System information

The relationship between Linux, DAHDI, and Asterisk (and therefore /etc/dahdi/sys-
tem.conf and /etc/asterisk/chan_dahdi.conf) is shown in Figure 9-5.
Figure 9-5. The relationship between Linux, DAHDI, and Asterisk
Asterisk
With everything set at the Linux level, we now only need to configure Asterisk to make
use of the channels we just enabled at the Linux level and to customize the way that
Asterisk interprets and generates information that comes in from, or goes out over,
these channels. This work is done in /etc/asterisk/chan_dahdi.conf.
In this file we will not only tell Asterisk what sort of channels we have (these settings
will fit with what we already did in DAHDI), but also configure a number of things that
will ensure Asterisk is well suited to its new home.
Caller ID
A key component of this change is caller ID. While caller ID delivery methods are pretty
much standard within the BRI and PRI world, they vary widely in the analog world;
thus, if you plugged an American analog phone into the UK telephone network, it
would actually work as a phone, but caller ID information would not be displayed.
This is because that information is transmitted in different ways in different places
around the world, and an American phone would be looking for caller ID signaling in
the US format, while the UK telephone network would be supplying it (if it is enabled—
it is not standard in the UK; you have to pay for caller ID!) in the UK format.
Not only is the format different, but the method of telling a telephone (or Asterisk) to
look out for the caller ID may vary from place to place too. This is important, as we do
not want Asterisk to waste time looking for caller ID information if it is not being
presented on the line.
Asterisk | 189