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10 Zntegrated Services Digital Network ISDN Most public telecommunication operators throughout the world have now at least commenced the modernization of their networks by introducing

PART 2

MODERN TELEPHONE

NETWORKS

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

10

Zntegrated Services Digital

Network (ISDN)

Most public telecommunication operators throughout the world have now at least commenced the modernization of their networks by introducing digital transmission and digital switching into the public switched telephone network (PSTN) Simultaneously many operators are converting their networks to integrated services digital networks (ISDN) which will allow customers access

to a variety of services while reducing the cost of provision of these services both to the admin- istration and to the customer So what is an ISDN? What is its value? This chapter answers these questions, and goes on to explain the technical detail of how ISDN operates, and the user benefits that can be gained

The internationally agreed definition of ISDN is a ‘network evolved from the telephony

integrated digital network (ZDN) that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which the users have access by a limited set of standard multi-purpose customer interfaces’ The above definition of ITU-T makes a number of points First, that ISDN requires a digital network Second, that this digital network is not one between exchanges, but it extends to the customers Third, it provides not only telephony, but a variety of services Fourth, a customer does not require a separate interface for each service provided by ISDN, but can use all services via one access point or, at worst, via ‘a limited set of standard multi-purpose customer interfaces’ Figure 10.1 illustrates these principles

ISDN (integrated services digital network) will remove the need for telecommunica-

tion customers to have separate physical links to telephone and low speed networks Under ISDN, a single physical connection (or rather one of two types) is provided to a customer’s premises and a range of services can be made available from it Furthermore,

if required, the services may be used simultaneously, because access is not restricted to each individual service in turn However, as well as greatly enhancing the established services, and reducing their cost of provision, ISDN will introduce a powerful range of new ones

213

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

214 INTEGRATED SERVICES DIGITAL NETWORK (ISDN)

I 1

l 1

I I

I

I L IDN-!integrated dig>al ’ I

exchange exchange

I , D i g i t a l D i g i t a l I I

Telephone I

Facsimile L ISDN-integrated services digital network 1

Customer access access

Facsimile

Figure 10.1 ISDN

The services of ISDN are classified into three types, bearer services, supplementary

services and teleservices An introduction to each of these precedes our technical and

business benefits discussion of ISDN, which takes up the remainder of this chapter

The new ISDN interfaces enable the integration of telecommunications services or

teleservices by breaking them down into smaller component parts, called bearer and

supplementary services A teleservice comprises all elements necessary to support an

end user’s particular purpose or application Thus examples of teleservices are

telephony, facsimile, videotelephone, etc The specification of a teleservice usually calls

upon bearer services and supplementary services but may include additional informa-

tion specific to the particular application Thus the Group 4 facsimile teleservice

specification calls up a 64 kbit/s bearer capability and the ITU-T T.90 recommendation

on image-encoding

A bearer service is a simple information carriage service, at one of a number of avail-

able bandwidths (or rather bit-rates) At its simplest, this might be a normal switched

speech (public switched telephone network) service At its most advanced, it could be

a high-speed, 64 kbit/s data service, capable of carrying a switched ‘picture phone’ or

ISDN INTERFACES AND END-USER APPLICATIONS 215

slow-speed video service The range of bandwidths available also gives scope for

carriage of today’s voice-band data (e.g facsimile), telex and packet mode (Chapter 18)

or frame mode (Chapter 20) services

Supplementary services are so-called because, on their own, they have no purpose or value They are always provided in conjunction with one of the bearer services, to which

they add a supplementary value Examples of supplementary services are call diversion (redirecting incoming calls to another number) and calling-line identity (the identifica-

tion of the caller to the called party before or during answer) Another example is the

ring back when free (correctly called call connection to busy subscriber, C C B S ) service, which saves callers from futile repeat attempts when the called party is busy, by leaving the network to establish the call as soon as both parties are free In all of these examples,

the associated bearer service is speech In some networks, some supplementary services

are available even in the PSTN, but for many more ISDNs supplementary services will add a powerful new dimension

