

Figure 7.24 Digital switching units: (a) time-space switch; (b) time-space-time switch.


Y

13

Y

TS0

1

2

24

FS

513

CS

5

TS1

X

8

X

TS2

X

1

2

5

23

FS

58

CS

5

Output

X

5

C A B DD C B A

2 0 1 3

H G F E

t3 t2 t1 t0

0

1

Inputlines

(a)

t3 t2 t1 t0

CS

TS0

F H E G

1 1 1 1

t3 t2 t1 t0

CS

SS

0 0 0 0

F H E G

1 3 0 2 CS

TS1

C A B D

0

1

Outputlines

t3 t2 t1 t0

Digital switching unit

X

X

0

(b)

TS0

Digital switching unit

1

2

24

FS

15

CS

1

1

TS1

Y

5

Y

2

TS2

Y

1

2

20

24

FS

205

CS

20

Input

SS

2

t5

–

0

CS

Y5 0

1

2

TS = time switch SS = space switch FS = frame store CS = connection store

Figure 7.25 Signaling system components: (a) access and coretrunk network components; (b) access network components.


Trunknetwork

SubscriberTE

Local exchange(end office)

Local exchange(end office)

SubscriberTE

Trunk networksignaling

Access networksignaling

Access networksignaling

(a)

(b)

Information interchange

Time

Callsetup

Callclearing

Figure 7.26 Analog access signaling: (a) a selection of thesignals used; (b) sequence of signals exchanged to set up andclear a call.


CallingTE

LE/EO

CalledTE

(a)

(b)

Time

Off hook(current flow)

Dialed digits(pairs of tones)

LE/EO

Dial tone

Ringing tone

Remove ringing tone

Ringing signal

Answer (current flow)

On hook(current ceases) On hook

(current ceases)

Callsetup

Callclearing

Dial tone:Time

Ringing tone:

Busy tone:

Congestion tone:

1s

0.5s

0.25s

0.25s

1s5s

~425 Hztones

Information interchange

Figure 7.27 LAPM: (a) operational scope; (b) user serviceprimitives and corresponding frame types.


V.24V.24(a)

DTE

Modem (DCE)

LAPM

PSTNUIP ECP

Modem (DCE)

ECP DTEUIP

ECPUIP

Modem (DCE)

ECP UIP

Modem (DCE)

L_SETPARM.reqL_SETPARM.ind

L_SETPARM.resp

L_ESTABLISH.ind

L_ESTABLISH.resp

L_DATA.ind

L_SIGNAL.ind

L_SIGNAL.resp

L_RELEASE.ind

L_RELEASE.resp

L_SETPARM.conf

L_ESTABLISH.req

L_ESTABLISH.conf

L_DATA.req

L_SIGNAL.req

L_SIGNAL.conf

L_RELEASE.req

L_RELEASE.conf

XID

XID

SABME

RR

UA

I(DATA)

UI(BRK)

DISC

UI(BRKACK)

UA

ECP = error control partDCE = data circuit-terminating equipment

UIP = user interface partDTE = data terminal equipment

DTE DTE

(b)

Figure 7.28 ISDN digital access signaling protocol set.


Primary rateinterface

Basic rateinterface

Q.921 (LAPD)

Q.931

Layer 1

Layer 2

Layer 3

DSS1 signaling protocol stack

DSS1 = digital subscriber signaling number 1

Figure 7.29 LAPD user service primitives: (a) connectionless; (b) connection-oriented.


(a)

TELayer 3

NT2, ISDN, NT2Layer 2

DL_UNITDATA.indication

TELayer 3

DL_ESTABLISH.request

DL_UNITDATA.request

DL_ESTABLISH.confirm

DL_DATA.request

DL_DATA.indication

DL_RELEASE.request

DL_UNITDATA.request

DL_ESTABLISH.indication

DL_ESTABLISH.response

DL_DATA.indication

DL_DATA.request

DL_RELEASE.indication

(b)

DL_UNITDATA.indication

TE =NT =DL =

terminal equipmentnetwork terminationdata link

Figure 7.30 LAPD: (a) frame format; (b) address field usage.


