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The integrated services digital network (ISDN) is a concept that has been about to become a reality for many years. In the mid-1980s, British Telecom (BT) tried a pilot version, known as IDA, which used a proprietary structure and protocol. More recently, DASS II has been available from BT and Mercury Communications Limited (MCL) as a primary rate access protocol, but this is also proprietary.
BT launched its CCITT-compliant basic-rate ISDN service, ISDN 2, at the end of 1990. This article discusses the following what basic-rate ISDN is, who will use it, who will supply it, and for what it will be used.
THE MARKET FOR ISDN
The basic-rate interface operates at 144 kbps and comprises three channels, two at 64 kbps (the B channels) and one at 16 kbps (the D channel). The B channels are used for voice, data, or text, while the D channel is used for signalling.
The basic-rate interface is often referred to as "2B+D". This name can lead to confusion as many telephone instruments now have a 2B+D structure, but are not capable of connecting directly to a basic-rate interface. The CCITT recommendation covering the basic-rate interface is I.420.
Looking at the question of who will use basic-rate access, the simple answer is that the basic-rate users will be those for whom ISDN is attractive, but who do not need the 30 lines of primary rate access. The market is likely to be dominated by small to medium-seed companies in niche industries and by the smaller outlets or branches of large national companies.
Large, single-site companies are unlikely to be basic-rate users, as they generally have certain communication capabilities. These include PABXs capable of supporting primary rate (30B+D), digital feature phones giving dialup data access to the desk using proprietary protocols (these use the PABX primary rate access to the public ISDN), and local area networks that give comprehensive data access via wide area network gateways. Hence, any anticipated potential requirements for basic-rate ISDN can already be met. The exception to this is where I.420 ISDN terminal equipment is required, but where the PABX can support only a proprietary 2B+D implementation.
One of the constraints in the development of the basic-rate interface was the requirement for PTTs to...