Summarizing, examples of the three types - of services included as component parts of ISDN are:

0 bearer service:

0 supplementary services:

0 teleservices:

speech, at an appropriate bit-rate 3.1 kHz (the bearer service required for

voiceband data, i.e dial-up modems and analogue facsimile machines)

64 kbit/s high-speed data call diversion

calling line identity ring back when free facsimile

telephony videotelephone

10.3 ISDN INTERFACES AND END-USER APPLICATIONS

To use the ISDN, customers either have to re-equip themselves with PBXs and user terminals designed with one of the new ISDN interfaces (of which there are currently two as explained below), or use terminal adaptors ( T A ) in conjunction with existing

terminals A simple terminal might be an ISDN telephone, or an ISDN telex terminal

A more advanced device might be a personal computer with sophisticated file-transfer

capabilities, or a group 4 facsimile machine offering high-speed facsimile service of

almost photocopier quality

Worldwide standards for the new ISDN network interfaces are set by the I (ISDN),

G and Q series of ITU-T recommendations Two types of customer-to-network interface are specified, these being the basic rate interface (BRZ or B R A , basic rate access) and primary rate interface ( P R I or P R A , primary rate access)

Both basic and primary rate interfaces comprise a number of B (bearer or user information) channels plus a D (data, but best thought of as a signalling) channel The

216 INTEGRATED SERVICES DIGITAL NETWORK (ISDN)

B channels carry the bearer services while the D channel enables the customer to signal

to the network how each bearer channel is to be assigned and used at time (for example,

‘connect bearer (B) channel number 1 to M r A for telephone service’)

10.4 BASIC RATE INTERFACE (BRI)

The BRI (or BRA, basic rate access) comprises three channels, so-called 2B + D, and is the interface that is used to connect most end-user terminal equipments, either directly

to the public network or via a company private branch exchange ( P B X ) The bandwidth

itself can be made available to the terminal in a number of different ways, using any of

the S, T o r U variants of the interface, as the diagram of Figure 10.2 shows Each of the

variants S , T , and U are basic rate interfaces but they differ in their physical realization

The T interface provides for a connection point between new ISDN terminal equip-

ment ( T E ) and the ISDN network terminating equipment ( N T I ) The NT1, generally

provided as part of the network operator responsibility in Europe, is merely an exchange line terminating device

The U interface, often known as the wires only interface, is a two-wire line interface,

up to about 7 km in length, incoming from the public exchange, and designed to work

as far as possible on existing two-wire copper pairs Its definition came about because of regulatory requirements in the United States which debar PTOs from offering the S/T interface which the European PTTs intend to offer, because in the United States the

N T l is deemed to be customer premises equipment ( C P E ) The U-interface will also be

important for cellular radio based ISDNs

The S interface is physically and electrically identical to the T interface, but for the

academics among us it is the name we give to the interface after it has passed through a

call routing equipment (NT2) An example of an NT2 is an ISDN PBX Because the S and T interfaces are the same, some people refer to the SIT interface

The final interface shown in Figure 10.2 is the R interface This is the general nomen- clature used to represent any type of existing telecommunication interface (such as V.24, RS-232 or X.21, as we discussed in Chapter 9) By using a terminal adaptor ( T A )

we make possible the use of the ISDN to carry information between existing data, and telephone terminals thus easing the user’s problem of deciding when to changeover from older style networks (e.g the telephone or public packet data network)

The S/T interface is a four-wire or equivalent interface, allowing 192 kbit/s data carriage in both directions The 192 kbit/s comprises 144 kbit/s of user information

(data supplied by the terminal), together with an overhead of 48 kbit/s which is needed

to administer and control the interface, to control the passive bus, for example, as

described below

The 144 kbit/s is used by the terminal as two B channels (both 64 kbit/s) and one