Flag

Address (1)

Address (2)

Control (1)

Control (2)

Information

FCS (1)

FCS (2)

Flag

Octet 1

2

3

4

5

N – 2

N – 1

N

Octet 2

3

(a)

(b)

Data link connection identifier (DLCI)

1 octet: modulo-8 sequence numbers2 octets: modulo-128 sequence numbers

Generator polynomial = x16 + x12 + x5 + 1Generator polynomial = CCITT-16

EA bit

Command/response bit

Service access pointidentifier, which identifiesthe layer 3 serviceaccess point to whichthe frame relates;(0 = call control SAP;16 = packet data SAP;all others are reservedfor other uses)

Extended address bit(EA = 1 in octet 2 forLAPD operation)

Terminal endpoint identifier, which identifies aspecific connection endpoint (terminal) within aSAPI and is normally fixed at the time ofmanufacture/installation of the terminal equipment

SAPI

TEI

C/R 0

1

Figure 7.31 LAPD control field bit definitions: (a) information;(b) supervisory; (c) unnumbered.


Figure 7.32 ISDN layer 3 signaling: (a) example messagesequence to set up a conventional telephone call; (b) messageformat.


CallingTE

LE/EO

CalledTE

(a)

(b)

User enters addressof called TE

LE/EO

(Ringing)

Contains operatingparameters of calling TE

Operating parametersof called TE compatible

Callsetup

Callclearing

Trunk network

SETUP

CALL PROCeeding

ALERTing

CONNECT

CON ACK (optional)

SETUP

ALERTing

User answersCONNECT

CON ACK

User initiatesclearing

DISConnect

RELEASE

DISConnect

RELEASE

REL COMPREL COMP

(Ringingremoved)

= channel/connection established = channel/connection released

Message parameters (e.g. address/number ofcalled TE) and proposed operational parameters

Message type (e.g. SETUP)0

Call reference field

0

Protocol discriminator

0 0 0 Length of callreference field

8 7 6 5 4 3 2 1

N

3

Octet 1

2

Bit

Information interchange over B-channel

Figure 7.33 Signaling in PCM circuits: (a) 24-channel system;(b) 32-channel system.


TS1

24

One multiframe = 24 frames(a)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1

1 2 3 4 5 6 7 S1 1 2 3 4 5 6 7 SZ 1 2 3 4 5 6 7 S24

125µs

Info. bits Info. bits Info. bits

Framing bit

F

Channel-associated signaling bits

TS2 TS24

15

One multiframe = 16 frames(b)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0

TS17

125µs

Info. time slots Info. time slots

Frame alignmentChannel-associated

signaling bits

TS16TS15 TS30 TS31TS3TS2TS1TS0

Signaling time slot/channel

Frame format

1 2 3 4 5 6 7 8

Framenumber

multiframe alignment0

Ch 11 Ch 17

Ch 22 Ch 18

Ch 33 Ch 19

Ch 1515 Ch 31

0

Figure 7.34 Analog access signaling through an RCU and PCMcircuit.


Callclearing

Off hook

Dial tone

LE/EORCU RCU Called TECalling TE

Time slot seizure

Seizure ack.

Time slot seizure

Seizure ack.

Ringing signal

Answer

Dial message

Dialed digits

Address info.

Address info.

Answer messageRemove signaling

messageRemove ringing tone

Ring messageRinging tone

Time slot release

Release ack.On hook

Time slot release

Release ack.

On hook

PCM circuit PCM circuit

Calling TE access network Calling TE access network

Information interchange over assigned time slot

Callsetup

Figure 7.35 SS7 components and terminology.


LE/EO

SP

Tandem E/O

STP

LE/EO

SP

Switchingunit

Switchingunit

ARP OMAPUPLSI

MTP

OMAPUP

MTP

ARPOMAP

Switchingunit

UP LSI

MTP

MTP

OMAP

Operations,maintenance +administrationapplication

Database

MTP

ARP

Addressresolution

applicationDatabase

SPSP

Signalingnetwork

PCM circuitswith CAS

PCM circuitswith CAS

ARP = address resolution partUP = user partOMAP = operations, maintenance, and administration partLSI = local signaling interface

MTP = message transfer partSP = signaling pointSTP = signaling transfer point

Figure 7.36 SS7 protocol architecture: (a) protocol components;(b) format of MTP message units.


