System information

supported) has to be sent as part of the audio stream. CAS is commonly used on the
T1 link in channel banks.
ISDN
The Integrated Services Digital Network (ISDN) has been around for more than 20
years. Because it separates the channels that carry the traffic (the bearer channels, or
B-channels) from the channel that carries the signaling information (the D-channel),
ISDN allows for the delivery of a much richer set of features than CAS.
In the beginning, ISDN promised to deliver much the same sort of functionality that
the Internet has given us, including advanced capabilities for voice, video, and data
transfer. Unfortunately, rather than ratifying a standard and sticking to it, the respective
telecommunications manufacturers all decided to add their own tweaks to the protocol,
in the belief that their versions were superior and would eventually come to dominate
the market. As a result, getting two ISDN-compliant systems to connect to each other
was often a painful and expensive task. The carriers who had to implement and support
this expensive technology, in turn, priced it so that it was not rapidly adopted. Cur-
rently, ISDN is rarely used for much more than basic trunking—in fact, the acronym
ISDN has become a joke in the industry: “It Still Does Nothing.”
Having said that, ISDN has become quite popular for trunking, and it is now (mostly)
standards-compliant. If you have a PBX with more than a dozen lines connected to the
PSTN, there’s a very good chance that you’ll be running an ISDN-PRI (Primary Rate
Interface) circuit. Also, in places where DSL and cable access to the Internet are not
available (or are too expensive), an ISDN-BRI (Basic Rate Interface) circuit might pro-
vide you with an affordable 128-Kbps connection. In much of North America, the use
of BRI for Internet connectivity has been deprecated in favor of DSL and cable modems
(and it is never used for voice), but in many European countries it has almost totally
replaced analog circuits.
The Basic Rate Interface (or Basic Rate Access) flavor of ISDN is designed
to service small endpoints such as workstations.
This flavor is often referred to simply as “ISDN,” but this can be a source of confusion,
as ISDN is a protocol, not a type of circuit (not to mention that PRI circuits are also
correctly referred to as ISDN!).
A Basic Rate ISDN circuit consists of two 64-Kbps B-channels controlled by a 16-Kbps
D-channel, for a total of 144 Kbps.
Basic Rate ISDN has been a source of much confusion during its life, due to problems
with standards compliance, technical complexity, and poor documentation. Still, many
European telecos have widely implemented ISDN-BRI, and thus it is more popular in
Europe than in North America.
The Primary Rate Interface (or Primary Rate Access) flavor of ISDN is used
to provide ISDN service over larger network connections. A Primary Rate ISDN circuit
ISDN-BRI/BRA.
ISDN-PRI/PRA.
The Digital Circuit-Switched Telephone Network | 613