D channel (16 kbit/s) Either or both of the B channels may be in use at any one time, and when used simultaneously are not constrained to be connected to the same destination

218 INTEGRATED SERVICES DIGITAL NETWORK (ISDN)

The passive bus is a feature of the SIT interface that enables up to eight terminal devices to be connected simultaneously to the same basic-rate interface It is shown in Figure 10.3

The capability for multiple-terminal connection to the BR1 in this manner is crucial

to the support of incoming calls because, prior to the receipt of an incoming call, it is not possible to know which device (e.g facsimile machine, telex machine, telephone) should be connected to the line, because the type of the sending device is not known

A procedure called terminal compatibility checking which is part of the D channel signalling ensures that the call is answered only by the correct terminal device (i.e the one of up to eight connected to the bus which is compatible with the sending device)

The SIT interface and the passive bus are intended for within-building use on customer premises They are defined by ITU-T Recommendation 1.420 They are

provided directly from the NT1 On the terminal side of the NT1, there may be one of

three wiring configurations:

e a short and full facility passive bus up to 150m in length (the So-bus)

e an extended passive bus up to 500m in length

e a single point access line up to l000m in length

Protocols defined for use over the basic rate interface are defined by ITU-T Recom-

mendation 1.420 (and the European version of it is called NET3 or Euro ZSDN) The

B channels are left as clear channels for users, while a three-layer protocol stack, con- forming to the first three layers of the OS1 model, is tightly defined for the D channel, as Figure 10.4 shows

The physical layer protocol (ITU-T Rec 1.430) provides a contention resolution scheme to ensure that messages from different terminals on the passive bus do not

collide (compare with the C S M A j C D technique for L A N s , discussed in Chapter 19) At the second layer (1.441 or Q.921) the protocol is called LAPD (link access procedure in the D-channel) Like H D L C (see Chapter 9), the procedure controls the data flow over

Customer’s premises

I

D i g i t a l telephone Group

G

facsimi Le

NT1 -

S I T - p a s s i v e bus

Exchanae Line

Figure 10.3 Passive bus at a customer’s premises

USE OF THE BASIC RATE INTERFACE 219

L a y e r 3 ( N e t w o r k l a y e r )

L a y e r 2

( L i n k l a y e r )

Layer l

( P h y s i c a l l a y e r )

ILL1 (Q9211 L A P D

I4 30

Figure 10.4 Protocol stack in the ISDN D channel

the D channel, ensuring that the data buffers are not overfilled and that the synchron- ization of data is maintained It includes some degree of data error checking In parti- cular, LAPD provides for a number of connections to be simultaneously maintained with the multiple terminals on the passive bus Finally, the network layer protocol (layer

3 protocol called digital subscriber signalling l or D S S l ) is defined by ITU-T Recom-

mendation 1.451 (and duplicated in recommendation 4.931) This protocol allows the transfer of dial-up signalling information and for the set-up, control and clearing of

B channel connections It allows the user’s terminal to negotiate with the network set up

of an appropriate terminal device at the destination end It is this protocol that includes the terminal compatibility checking procedure

Given the small number of channels, the BR1 is well-suited for digital telephone customers, computer networks, small-business users and sophisticated residential-cus- tomers (e.g users of the Internet, or a businessman accessing his company computers from home).Circuit-switched calls at any of the three bit rates corresponding to speech, voice-band data (e.g facsimile) or 64 kbit/s data (e.g inter-computer links) are set up over one of the B channels, using the D channel to signal the t-equest and managing the connection The ISDN exchange responds to such requests by extending the relevant

B channel using a circuit-switched inter-exchange connection through to the destina- tion, as indicated by the dialled ISDN number The ISDN number is similar to the PSTN number; see Chapter 28) However, although the customer-to-customer interface will use a B channel with 64 kbit/s capability, not all of this bit rate will necessarily be extended Instead, within the inter-exchange part of the ISDN, a connection with a bit rate appropriate to the requested bearer service is provided Thus in the case of a