Signaling connection control part

Transaction capabilities/support

ARPOMAP

ISUPDUPTUP

Signaling network

Signaling link

Signaling data link

Link layer

Physical layer

Network layer

Applicationparts

Message transferpart (MTP)

(a)

TUP = telephone user partDUP = data user partISUP = ISDN user part

ARP = address resolution partOMAP = operations, maintenance, and administration part

(b)

Flag FSN

8 8

BSN

8

LI

8

SIO

8

SIF

32

Application data

8 × N

FCS

16

Flag

8

Destination pointcode (DPC)

14

Originating pointcode (OPC)

14

SLS

4

Serviceindicator

4

Subservicefield

4 Bits

Bits

I-frameformat

FSN = forward sequence numberBSN = backward sequence numberLI = length indicator

SIO = service information octetSIF = signaling information fieldFCS = frame check sequenceSLS = signaling link selection

Signaling network ISUP, ARP, etc.

Signaling link user data Signalinglink trailer

Signaling link header

Figure 7.37 Network signaling message sequence to set up anISDN channel/connection using SS7.


Callsetup

Callclearing

SETUP

CALL PROC

IAM

ACM

= channel/connection established = channel/connection released

SETUP

ALERTing

IAM

ACM

CPGCPG

CONNECT

CON ACK

ALERTing

ANMANM

CONNECT

CON ACK

DISConnectREL

RELDISConnect

RELEASERLC

RLCRELEASEREL COMP

REL COMP

CallingTE

CallingLE/EO

TandemEO

CalledLE/EO

CalledTE

Access signaling messagesDSS1

Network signaling messagesSS7

Access signaling messagesDSS1

Information interchange over allocated channel

Figure 7.38 Public circuit-switched network summary: (a)access network; (b) core transmission and switching network.


Summary

Public circuit-switched networks

Access network

ISDN

Digital subscriber lines

Primary rate(23B + D/30B + D)

Basic rate(2B + D)

PSTN

Analog subscriber lines

Modems

Interfacestandards

Modulationmethods

Subscribersignaling

Signaling system

(a)

Core transmission and switching network

Signaling system

SS7CCS

Transmission system

SDH/SONETPDH

(b)

Switching system

Spaceswitching

Timeswitching

Digital switching units

Example 7.1


Deduce the bit rate and baud rate of the subscriber line of an ISDNbasic rate access circuit assuming the 2B1Q line code and the frameformat shown in Figure 7.14(c).

Answer:

Each 240-bit frame comprises 12 × 8 = 96 bits per B-channel. Hence,since this is equivalent to a bit rate of 64kbps, the total bit rate is64 × 240/96 = 160kbps.

Since there are 2 bits per signal element, the signaling rate = 80 kbauds.

Example 7.2


A simple time switch is to be used to switch the time slot positionsassociated with (i) a 24-channel and (ii) a 32-channel PCM line. Derivethe size of the frame store, address counter, and connection store ineach case.

Answer:

(i) 24-channels:Frame store = 24 locations each of 8 bitsAddress counter = Modulo-24 counter (00000–10111 then repeat)Connection store = 24 locations each of 5 bits (00000–10111 used)

(ii) 32-channels:Frame store = 32 locations each of 8 bitsAddress counter = Modulo-32 counter (00000–11111 then repeat)Connection store = 32 locations each of 5 bits (00000–11111 used)

Example 7.3


A 7-input, 7-output line space switch is to be used to switch the con-tents of each set of time slot positions associated with (i) a 24-channeland (ii) a 32-channel PCM system. Derive the size of the connectionstore and the number of bits required for each entry in the store forboth systems.

Answer:

(i) 24-channels:Connection store has 7 × 24 = 168 locationsEach entry must have 3 bits: entries 000–110 to indicate the output line number (0–6) and hence the crosspoint position to be acti-vated. Entry 111 is then used to indicate no crosspoint is to beactivated; that is, the time slot is not in use.

(ii) 32-channels:Connection store has 7 × 32 = 224 locationsEach entry must have 3 bits as for (i).

Example 7.4


Assuming channel associated signaling is being used, derive the bit rateof the signaling channel associated with each of the time slots in (i) a24-channel PCM circuit and (ii) a 32-channel circuit.

Answer:

(i) As we can deduce from Figure 7.33(a), for each time slot, 1 bit inevery sixth frame is used for signaling. Hence, since each frame isof 125µs duration, we have 1 bit every 6 × 125 = 750 µs.Thus the signaling rate per channel = 1/750 × 10-6 = 1.333kbps.

(ii) As we can deduce from Figure 7.33(b), for each time slot, 4 bitsevery 16 frames are used for signaling. Hence we have 4 bits every16 × 125 = 2000 µs.Thus the signaling rate per channel = 4/2 × 10–3 = 3kbps.

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Figure 7.1 General architecture of a national circuit-switched...

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