30 years of ISDN

The ISDN (Inegrated Services Digital Network) was launched in the UK on June 25th 1985. Originally called Inegrated Digital Access (IDA) it was an extending of the digital phone network right through from the exchange to the customers premises enabling faster data transfer than had previously been possible over phone lines and new services such as video calling. It was the first ISDN system in the world and BP was the first customer on the new network.

Since the 1960s the network linking telephone exchanges together had gradually been going digital. Research was also undertaken to introduce digital exchanges such that calls would eventually be switched between exchanges using 64kbit/sec digital paths in what was termed an Integrated Digital Network.

With the introduction of System X digital exchanges starting in the early 80's the only analogue part remaining was the 'last mile' between the customer and their local exchange – the so called 'access network'.

Extending the Integrated Digital Network over the access network to the customer to give them 'Integrated Digital Access' (IDA) had been the vision from the 1960s and the term Integrated Services Digital Network (ISDN) had been defined back in 1971 by the telecoms standards body CCITT (Consultative Committee for International Telephone & Telegraphy) to describe such a system and the facilities it could offer.

Originally the twisted copper wire network used for analogue phones was thought unsuitable for such a system and it was expected that a new access network made up of either coaxial cable (fibre was still too expensive for such a mass installation) or wireless would be needed. Research however into improved transmission techniques meant that speeds of over 100 kbits/sec could be achieved over the existing copper network by the early 80s where before 4kbits/sec was though to be the maximum!

Research model testing higher speed digital transmission over twisted pair copper telephone wire. The drum of cable gives a reasonable line length between the customer and the exchange equipment.

With only a handful of System X digital exchanges in service by 1985 British Telecom launched a pilot of IDA in select locations using remote IDA multiplexers installed in the local exchange linked back to a System X exchange in that region. (see sidebox)

Initially one System X exchange in Baynard House London was used with remote multiplexers at Martlesham Heath (BTs Research Labs), Croydon, Bracknell, Brighton, Reading, Bristol and Slough.

Later in 1985 a second exchange in London (Maida Vale) was used to serve additional locations including Watford & Milton Keynes, and two other exchanges, one in Birmingham (Midland) and one in Manchester (Blackfriars) were used to serve locations in the midlands and north west (see map).

As the rollout of System X exchanges progressed so the availability of ISDN expanded nationwide and was soon available in places served by BTs second digital exchange system (Ericssons AXE10) as well.

ISDN offers a number of digital 64kbit/sec channels which the customer can make use of. These are referred to as B channels. The old analogue channels offered by the traditional phone system were often referred to as A channels, hence the next logical step was a B (binary) channel.

In addition there is a signalling channel used for communication between the exchange and the customer's equipment for call setup and clear down. This is called the Delta (D) channel.

Prior to the publication of full international standards early systems such as BTs IDA and other trials throughout the world used their own signalling protocols and range of channel offerings.

The CCITT standards offer two types of line referred to as basic rate access & primary rate access, offering 2 and 30 B channels respectively. These are marketed in the UK as ISDN2 and ISDN30. ISDN2 is delivered over a standard twisted copper pair of wires just like an analogue phone line. ISDN30 was initially only deliverable over coax or fibre. In later years a new transmission system called Single Pair High Speed Digital Subscriber Line (SHDSL) allowed digital transmission of speeds in excess of 2MBits/sec in both directions over standard twisted pair phone lines, however, by then there was little market demand to produce the hardware necessary to use it for ISDN30 delivery.

Each 64kbit/sec B channel on an IDSN is routed through the network as an individual call (and billed accordingly!) hence using all 30 channels of a primary rate line for 2Mbit data transfer can work out expensive. Switching dedicated channels like this (referred to as circuit switching) for data transfer is not an effeicient use of network capacity, but, it has the advantage of guranteed constant throughput, unlike the variable and unpredictable nature of packet switching. This means ISDN is still popular for impromptu interviews by TV & radio stations where it gives much better sound and picture quality than can be guaranteed using the Internet.

As the popularity of the web exploded in the mid to late 90's a lot of businesses and schools / libraries used basic rate ISDN (ISDN2) as a means of achieving what was at the time quite fast access. Most analogue dial up connections at the time were 28kbits/sec or 33.6kbits/sec. so even a single ISDN channel was upto or in excess of twice as fast. V.90 modems introduced in the late 90's were theoretically capable of 56kbits/sec but this was pushing the 4khz analogue telephony channel to its absolute limits and required a digital (ISDN) line at the other (ISP) end in order to reduce the number of digital-analogue conversions it passed through. Even then achieving 56k was extremely unlikeley, 46 or 48k was far more common. In addition an ISDN call is established in less than 1 second rather than the rather nosiy 10 to 15 second setup procedure used by analogue modems.