The configuration of an FXS channel is similar to that of an FXO channel. Let’s take a look.
Recall from our earlier discussion that the opposite type of signaling is used for FXO and FXS channels, so we will be configuring FXO signaling for our FXS channel. In the example below we are configuring channel 1 to use FXO signaling, with the kewlstart signaling protocol:
fxoks=1 fxsks=2 loadzone=us defaultzone=us
After loading the drivers for your hardware, you can verify
their state with the use of
Zaptel Configuration ====================== Channel map: Channel 01: FXO Kewlstart (Default) (Slaves: 01) Channel 02: FXS Kewlstart (Default) (Slaves: 02) 2 channels configured.
The following configuration is identical to that for the FXO
channel, with the addition of a section for our FXS port and of the line
immediate=no. The context for our FXS port
internal, the signaling is
fxoks (kewlstart), and the channel
number is set to
FXS channels can be configured to perform one of two different
actions when a phone is taken off the hook. The most common (and often
expected) option is for Asterisk to produce a dial tone and wait for
input from the user. This action is configured with
immediate=no. The alternative action is for
Asterisk to automatically perform a set of instructions configured in
the dialplan instead of producing a dial tone, which you indicate by
immediate=yes. The instructions to be performed are found in the
context configured for the channel and will match the
s extension (both of these topics will be
discussed further in the following chapter).
Here’s our new zapata.conf:
[trunkgroups] ; define any trunk groups [channels] ; hardware channels ; default usecallerid=yes hidecallerid=no callwaiting=no threewaycalling=yes transfer=yes echocancel=yes echotraining=yes immediate=no ; define channels context=internal ; Uses the [internal] context in extensions.conf signalling=fxo_ks ; Use FXO signalling for an FXS channel channel => 1 ; Telephone attached to port 1 context=incoming ; Incoming calls go to [incoming] in extensions.conf signalling=fxs_ks ; Use FXS signalling for an FXO channel channel => 2 ; PSTN attached to port 2
To test our newly created Zap extension, we need to create a
basic dialplan. The following dialplan contains a context called
internal. This is the same context
name that we configured in zapata.conf for
channel 1. When we configure
zapata.conf, we are telling Asterisk where to
look for instructions when a user presses digits on his telephone. In
this case, the only extension number that will work is 611. When you
dial 611 on your telephone, Asterisk will execute the
Echo() application so that when you talk
into the phone whatever you say will be played back to you, thereby
verifying bidirectional voice.
The dialplan looks like this:
[internal] exten => 611,1,Answer() exten => 611,2,Echo()
 Also referred to as the BatPhone method, and more formally known as an Automatic Ringdown or Private Line Automatic Ringdown (PLAR) circuit . This method is commonly used at rental car counters and airports.