30 years of ISDN

30 years of ISDN

ISDN (Integrated Services Digital Network) was launched in the UK on 25th June 1985. Originally called Integrated Digital Access (IDA), it extended the digital phone network right through from the exchange to the customer's 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, so that calls would eventually be switched between exchanges using 64kbit/sec digital paths — what was termed an Integrated Digital Network.

With the introduction of System X digital exchanges starting in the early '80s, 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, giving them "Integrated Digital Access" (IDA), had been the vision since 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 of either coaxial cable (fibre was still too expensive for mass installation) or wireless would be needed. Research into improved transmission techniques, though, meant speeds of over 100kbit/sec could be achieved over the existing copper network by the early '80s, where 4kbit/sec had previously been thought the maximum.

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 at Baynard House, London, was used, with remote multiplexers at Martlesham Heath (BT's Research Labs), Croydon, Bracknell, Brighton, Reading, Bristol and Slough.

Later in 1985, a second London exchange (Maida Vale) served additional locations including Watford & Milton Keynes, and two more exchanges — one in Birmingham (Midland) and one in Manchester (Blackfriars) — served locations in the Midlands and North West (see map).

As the rollout of System X exchanges progressed, ISDN availability expanded nationwide, soon reaching places served by BT's second digital exchange system too (Ericsson's AXE10).

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

There's also a signalling channel used for communication between the exchange and the customer's equipment, for call setup and clear-down — the Delta (D) channel (see sidebox).

Prior to full international standards being published, early systems such as BT's IDA, and other trials worldwide, used their own signalling protocols and range of channel offerings.

The CCITT standards offer two types of line: basic rate access and primary rate access, offering 2 and 30 B channels respectively — marketed in the UK as ISDN2 and ISDN30. ISDN2 is delivered over a standard twisted copper pair, just like an analogue phone line. ISDN30 was initially only deliverable over coax or fibre. Later, a transmission system called Single Pair High Speed Digital Subscriber Line (SHDSL) allowed digital transmission in excess of 2Mbit/sec in both directions over standard twisted-pair phone lines — though by then there was little market demand to build the hardware needed to use it for ISDN30 delivery.

Each 64kbit/sec B channel on an ISDN line is routed through the network as an individual call (and billed accordingly), so using all 30 channels of a primary rate line for 2Mbit data transfer could work out expensive. Circuit-switching dedicated channels like this for data transfer isn't an efficient use of network capacity, but it has the advantage of guaranteed constant throughput, unlike the variable, unpredictable nature of packet switching. This is why ISDN is still popular for impromptu interviews by TV & radio stations, where it gives much better sound and picture quality than can be guaranteed over the Internet.

As the popularity of the web exploded in the mid-to-late '90s, a lot of businesses, schools and libraries used basic rate ISDN (ISDN2) to get what was, at the time, quite fast access. Most analogue dial-up connections then ran at 28kbit/sec or 33.6kbit/sec, so even a single ISDN channel was up to twice as fast, or more. V.90 modems, introduced in the late '90s, were theoretically capable of 56kbit/sec, but this pushed the 4kHz analogue telephony channel to its absolute limits, and required a digital (ISDN) line at the ISP end to reduce the number of digital-analogue conversions along the way. Even then, achieving 56k was unlikely — 46 or 48k was far more common. On top of that, an ISDN call was established in under a second, rather than the noisy 10–15 second setup procedure used by analogue modems.