ISDN - Integrated Services Digital Network

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ISDN is an international telecommunications standard for transmitting digital and analog information, over ordinary telephone copper wire as well as over other media. It facilitate digital transmission of voice, data, and video at a much faster rate than with a traditional modem and allows multiple digital channels to be operated simultaneously through a single, standard interface.

ISDN Channel

There are two two pre-defined configurations:

Basic Rate Interface (BRI)
BRI is the most common ISDN service for Internet access. BRI is often known as "2B+D" referring to its two B-channels and one D-channel. This connection supports two 64 kbps B-channels and one 16 kbps D-channel over a standard phone line. In another word, a single BRI line can support up to three calls at the same time. Two voice, fax or data "conversations," and one packet switched data "conversation" can take place at the same time. Depending on the equipment use, multiple channels or even multiple BRI lines can be combined into a single faster connection. Channels can be combined as needed for a specific application (example large multimedia file transfer), then broken down and reassembled into individual channels for different applications (normal voice or data transmissions).

The BRI physical layer specification is International Telecommunication Union-Telecommunications Standards Section (ITU-T) (formerly the Consultative Committee for International Telegraph and Telephone [CCITT]) I.430.

Primary Rate Interface (PRI)
This service is used primarily by large organizations with intensive communications needs. An ISDN PRI connection in North America and Japan supports 23 64 kbps B-channels and one 64 kbps D-channel (or 23B+D) over a high speed DS1 (or T-1) circuit with a total bit rate of 1.544 Mbps. ISDN PRI in Europe, Australia, and other parts of the world supports 30 B channels plus one 64-kbps D channel and a total interface rate of 2.048 Mbps.

The PRI physical layer specification is ITU-T I.431.


About B-channel and D-channel

The Bearer ("B") channel is a 64 kbps channel which can be used for voice, video, data, or multimedia calls. B-channels can be aggregated together for even higher bandwidth applications.

The Delta ("D") channel can be either a 16 kbps or 64 kbps channel used primarily for communications (or "signaling") between switching equipment in the ISDN network and the ISDN equipment at user site. The D-channel on a BRI line can even support low-speed (9.6 kbps) X.25 data, however, this is not a very popular application in the United States.


ISDN Application

ISDN offers the speed and quality that previously was only available to people who bought expensive, point-to-point digital leased lines. ISDN has become the service of choice for many communications applications due to its flexibility as a dial-up service. Popular ISDN applications include:

  • Internet access

  • Video conferencing

  • Telecommuting/remote access to corporate computing

  • Small and home office data networking


Advantages of ISDN

As the Internet becomes more and more information-intensive with graphics, sound, video and multimedia, the ability to take advantage of these new resources depends on the speed of the Internet connection.

With ISDN,

  • Fast Access
    By combining two B-channels, home and business users have access to up to 128 kbps -- more than four times as fast as a 28.8 kbps modem on a standard phone line. And ISDN's digital technology assures user the cleanest connection to the Internet preventing slowed down by re-transmissions because of old analog technology.

  • Synchronize
    ISDN is synchronized so that all digital elements communicate in the same protocol at the same speed.

  • Economical
    ISDN increased capabilities, reduced costs and improved productivity to organizations both large and small. It eliminates the high cost of maintaining multiple Internet accounts, phone lines, and modems for multiple users by consolidating Internet traffic onto a single, high-speed connection.

  • Efficiency
    Dedicated ISDN service not only lowers costs and maintenance headaches, it also increases productivity. Users no longer have to wait for a modem to "dial".

  • Availability
    ISDN service keeps user network connected to the Internet 24 hours a day, 7 days a week. Because this connection is always open, servers such as mail, web, and FTP may be hosted through the connection. Remote users may even use the Internet to "tunnel" in and access files, databases, and other corporate resources.


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