System information

You can then restart your DAHDI process to unload any existing drivers that were
loaded, and load just the dahdi_dummy module with the init script:
$ sudo /etc/init.d/dahdi restart
Unloading DAHDI hardware modules: done
Loading DAHDI hardware modules:
No hardware timing source found in /proc/dahdi, loading dahdi_dummy
Running dahdi_cfg: [ OK ]
Before you can start using your hardware, though, you’ll need to configure the /etc/
dahdi/system.conf file; this process is described in “Configuring digital cir-
cuits” on page 138 and “Configuring analog circuits” on page 142.
Configuring digital circuits
Digital telephony was developed by carriers as a way to reduce the cost of long-distance
circuits, as well as improve transmission quality. The entire PSTN backbone has been
fully digital for many years now. The essence of a digital circuit is the digitization of
the audio, but digital trunks also allow for more complex and reliable signaling. Several
standards have been developed and deployed, and for each standard there may be
regional differences as well.
You can use dahdi_hardware and lsdahdi to help you determine what
telephony hardware your system contains. You can also use dahdi_gen-
conf modules to build an /etc/asterisk/modules file for you based on the
found hardware.
Primary Rate Interface ISDN (commonly known as PRI) is a protocol designed
to run primarily on a DS1 circuit (a T1 or E1, depending on where you are in the world)
between a carrier and a customer. PRI uses one of the DS0 channels as a signaling
channel (referred to as the D-channel). A typical PRI circuit is therefore broken down
into a group of B-channels (the bearer channels that actually carry the calls), and a
D-channel for signaling. Although it is most common to find a PRI circuit being carried
across a single physical circuit (such as a T1 or E1), it is possible to have a PRI circuit
span multiple DS1s, and even to have multiple D-channels.
†
While there are many different ways to configure PRI circuits, we are hoping to avoid
confusing you with all of the options (many of which are obsolete or at least no longer
in common use), and instead provide examples of the more common configurations.
PRI ISDN.
† Sometimes circuits are referenced by the number of B- and D-channels they contain, so a single T1 running
the PRI protocol in North America might be referred to as 23B+D, and a dual T1 circuit with a backup
D-channel would be a 46B+2D. We’ve even seen PRI referenced as nB+nD, although this can get a little bit
pedantic.
138 | Chapter 7: Outside Connectivity