communications

One network for everything, eve’rywhere by KATHY LAWRENCE

I t doesn’t mean much the first time you hear it, but consultant and lecturer Robert Blackshaw* says

that ‘integrated services digital net- work’ is one of the most descriptive names possible. It is the title for the future of communications on a world- wide scale. It means bringing together voice and data communications (inte-

grated) as offered by PTTs and sup- pliers to users (services) in a single digital network, rather than a mixture of various analogue, digital and hy-

brid networks as at present. The idea of such a network has

been around for some time. The value to users and service suppliers is not difficult to envisage. Users will get digital services at reasonable prices, and suppliers will find it easier and cheaper to maintain existing services

and introduce new ones.

Abstract: In the neuj integrated services

digital network (ISDN) all users services

will be available over one network, supplied

by national PTTs. This should benefit

PTTs, users a& equipment suppliers.

Although it will be some years before ISDN

is in place, it is time to start planning for its

use. Field trials are already in progress.

Keywords: data processing, computer

networks, communications.

*Robert Blackshaw and Harold Folts were speaking at a seminar on Integrated Services Digital Networks, organized in London by the CGS Institute.

The possibility of a set of standards governing the network will also help

suppliers of user equipment for ex- ample, who will be able to manufac- ture at levels sufficient to introduce economies of scale, because require- ments will be the same the world over.

What has held up the advent of ISDN so far, says Blackshaw, is the ‘last mile dragon’. Actually getting down to designing the network, pro- ducing standards and implementing it has been hard for suppliers, who

constantly found new technology which should also be incorporated. At last, though, says Blackshaw, the time has finally come to ‘shoot the engineer and get started on ISDN’. A series of recommendations for ISDN was re- leased in October 1984 by the Euro- pean standards body, the CCITT, and suppliers are now beginning field trials. ISDN is now on the path to

becoming a reality, and will in time be highly important to any user of busi- ness communications.

History

Data communications has developed in an ad hoc fashion from the analogue telephone network.

At present the national service pro- viders, the PTTs, offer a mixture of telephone networks, analogue and digital, and circuit-switched and packet-switched data networks. It has been felt for some time though that a properly-designed network could carry all voice and data communica-

tions, plus telemetry and any other services required. Some work was carried out on imroducing an inte-

grated digital network (IDN), but ISDN supersedes that idea because it involves services and is therefore

more in tune with market require- ments.

As far as the user is concerned, all services (teleservices) will be available on one network, ISDN. There will no

longer be a need for separate circuits for voice, data, telex, teletex and so on. Teleservices relate to all layers of the Open Systems Interconnection

(OSI) standards model. However, the top four layers of this model are

application-dependent, so as far as the terminal is concerned, ISDN deals with the lowest three levels, known in ISDN terms as the bearer services (see Figure 1).

Figure 1. ISDN deals with the bearer

semices

Speed of transmission has been one of the points of contention, with a decision finally being made on 64 kbit/s. In a basic interface there will be two switched channels operating at 64 kbitis, called B channels and a signalling channel, ‘called a D channel. This gives a basic interface of 2B+D, although it is possible to operate on B+D or just D. The customer equipment plays a large part in deciding which channel will be used for which transmissions. For example, the digital telephone system might use a B channel, as might the main computer, but desktop com- puters or telemetry systems could operate throughout the D channel, which will carry transmissions as well

~0127 no 6 jLlly/august 1985 0011-684)3/851060035-02$03.00 0 1985 Butterworth & Co (Publishers) Ltd. 35

as operating as the signalling channel (see Figure 2).

Customer premises

Digital telephone

i.1 .

.; . . . . .y m Main computer

Telemetry

B channel

B channel _

ISDN

D channel

Figure 2. The basic interfhce

This basic interface therefore offers a bandwidth of 144 kbit/s, which would not be anything like sufficient

for all services. The primary rate interface therefore offers a much greater bandwidth, being many B channels plus a single D channel. In fact, if you have many primary rate interfaces a single D channel will be sufficient for signalling for all of them. In Japan and the USA the

primary rate interface is 23B+D, giving a total of 1.544 Mbit/s. Europe is more in favour of 30B + D, giving a total of 2.048 Mbit/s.

I I

Figure 3. Interfaces between ZSDN

and customer premises equipment

There are a number of interfaces where the ISDN network meets the customer’s premises (see Figure 3). The outside transmission line inter- faces with what is called network termination 1 (NTl) on the cus- tomer’s premises. This is where the network terminates physically and electrically, and can therefore be re- lated to the lowest level of the OS1

model - the physical layer. NT2 might be a PABX or local area net-

work, and relates to the lower three levels of the OS1 model - physical, data link and network. TEl is a terminal suitable for ISDN use, with substantial intelligence, which handles the higher levels of the OS1 model. A terminal not directly suit-

able for use with the ISDN network (TE2) can be adapted through the terminal adapter (TA).

There is currently argument as to which interfaces belong to ISDN and which to customer premises. The standpoint taken seems to depend on the degree of competition in a country’s ISDN market, and this in turn depends on the amount of regu- latory control. Where the PTT is a

monopoly, there is no need to com- pete for ISDN supply contracts. In the USA though, for example, with the divestiture of AT&T, the market is far more competitive. The mood in the USA seems to be for the ISDN responsibility to end at the very first interface.

A further area of contention which could hold up standardization is the transmission method for the basic rate interface. There are two possibilities, both using two-wires: echo cancelling and time compression multiplexing. TCM is known as the ‘ping pong’ method, whereby transmission speed is doubled, so that full duplex can still be achieved, transmitting first one way and then the other. Echo cancel- ling is also full duplex, but both transmissions occur at once, with a complex procedure for separating sig- nals.

The future

Folts and Blackshaw believe the ad- vent of ISDN will open up for more use all sorts of services which are currently too expensive to use very often now. For example, slow-scan video can be used over the network, using the primary rate interface.

The telephone could take on a new lease of life. Equipped with a keypad,

hookswitch, handset and alpha- numeric display, the digital telephone could be used for online transactions, such as ticket reservations, banking and telephone ordering. On the PABX side, the primary rate interface can bring down the cost of loops. It can also provide the X.25, X.3 packet assembly/disassembly (PAD) func-

tions for supporting asynchronous terminals.

The cost of interfacing PCs will fall, allowing even greater use of personal computing. Online scanning, using very high-speed terminals will be- come more feasible. Greater use of transaction terminals might even lead eventually to one ATM being used for access to several banks.

All these services on one network

will not be properly available until ISDN is in place. This is not expected to happen until the early decades of the next century. Field trials have started in Europe, Japan and the USA but there are still many problems to overcome, such as how to set tariffs

for the services. In any case, the recommendations will probably not be fully approved until 1986, and it will then take a couple of years for terminal equipment suitable for ISDN to appear on the market.

Felts and Blackshaw see many benefits in the ISDN recommenda- tions. Users will get the digital ser- vices they want. Suppliers of services will fYind it easier to maintain existing services and introduce new ones, with only changes to the software of the stored program control of the switch- ing exchange being necessary. This might lead to a more adventurous attitude from the service suppliers.

Of particular importance though, is the need for the market to be user- driven. Despite all the possibilities of new, improved services over ISDN, there has been little market research into what users really want. It is important that everyone is involved in the development of the services, and that they become involved at as an early a stage as possible. q

data processing