64 kbit/s bearer service, a clear digital 64 kbit/s path will be chosen However, when only speech bearer service is requested, the exchange may select either a standard digital telephone channel of 64 kbit/s or may choose instead a low bit rate digital connection,

220 INTEGRATED SERVICES DIGITAL NETWORK (ISDN)

routed via circuit multiplication equipment ( C M E ; see Chapter 38) or perhaps even a

link carried over analogue transmission plant Similarly, for a 3.1 kHz bearer service, a

connection of at least 3.1 kHz bandwidth will be selected Calls connected over ISDN in

a circuit-switched manner include telephone calls, telex calls and 64 kbit/s circuit-

switched data calls

Packet-switched connections (Chapters 9 and 18) may also be established over ISDN,

using either the B channel or the D channel In the B channel mode (ITU-T recom-

mendations X.31 case A and X.32), a circuit switched (B channel) connection is first

established to a nearby P A D ( p a c k e t assemblerldissembler) on the packet network, and

then X.25 protocols are used during the ‘conversation’ phase of the call However, a

more efficient method of packet-switched connection is to use the D channel (ITU-T

recommendation X.31 case B) In this instance the user’s data packets are interleaved

with other signalling messages on the D channel, and are transferred between ISDN

exchanges either using the SS7 inter-exchange signalling network (Chapter 12) or via

the established X.25 packet network Figure 10.5 illustrates both cases Figure 10.5(a)

illustrates access to an existing packet network using dial-in via a B-channel of ISDN

(minimum integration scenario) X.32 defines this procedure and the user identification

measures which should be conducted when either dialling-in or dialling-out of the

packet network in this way

Figure 10.5(b), meanwhile, illustrates a maximum integration scenario, in which data

information is sent from the user device (either DTE or TA) via the D channel The

data information is removed by the signalling terminal ( S T ) at the end of the D signal-

ling channel and concentrated via the SS7 network to a packet handler ( P H ) which

provides for an X.75 gateway connection to the packet network

In addition to the support of packet relaying services (or packet mode service), ISDNs

have lately been further developed also to support frame relaying service (frame mode, or

FHW oG - - - _ _ _ _ _ _ _ _ _ _ _ _ dialled-up (clear channel) B-channel- network

a) X.31 case A (minimum intearation)

I,,,HTHFt

bl X.31 case B (maxi,mum intearation)

DTE = data terminal equipment NTl=network termination 1 PH=packet handler

ST= signalling terminal TA=terminal adaptor

Figure 10.5 Carriage of packet data via ISDNs (ITU-T recommendation X.31)

ISDN TERMINALS 22 1

frame relay) as defined ITU-T recommendation Q.92 (Chapter 20) The optional

configurations for support o f f r a m e relaying services closely align with those of ISDN packet mode services Thus a frame handler replaces the packet handler of Figure 10.5(b)

Terminals used in conjunction with the BR1 must conform with the 1.420 S/T interface This interface alone is not sufficient to ensure the correct operation of terminals for particular applications, because it only ensures correct establishment of the connection between like terminals In addition, higher layer protocols (corresponding to the higher application layers of the OS1 model) must be defined so that the two end terminals can interpret the data they are sending to one another For example, two TV-phones operating over the ISDN must use the same picture coding technique (e.g ITU-T’s H.261 code) if the picture is to be transferred successfully Other potential application

1420

SIT i n t e r f a c e

I

v 2 1 I V 2 0

or

( C C I T T Rec V.110) I

I

M 1 ylrclllurluua

t e r m i n a l

I

I

t

I

I C i r c u i t

1 t e r m i n a l data 1 1 Rec X.30) 1 I

I

Figure 10.6 Common ISDN terminal adapters

1 S D N

exchange