British Telecom's IS DN

Howard Brown reviews the ISDN concept in relation to customer needs and the introduction of three British Telecom ISDN services

In this paper ISDN is discussed in relation to existing and projected future customer requirements. Such a communi- cations innovation is ultimately bound to have a major effect on all existing customer networks, but in the meantime ISDN also has to be fully integrated with existing network environments. The paper discusses British Telecom's three ISDN services, Single- and Multi-line Integrated Digital Access and 1.420 Single-line Integrated Digital Access services, commenting on their uses and technical specifications. Hybrid network configurations, the paper concludes, could blend private and public ISDNs.

Keywords: PABX, ISDN, network environment, Integrated Digital Access

British Telecom's (BT) Integrated Services Digital Network (ISDN) is marketed as the Integrated Digital Access (IDA) (see Figure 1), showing BT's focus on the customers' access point to the ISDN, and indicating its relationship to other services such as Packet Switching Services (PSS). IDA meets the definition, and hence offers the benefits originally identified by the Comit~ Consultatif Inter- nationale de T~Mphonique et T~Mgraphique (CCITT) for ISDN. That definition has two key points: first, that it should be based on an 'all purpose' network suitable for both voice and data, enabling ISDN to be provided as an inherent part of the modernization programme being introduced for telephony; and second, it should offer to customers a very limited set of interfaces suitable for both voice and data, which may be found both on direct connections to the network, and ultimately on the extensions of Private Automatic Branch Exchanges (PABX). This latter point obviously offers a customer benefit in terms of procurement.

British Telecom, Proctor House, 100-110 High Holborn, London WCIV 6LD, UK

ISDN CONCEPT

The ISDN concept is probably the most significant development in communications this decade, and it will certainly impact on communications to a significant degree for the rest of this century. The first justification for this view is that ISDN applies to large and small customer sites, so it addresses both the PABX market and the non- PABX key system market as well as the simple telephone in small premises. The second main justification is that it enables access to telephony, circuit switched data, packet switching and ultimately to digital private circuits. Therefore, it does address both the voice and data market, and recognizes that customer communications will, for many years to come, still be dominated by voice communications, even though there is a very strong growth in data.

CUSTOMERS' NETWORK ENVIRONMENT

If we look at the current network environment we would typically see a corporate network with a digital core using BT's MegaStream circuits to interconnect a number of digital PABXs. The traffic between these digital PABXs would be voice and data, with Digital Private Network Signalling Systems (DPNSS) type facilities.for enhancing these telephony and data applications. This approach can be considered as 'private ISDN'. When customers wish to communicate between extension users on this corporate core network, they can enjoy features and facilities such as fast call set-up and call-waiting, which makes communi- cation far more effective and staff far more productive. When calls need to be made to branch or high street offices, in most cases this means going to a less advanced, slower communications media using modems on the switched telephone network, or analogue private circuits. Although customers can use BT's KiloStream services to

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vol 11 no 4 august 1988 177

Customer

Figure 1.

IDA

Integrated Digital Access (IDA) service

Digital Local

Exchange

Integrated .~ Digital

Network

feed these offices, the low levels of traffic that are involved, and the fact that most communications would still be voice, makes the economics difficult in many situations.

ISDN IMPACT: CREATING THE HYBRID SOLUTION

If we look at how the ISDN can impact on this current network environment, we can see that by augmenting the existing corporate digital networks with a distribution system of switched ISDN circuits, the branch and high street offices can be brought into the advanced communi- cations network that exists within the core (see Figure 2). This enables a wide area networking capability to be given to the private ISDN. The impact on the customer is that extension users will receive the same quality and similar feature rich service whether they are located at a high street office or on an extension directly served by one of the headquarters' PABXs. The switched ISDN will there- fore help to provide a total solution encompassing both the high street branch offices as well as the headquarters' core network. For the first time the company's network will appear transparent to the way that a particular office site is served.

In addition to the total solution benefit already described, the benefits to customers include the freedom and greater flexibility to select between telephony and data calls. This applies both on the lines to the network, as well as in the wiring system used within a customers' site. This is particularly true in the case of the Integrated Services Private Branch Exchanges (ISPBX) where the normal telephone wiring can be used to distribute voice and data calls, rather than having to install special coaxial or fibre LANs. Other benefits include the better utilization of lines, as separate circuits do not need to be provided to cater for peaks in individual services. Improved manage- ment is also a benefit of ISDN, as only one set of diagnostics needs to be learnt and operated. In addition, ISDN's benefit of'future proofing' a customer's service, by allowing access to the public switched network, enables customers to enjoy and sample new network services that may become available in the future, such as Virtual Private Networks (VPN).

A typical scenario could be considered of, say, a small building society, a bank, or even a retail chain, and the way ISDN could impact upon it. The headquarters of such an organization could be equipped with an ISPBX, which would enable both telephony and at least the PCs of the company to be distributed around the building by means of the ISPBX. The main data processing equipment can either be connected by a front end processor to the ISPBX, or use a 'drop and insert' type multiplexer to share the group's transmission systems with the ISPBX. Which- ever way is chosen, easier management and economies can be achieved by using common 2 Mbit/s digital ports. At the high street or branch office end, the current telephone lines can be used to support single line IDA, and enable voice and data communications to be sent either to other offices, or to the headquarters' ISPBX. This arrangement removes the need for separate pairs of wires, separate terminating boxes and separate protocols for each of the services that are currently required to go into the branch office.

