ISDN (Integrated Services Digital Network) has been regarded by many as the best kept secret of the computer networking world for too long. The continuing growth of the Internet and particularly the web seems to have finally pushed ISDN out into the open, as PC users have become increasingly frustrated as they wait for graphic-intensive web pages to download and want more speed from their dial-up net connection. Businesses are also looking for cost-effective ways to provide their staff with good-quality connections to the net.

The irony is that ISDN has been around for many years in the shape of the UK’s telephone network which has slowly been migrating away from being a public switched telephone network (PSTN), towards having an all digital infrastructure. What is still analogue, however, is the local loop – the copper telephone cable that runs from the typically digital telephone exchange to the home or business. So in fact, ordinary voice telephone calls go through an ISDN, but the real benefits of ISDN are not available until users pay for their particular strand of the local loop to be upgraded to ISDN.

ISDN was initially available in two versions, Basic Rate ISDN (BRI) which is also known as ISDN-2, and Primary Rate ISDN (PRI) or ISDN-30:

  • A Basic Rate installation, suitable for the home user or small business, consists of two 64 Kbit/s B channels for data transmission and one hidden 16 Kbit/s D channel for control information. The two 64 Kbit/s B channels can be used separately or bonded together to give one channel of 128 Kbit/s.
  • A Primary Rate ISDN installation consists of 30 B channels (although a minimum of six can be installed) of 64 Kbit/s each, plus a 64 Kbit/s D channel for control data which will normally be installed into a company’s PABX for maximum flexibility. As with Basic Rate the B channels can be bonded to give a single pipe of 1.92 Mbit/s.

Late 1998 saw BT making the first serious attempt to market ISDN technology to the home user with the announcement of the BT Highway services. When a customer subscribes to one of these services, their existing telephone line is retained but the old master socket is replaced by a Highway unit. This has four sockets, two analogue and two ISDN, and can support up to three calls simultaneously. Subscribers retain their old analogue number while receiving two additional numbers, one for a second analogue port and one for the ISDN lines. Two major differences between the Home and Business services are that the latter supports Multiple Subscriber Numbering (MSN) – whereby different devices attached to one ISDN line can have different numbers – as well as BT’s new ISDNConnect data service – a permanent low-speed link that uses ISDN’s signalling channel.

At the same time as BT Highway was launched BT’s ISP operation,BT Internet, announced support for 128 Kbit/s access, allowing users to use their two ISDN lines as one high-bandwidth link. Previously, UK-based ISPs had not supported this option – undermining BT’s efforts to promote ISDN to the Internet community.