Integrated Services Digital Network (ISDN) is essentially digital
telephone service. ISDN envisions telephone companies providing
"Integrated Services," capable of handling voice, data, and video
over the same circuits. The core of the telephone network is now
digital, so most ordinary telephone calls are now converted into
bits and bytes, transported through digital circuits, and converted
back into analog audio at the remote end. The international standard
for the digital telephone network is Signaling System 7 (SS-7), a
protocol suite in its own right, roughly comparable to TCP/IP.
End users never see SS-7, since it is only used between telephone
switches. ISDN provides a fully digital user interface to the
SS-7 network, capable of transporting either voice or data.
BISDN (Broadband ISDN) uses ATM instead of SS-7 as the
underlying networking technology.
ISDN is a complete networking technology in its own right,
providing clearly defined Physical, Data Link, Network and
Presentation layer protocols. For most Internet applications,
though, ISDN is regarded as a fancy Data Link protocol used
to transport IP packets.
An ISDN interface is time division multiplexed into channels.
In accordance with SS-7 convention, control and data signals
are seperated onto different channels. Contrast this to TCP/IP,
where control packets are largely regarded as special cases
of data packets and are transported over the same channel.
In ISDN, the D channel is used for control, and
the B channels are for data. B channels are always
bi-directional 64 kbps, the standard data rate for transporting
a single audio conversation; D channels vary in size.
The two primary varients of ISDN are BRI (Basic Rate Interface)
and PRI (Primary Rate Interface). BRI, sometimes referred to
as 2B+D, provides two 64 kbps B channels and a 16 kbps D channel
over a single 192 kbps circuit (the remaining bandwidth is
used for framing). BRI is the ISDN equivalent of a single
phone line, though it can handle two calls simultaneously
over its two B channels. PRI, essentially ISDN over T1,
is referred to as 23B+D and provides 23 B channels and a
64 kbps D channel. PRI is intended for use by an Internet
Service Provider, for example, multiplexing almost two dozen
calls over a single pair of wires.
A number of international standards define ISDN. I.430 describes
the Physical layer and part of the Data Link layer for BRI.
Q.921 documents the Data Link protocol used over the D channel.
Q.931, one of the most important ISO standards, documents
the Network layer user-to-network interface, providing call
setup and breakdown, channel allocation, and a variety of
optional services. Varients of Q.931 are used in both ATM
and voice-over-IP. G.711 documents the standard 64 kbps
audio encoding used by telcos throughout the world.