System information

CHAPTER 5
User Device Configuration
I don’t always know what I’m talking about,
but I know I’m right.
—Muhammad Ali
In this chapter we’ll delve into the user devices that you might want to connect to
Asterisk, typically VoIP telephones of some sort. Configuring a channel in Asterisk for
the device to connect through is relatively straightforward, but you also need to con-
figure the device itself so it knows where to send its calls.
*
In other words, there are two
parts to configuring a device on Asterisk: 1) telling Asterisk about the device, and
2) telling the device about Asterisk.
How Asterisk Relates to the SIP Protocol
SIP is a peer-to-peer protocol, and while it is common to have a setup where endpoints
act as clients and some sort of gateway acts as a server, the protocol still thinks in terms
of peer-to-peer relationships. What this means is that a SIP telephone expects to make
a direct connection to another SIP telephone, without a PBX in between.
The reality is that many SIP transactions happen through a server, and in the case of
Asterisk, it is common to have the PBX in the middle of all connections. When a SIP
call is made from a telephone to another telephone through Asterisk, there are actually
two calls happening: one from the originating set to Asterisk, and another separate call
from Asterisk to the destination set. Asterisk bridges the two channels together.
From the perspective of the SIP telephone, therefore, you need to configure it to send
all its calls to Asterisk, even though the device is quite capable of directly connecting
to another SIP endpoint without the Asterisk server. The SIP protocol is complex and
very flexible, and configuring endpoints can seem difficult because they have much
more flexibility than we require of them for an Asterisk implementation.
* This has nothing to do with Asterisk configuration, and each hardware manufacturer will have its own tools
to allow you to configure its devices.
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