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.
There are two two
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.
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 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:
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.
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
ISDN is synchronized so that all digital elements communicate in the same
protocol at the same speed.
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.
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".
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