BROADBAND ISDN

BROADBAND ISDN

In 1988, as part of its I-series of recommendations on ISDN, CCITT issued the first

two recommendations relating to broadband (B-ISDN): 1.113, Vocabulary of Terms

for Broadband Aspects of ISDN; and 1.121, Broadband Aspects of ISDN. These

documents provided a preliminary description and a basis for future standardization

and development work, and from those documents a rich set of recommendations

has been developed. Some of the important notions developed in these

documents are presented in Table A.5.

CCITT modestly defines B-ISDN as "a service requiring transmission channels

capable of supporting rates greater than the primary rate." Behind this innocuous

statement lie plans for a network and a set of services that will have far more

impact on business and residential customers than ISDN. With B-ISDN, services,

especially video services, requiring data rates in excess of those that can be delivered

by ISDN will become available. To contrast this new network and these new

services to the original concept of ISDN, that original concept is now being referred

to as narrowband ISDN.

Broadband ISDN Architecture

B-ISDN differs from a narrowband ISDN in a number of ways. To meet the

requirement for high-resolution video, an upper channel rate of approximately

150 Mbps is needed. To simultaneously support one or more interactive and distributive

services, a total subscriber line rate of about 600 Mbps is needed. In terms

of today's installed telephone plant, this is a stupendous data rate to sustain. The

only appropriate technology for widespread support of such data rates is optical

fiber. Hence, the introduction of B-ISDN depends on the pace of introduction of

fiber subscriber loops.

Internal to the network, there is the issue of the switching technique to be

used. The switching facility has to be capable of handling a wide range of different

bit rates and traffic parameters (e.g., burstiness). Despite the increasing power of

digital circuit-switching hardware and the increasing use of optical fiber trunking, it

is difficult to handle the large and diverse requirements of B-ISDN with circuitswitching

technology. For this reason, there is increasing interest in some type of

fast packet-switching as the basic switching technique for B-ISDN. This form of

switching readily supports ATM at the user-network interface.

Functional Architecture

Figure A.12 depicts the functional architecture of B-ISDN. As with narrowband

ISDN, control of B-ISDN is based on common-channel signaling. Within the network,

an SS7, enhanced to support the expanded capabilities of a higher-speed network,

is used. Similarly, the user-network control-signaling protocol is an enhanced

version of I.451lQ.931.

B-ISDN must, of course, support all of the 64-kbps transmission services, both

circuit-switching and packet-switching, that are supported by narrowband ISDN;

this protects the user's investment and facilitates migration from narrowband to

broadband ISDN. In addition, broadband capabilities are provided for higher datarate

transmission services. At the user-network interface, these capabilities will be

provided with the connection-oriented asynchronous transfer mode (ATM) facility.

User-Network Interface

The reference configuration defined for narrowband ISDN is considered general

enough to be used for B-ISDN. Figure A.13, which is almost identical to Figure A.4,

shows the reference configuration for B-ISDN. In order to clearly illustrate the

broadband aspects, the notations for reference points and functional groupings are

appended with the letter B (e.g., B-NT1, TB). The broadband functional groups are

equivalent to the functional groups defined for narrowband ISDN, and are discussed

below. Interfaces at the R reference point may or may not have broadband

capabilities.

Transmission Structure

In terms of data rates available to B-ISDN subscribers, three new transmission services

are defined. The first of these consists of a full-duplex 155.52-Mbps service.

The second service defined is asymmetrical, providing transmission from the subscriber

to the network at 155.52 Mbps, and in the other direction at 622.08 Mbps;

and the highest-capacity service yet defined is a full-duplex, 622.08-Mbps service.

A data rate of 155.52 Mbps can certainly support all of the narrowband ISDN

services. That is, such a rate readily supports one or more basic- or primary-rate

interfaces; in addition, it can support most of the B-ISDN services. At that rate, one

or several video channels can be supported, depending on the video resolution and

the coding technique used. Thus, the full-duplex 155.52-Mbps service will probably

be the most common B-ISDN service.

The higher data rate of 622.08 Mbps is needed to handle multiple video

distribution, such as might be required when a business conducts multiple simul

taneous videoconferences. This data rate makes sense in the network-to-subscriber

direction. The typical subscriber will not initiate distribution services and thus

would still be able to use the lower, 155.52-Mbps service. The full-duplex, 622.08-

Mbps service would be appropriate for a video-distribution provider.

Broadband ISDN Protocols

The protocol architecture for B-ISDN introduces some new elements not found in

the ISDN architecture, as depicted in Figure A.14. For B-ISDN, it is assumed that

the transfer of information across the user-network interface will use ATM.

The decision to use ATM for B-ISDN is a remarkable one; it implies that

B-ISDN will be a packet-based network, certainly at the interface, and almost certainly

in terms of its internal switching. Although the recommendation also states

that B-ISDN will support circuit-mode applications, this will be done over a packetbased

transport mechanism. Thus, ISDN, which began as an evolution from the

circuit-switching telephone network, will transform itself into a packet-switching

network as it takes on broadband services.

The protocol reference model makes reference to three separate planes:

User Plane. Provides for user-information transfer, along with associated controls

(e.g., flow control, error control).

Control Plane. Performs call-control and connection-control functions.

9 Management Plane. Includes plane management, which performs management

functions related to a system as a whole and provides coordination

between all the planes, and layer management, which performs management

functions relating to resources and parameters residing in its protocol entities.

Table A.6 highlights the functions to be performed at each sublayer.