ISDN

'ISDN' stands for Integrated Services Digital Network, a smart network technology intended to add new services to the Public switched telephone network by giving users direct access to end-to-end circuit-switched digital services.

Configurations

In ISDN, there are two types of channels, B and D:

• B channels are used for data, and
• D channels are used for signalling and control.

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 modems, most commonly for Internet access. The 'smart services' envisaged as being delivered over ISDN are now delivered over the Internet instead.

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Other references

See also:

• ADSL
• ATM
• B-ISDN
• Internet
• 1.TR.6 (outdated German national ISDN protocol)

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

• ISDN BRI: ITU-T I.430.
  
• ISDN PRI: ITU-T I.431.
  

External references:

• http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm
• http://www.alumni.caltech.edu/~dank/isdn/