Implementing a Channel

Implementing a Channel

A channel is a unit which brings in a call to the Asterisk PBX.  A channelcould be connected to a real telephone (like the Internet Phone Jack) orto a logical call (like an Internet phone call).  Asterisk makes nodistinction between "FXO" and "FXS" style channels (that is, it doesn'tdistinguish between telephone lines and telephones).

Every call is placed or received on a distinct channel.  Asterisk uses achannel driver (typically named chan_xxx.so) to support each type ofhardware.

* What do I need to create a channel?

In order to support a new piece of hardware you need to write a channeldriver.  The easiest way to do so is to look at an existing channel driverand model your own code after it.

* What's the general architecture?

Typically, a channel reads a configuration file on startup which tells itsomething about the hardware it's going to be servicing.  Then, itlaunches a thread which monitors all the idle channels (See the chan_modemor the chan_ixj for an example of this).  When a "RING" or equivalent isdetected, the monitoring thread should allocate a channel structure andassign all the callbacks to it (see ixj_new, for example), and then callast_pbx_start on that channel.  ast_pbx_start will launch a new thread tohandle the channel as long as the call is up, so once pbx_start hassuccessfully been run, the monitor should no longer monitor that channel.The PBX thread will use the channel, reading, writing, calling, etc., andmultiplexing that channel with others using select() on the channel'sfile descriptor (if your channel doesn't have an associated filedescriptor, you'll need to emulate one somehow, perhaps along the lines ofwhat the translator API does with its channel.

When the PBX is finished with the line, it will hang up the line, at whichpoint it the hardware should again be monitored by the monitoring thread.

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