Configuring an FXS Channel for an Analog Telephone

The configuration of an FXS channel is similar to that of an FXO channel. Let’s take a look.

Zaptel Hardware Configuration

The following is a minimal configuration for an FXS channel on a TDM400P. The configuration is identical to the FXO channel configuration above, with the addition of fxoks=1.

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:


After loading the drivers for your hardware, you can verify their state with the use of /sbin/ztcfg -vv:

Zaptel Configuration

Channel map:

Channel 01: FXO Kewlstart (Default) (Slaves: 01)
Channel 02: FXS Kewlstart (Default) (Slaves: 02)

2 channels configured.

Zapata Hardware Configuration

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 is phones, the signaling is fxoks (kewlstart), and the channel number is set to 1.

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 configuring immediate=yes.[56] 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:

; define any trunk groups

; hardware channels
; default

; define channels
context=phones       ; Uses the [internal] context in extensions.conf
signalling=fxo_ks       ; Uses 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

Dialplan Configuration

We will make use of our minimal dialplan we configured earlier in the chapter to test our FXS port with the use of the Echo() application. The relevant section, which should already exist in your dialplan, looks like this:

exten => 500,1,Verbose(1|Echo test application)
exten => 500,n,Echo()
exten => 500,n,Hangup()

include => internal

Whatever you say, the Echo() application will relay back to you.

[56] 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.