System information

the relevant channel configuration file and the extensions.conf file play a role in most
calls routed through the system. Figure 5-1 provides a graphical representation of the
relationship between the sip.conf and extensions.conf files.
When a call comes into Asterisk, the identity of the incoming call is matched in the
channel configuration file for the protocol in use (e.g., sip.conf). The channel configu-
ration file also handles authentication and defines where that channel will enter the
dialplan.
Once Asterisk has determined how to handle the channel, it will pass call control to
the correct context in the dialplan. The context parameter in the channel configuration
file tells the channel where it will enter the dialplan (which contains all the information
about how to handle and route the call).
Figure 5-1. Relationship of sip.conf to extensions.conf
Conversely, if the dialplan has been programmed to dial another device when the re-
quest for extension number 101 is being processed, a request to dial telephony device
0000FFFF0002 will use the channel configuration file to determine how to pass the call
back out of the dialplan to the telephone on the network (including such details as
authentication, codec, and so forth).
A key point to remember is that the channel configuration files control not only how
calls enter the system, but also how they leave the system. So, for example, if one set
calls another set, the channel configuration file is used not only to pass the call through
to the dialplan, but also to direct the call from the dialplan to the destination.
sip.conf
The SIP
#
channel module is arguably the most mature and feature-rich of all the channel
modules in Asterisk. This is due to the enormous popularity of the SIP protocol, which
#The SIP RFC is a long read, but about the first 25 pages are a good introduction. Check it out at http://www
.ietf.org/rfc/rfc3261.txt.
Configuring Asterisk | 89