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Integrated services digital network

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Integrated Services Digital Network (ISDN) is a type of telephone network system.

It is also used as a smart network[?] technology intended to add new services to the public switched telephone network (the PSTN) by giving users direct access to end-to-end circuit-switched digital services. ISDN has never gained popularity as a telephone network in the United States and today remains a niche product. In Japan, it became popular to some extent from around 1999 to 2001, but now that ADSL has been introduced, the number of subscribers is in decline.

In Japan, NTT, a dominant telephone company, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.

In the UK, British Telecom provides Home Highway[?] and Business Highway[?], which are BRI ISDN services which offer connection from analog devices (such as normal phones) as well as ISDN devices (such as PCs equipped with Terminal Adapters[?]). Home Highway has been bought by many home users, usually for Internet connection. Although not as fast as ADSL, it was available before ADSL, and in places where ADSL does not reach.

BT also offers PRI ISDN.

Table of contents


In ISDN, there are two types of channels, B and D:
  • B channels are used for data, and
  • D channels are intended for signalling and control (but can also be used for data).

B stands for Bearer and D stands for Delta.

There are two kinds of access to ISDN:

  • Basic Rate Interface (BRI) - consisting of two 64 kbps digital data channels and a 16 kbps digital signalling channel, designated as 2B+D, and,
  • Primary Rate Interface (PRI) - containing a greater number of channels, based on the country:
    • North America and Japan: 23B+1D, aggregate bit rate of 1.544 Mbps (T1)
    • Europe, Australia: 30B+D, aggregate bit rate of 2.048 Mbps (E1)

Calls are made over the data (B) channels, with the signalling (D) channels used for call setup and management. Once a call is set up, there is a simple 64 kbps synchronous bidirectional data channel between the users, lasting until the call is terminated. There can be as many calls as there are data channels, to the same or different end-points. Data channels may also be multiplexed into what may be considered single, higher-bandwidth channels.

Reference Points

A set of reference points[?] are defined in the ISDN standard to refer to certain points between the telco and the end user ISDN equipment.
  • R[?] - defines the point between a non-ISDN device and a Terminal Adapter (TA) which provides translation to and from such a device
  • S - defines the point between the ISDN equipment (or TA) and a Network Termination Type 2 (NT-2[?]) device
  • T - defines the point between the NT-2 and NT-1 devices1
  • U - defines the point between the NT-1 and the telco switch2

1 Most NT-1 devices can perform the functions of the NT-2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.
2 Inside North America, the NT-1 device is considered customer premises equipment and must be maintained by the customer, thus, the U interface is provided to the customer. In other locations, the NT-1 device is maintained by the telco, and the S/T interface is provided to the customer.

Types of communcations handled

Amongst the kinds of data that can be moved over the 64 kbps channels are pulse-code modulated voice calls, providing access to the traditional voice PSTN. This information can be passed between the network and the user end-point at call set-up time.

ISDN is nowadays mostly used as an alternative to analog connection, most commonly for Internet access. Some of the services envisaged as being delivered over ISDN are now delivered over the Internet instead.

Where an analog connection would require a modem an ISDN connection requires a Terminal Adapter, or TA.

Other references

See also:

The specifications defining the physical layer and part of the data link layers of ISDN are:

External references:

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