Asterisk: The Future of Telephony, 2nd Edition

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:

fxoks=1
fxsks=2
loadzone=us
defaultzone=us

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

Zaptel Configuration
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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