PACKET SWITCHING

Access to packet switching is being made available via IDA. A circuit switched call or a direct connection to PSS will allow applications that are more suited to packet switching to use IDA to provide a high speed, low error

0igital Exchange lines

Switched access to remote offices

' Digital Exchange l ines'

J to remote offices

Figure 2. Hybrid corporate network

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Figure 3.

PABX

Multi-line Integrated Digital Access (IDA)

2.048 Mbif/s

Multi -line IDA : Z o e

7

/

L rate connection. This form of packet access is called 'Minimum' integration by CCITT. 'Maximum' integration, which involves providing a packet facility at the local exchange, will only be provided when and if it is shown to be more economical, and when current equipment is ready for renewal. Access is via the 64 kbit/s B-channel of IDA. Access on the D-channel is being planned by some operating companies in the USA. This type of access is not being provided as part of the initial service by BT, or by other European countries.

The types of configuration described above for the headquarters and branch office locations could make use of IDA access to PSS for certain data applications.

BT ISDN SERVICES

BT's three ISDN services are:

1 Single-line IDA pilot service: this has been available since 1985. The most significant points are that it is provided over the ordinary twisted-pair copper networks, so no special fibre or cable to serve a customer site has to be provided. It offers 64 kbit/s digital exchange channel for voice and data communications, and a separate digital exchange channel at 8 kbit/s which could be used for data communications. BT customers are provided with the CCITT X.21 interface, which enables automatic call setup and clear down from a customer's terminal. Customers may attach terminal adapters to the X.21 interface to support existing V series terminals, or they can connect directly to the X.21 network terminals, such as

Group 4 facsimile machines. All such devices are available on the UK market. The IDA pilot service is currently available at some 100 business exchanges throughout the country. By the end of the summer of 1988 BT will have 200 business exchanges equipped with the ISDN capability. BT has commissioned 1 000 lines, each of 64 kbit/s and 8 kbit/s, and by the summer there will be a capacity of nearly 2 000 lines.

2 Multi-line IDA (see Figure 3): designated primary rate access by the CCITT, it enables the new generation of ISPBXs to be connected digitally to the public network over 2 Mbit/s links. Each 2 Mbit/s link offers 30 traffic channels, plus a common channel signalling link to the network. This signalling link is the first time that a high speed, duplex, signalling channel, with the power of an inter-exchange signalling system has been available to the customer. In the absence of a CCITT standard, BT has adopted a UK proprietary signalling system called Digital Access Signalling System (DASS). This system is fully compatible with DPNSS. The DASS protocol is fully specified and is being implemented by PABX manu- facturers in the UK. The CCITT equivalent Q931 is not yet finalized, and is unlikely to appear as an implementation anywhere in the world in this decade. Trials of ISPBXs were started in 1987, and approvals are due throughout 1988. The network service is being launched this year to meet customer demand and availability of approved ISPBXs.

The multi-line IDA service will have wide business coverage. Currently BT has thousands of 2 Mbit/s exchange ports for ISPBXs available in the network, spread over hundreds of business exchanges. It should therefore

vol 11 no 4 august 1988 179

Terminal

I I I - ,L

I i er 'no' equipment adaptor

Terminal equipment

l 192 kbit/s lw

I 420 passive bus 4 wire

NT I

Single pair 144 kbit/s

Terminal I

Figure 4. 1.420 terminal configurations: a total of eight terminal equipments or adaptors can be supported on a basic rate access passive bus

192 kbit/s 1 420 European standards

Figure 5.

144 kbitls

Methods of providing IDA national service

IMUX

I ISPBX 2 Mbitls (DASS 2)

Digital Exchange

be possible for BT's ISPBX customers to be offered service to their major sites with little difficulty.

3 1.420 Single-line IDA (see Figure 4): trials are planned for later this year. The service will offer two 64 kbit/s digital exchange channels; either one can be used for voice or data. The customer interface provided by the service will conform to the new 1.420 standard. The important points regarding the 1.420 terminal interface are that the interface enables up to eight different terminals to be connected to the network via one network connection. The interface uses a passive bus, imple- mented by means of the ordinary telephone wiring, to offer a simple LAN to customers. The two 64 kbit/s channels are piped around this LAN. Any terminals connected to the passive bus may select and use the channels, and use the common channel signalling feature to send call setup and facility information. The 1.420 interface can also be connected to terminal adapters, which are liberalized equipment, to provide support for telephones or V series data terminals. The new service will be available potentially at all System X and AXEI0 exchanges, and BT has already ordered, and is having installed, I 500 lines of 1.420 service.

Evolution

BT's three ISDN services fit together in an evolutionary plan where all services intenwork, so that a customer using the X.21 64 kbit/s service can communicate to either an 1.420, customer or an extension user on an ISPBX, without any complex interworking arrangements (see Figure 5).

CONCLUSION

ISDN is a concept based on using a general purpose digital network and providing a digital common access to all network services. BT's ISDN network services are marketed as IDA, which provides access to telephony, circuit switched data and packet switching. ISPBXs are an important part of ISDN, and BT is offering multi-line IDA to support them. Hybrid network configuration may well appear in the future, which will blend the benefits of private ISDNs with those of the public ISDN. These network configurations will be tailored to optimize communications in order to meet the customer's needs.

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