Page 211 - Asterisk™: The Future of Telephony

expensive technology, in turn, priced it so that it was not rapidly adopted. Currently,
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.
ISDN-BRI/BRA. Basic Rate Interface (or Basic Rate Access) is the flavor of ISDN, and is
designed to service small endpoints such as workstations.
The BRI flavor of the ISDN specification 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.
ISDN-PRI/PRA. 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
uses a single DS-0 channel as a signaling link (the D-channel); the remaining channels
serve as B-channels.
In North America, Primary Rate ISDN is commonly carried on one or more T1 circuits.
Since a T1 has 24 channels, a North American PRI circuit typically consists of 23 B-
channels and 1 D-channel. For this reason, PRI is often referred to as 23B+D. †

In Europe, a 32-channel E1 circuit is used, so a Primary Rate ISDN cir-
cuit is referred to as 30B+D (the final channel is used for
synchronization).

† PRI is actually quite a bit more flexible than that, as it is possible to span a single PRI circuit across multiple
T1 spans. This can give rise, for example, to a 47B+D circuit (where a single D-channel serves two T1s) or a
46B+2D circuit (where primary and backup D-channels serve a pair of T1s). You will sometimes see PRI
described as nB+nD, because the number of B- and D-channels is, in fact, quite variable. For this reason, you
should never refer to a T1 carrying PRI as “a PRI.” For all you know, the PRI circuit spans multiple T1s, as
is common in larger PBX deployments.

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