Integrated Services Digital Network


ISDN, or Integrated Services Digital Network, can be categorized as either narrowband ISDN or broadband ISDN. Narrowband ISDN is the original implementation of this technology, which was designed to provide a standard for an integrated voice and data network over standard telephone lines. The primary advantage of this technology was that modems would not be needed to convert the signal into analog and then back into digital again.

Narrowband ISDN, BRI and PRI

Narrowband ISDN is further classified into two categories, the Basic Rate Interface (BRI) or the Primary Rate Interface (PRI). The Basic Rate Interface uses standard twisted pair telephone lines, and provides two B (or Bearer) channels and a D (or Delta) channel. The B channels are 64kbps each and the D channel is 16kbps. This provides a total aggregate bandwith of 144kbps, of which 128kbps is available for actual data. The B channels are used to carry the actual information, whether it be voice, data or video, and the D channel is provided for control signaling.

Primary Rate Interface uses trunking technology to provide a total bandwith equivalent to a T1 line, or 1.544Mbps. This is divided up into 23 64kbps B channels, and one 64kbps D channel. These channels are used in the same way as with BRI, except that there is a lot larger total bandwith available. In Europe, the PRI actually provides a data transfer rate of 2.048Mbps, having 30 B channels and one D channel. Any of these B channels can be used independently, meaning that multiple types of user data can be multiplexed and then transmitted across the network simultaneously.

With the increasing need for bandwith in todays applications, standards for broadband ISDN emerged. As part of broadband ISDN, Asynchronous Transfer Mode (ATM) has emerged to provide a high speed packet switching network that uses short, fixed length cells of 53 bytes ( a 5 byte header, and a 48 byte payload). It is carried over SONET, and provides the high bandwith solution that todays networks need. Check back for more information on ATM and SONET in the near future.

Is ISDN for you?

So, who should use ISDN services? It all depends on your network needs, how much bandwidth you need now and how much you expect to grow in the near future, such as in one year. Also, the costs and fees that your local telephone company charges for their services, and how these fees compare to other solutions, such as ADSL, Frame Relay or leased lines like T-1 can help determine if ISDN is right for you. If you are interested in finding out if your business could benefit from ISDN, feel free to contact one of ComTest's engineers in our Staff Directory.

NOTE: This article is designed to introduce you to the concepts relating to ISDN so that you can understand how you may benefit from such a service. However, if you are interested in an in depth discussion of CPE, ISDN devices, reference points and other topics related to ISDN, visit the links below:

ComTest Technologies proudly represents a number of manufacturers offering ISDN connectivity. These include, but are not limited to, the following:


This page was written by Will Twiggs, an associate with ComTest Technologies, Inc. For more information or if you have questions about this material, you can contact the author at

Data and Telecommunications Systems and Applications by Charles N. Thurwachter, Jr.

Telecommunications, 3rd Edition, by Warren Hioki

This page created by Will Twiggs
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Last updated on 01/4/2001