System information

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
circuit is referred to as 30B+D (the final channel is used for
synchronization).
Primary Rate ISDN is very popular, due to its technical benefits and generally compet-
itive pricing at higher densities. If you believe you will require more than a dozen or so
PSTN lines, you should look into Primary Rate ISDN pricing.
From a technical perspective, ISDN-PRI is always preferable to CAS.
Signaling System 7
Signaling System 7 (SS7) is the signaling system used by carriers. It is conceptually
similar to ISDN, and it is instrumental in providing a mechanism for the carriers to
transmit the additional information ISDN endpoints typically need to pass. However,
the technology of SS7 is different from that of ISDN; one big difference is that SS7 runs
on a completely separate network than the actual trunks that carry the calls.
SS7 support in Asterisk is on the horizon, as there is much interest in making Asterisk
compatible with the carrier networks. An open source version of SS7 (http://www
.openss7.org) exists, but work is still needed for full SS7 compliance, and as of this
writing it is not known whether this version will be integrated with Asterisk. Another
promising source of SS7 support comes from Sangoma Technologies, which offers SS7
functionality in many of its products.
It should be noted that adding support for SS7 in Asterisk is not going to be as simple
as writing a proper driver. Connecting equipment to an SS7 network will not be possible
without that equipment having passed extremely rigorous certification processes. Even
then, it seems doubtful that any traditional carrier is going to be in a hurry to allow
such a thing to happen, mostly for strategic and political reasons.
* 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.
614 | Appendix A: Understanding Telephony