ISDN Explained

The introduction of an Integrated Services Digital Network (ISDN) by the telecommunications industry has revolutionised many business activities throughout the world. What was once impossible is now made possible by direct access to dial-up digital lines. The range of applications is constantly growing but already includes...

  • High Quality Audio - Broadcasters can establish their own digital quality audio links without having to book expensive analogue music circuits in advance.
  • Internet Access - Surf the internet up to four times faster than traditional modems.
  • Data Transfer - Move anything from just simple data to large video or image files.
  • Desktop conferencing - Real time on-line document discussion.
  • Group 4 fax - Transmit an A4 sheet in just three seconds with laser print quality.
  • LAN to LAN bridging - Establish fast, reliable inter-connection between small Local Area Networks.
  • Newspapers - Press photographers can instantly relay their pictures around the world.
  • Home / Remote Working - Remote Access is possible to your LAN or computer network from any ISDN line, be this at home or in one of the increasing number of hotels offering ISDN lines.
  • Video conferencing and video telephony- Opening a new world of opportunity - distance learning, business meetings and access to remote expertise.
  • Video Surveillance - Allowing police and security firms to monitor live pictures from remote surveillance cameras using video codecs.
ISDN has been designed by the telecommunications industry to provide a common communications platform, which will replace all existing types of line, from ordinary domestic phone lines to telex lines. It allows customers instant access and direct end to end connection into the public digital telephone network. The standard for ISDN in Europe is laid down in the EC ISDN Memorandum of Understanding, which ensures the twenty-two telephone companies in seventeen European countries conform to common ISDN standards.

Across the world two types of ISDN service are available to customers.

  • ISDN basic access. Branded as ISDN2, 2B+D or ISDN BRI.
  • ISDN primary access. Branded as ISDN30, 30B+D or ISDN PRI.

On this web site I always use the UK descriptions ISDN2 and ISDN30.

The ISDN2 service is carried from the telephone exchange along a traditional twisted pair copper cable. This single pair of wires carries two Bearer (B) channels and one Delta (D) channel. These three channels make up an ISDN basic rate line. The B channels carry communication at a rate of 64 kbit/s. The D channel carries all the control information to the digital exchange including dial up information. So passing back and forward along your single ISDN2 line are two 64 kbit/s B channel data streams and one 16 kbit/s D channel data stream. The only restriction with ISDN2 is the distance between the local telephone exchange and subscriber is limited to 5.5 kilometres.

ISDN30 uses a single coaxial or fibre optic cable to carry all the data. The ISDN30 line carries 30 64 kbit/s B channels and one 16 kbit/s D channel. This allows 1920 kbit/s of data to be transferred along a single cable in either direction. ISDN30 is mainly used to connect private telephone switchboards (PABX's) to the public digital telephone network.

Terminal Adapters for broadcasters

It goes without saying that to make a phone call you require a telephone. To make an ISDN call you require a terminal adapter (TA). The TA handles the interface between the ISDN line and any ISDN compatible digital equipment. In the broadcasting world this is usually a Digital Audio Codec.

The majority of TAs are manufactured for the general data communications industry and are not ideally suited to the broadcast environment. Making a call is a time consuming task, involving a lot of keyboard work, which has to be repeated every time you wish to make a call. To establish an ISDN link both 64 kbit/s B Channels have to be dialled separately. On the TA each B channel is referred to as a port.

If, for instance, you are making a stereo studio to studio link using a CDQ2000ED you will dial up both ports. If, on the other hand, you are handling a mono narrow bandwidth football commentary using a Glensound unit you will only require one port. Once the ISDN link is finished with, it is then a straightforward process to clear down both ports.

A number of companies have developed broadcast friendly, BT approved, TAs. Nicral have developed the "Easycall", APT "Prolink", and Canford Audio now have their own TA. These store ISDN numbers in their memory and dial up both B channels automatically.

Any digital audio codec distributor will recommend a suitable TA.

Digital Audio Codec

Research into data compression has resulted in the development of a range of units capable of using ISDN2 lines to relay broadcast quality audio. These Coder/Decoders, described as codecs, convert the analogue audio signal into a compressed digital audio data format that can be transferred over an ISDN2 line. Unfortunately, there is no common standard for this process and as a result there are at least six systems available, each using a different coding technique and none is compatible with any of the others.

The six main coding systems in use are apt-X100, DolbyFax, G.722, LayerII, LayerIII and MUSICAM.