Asterisk: The Future of Telephony, 2nd Edition

Configuring an FXO Channel for a PSTN Connection

We’ll start by configuring an FXO channel. First we’ll configure the Zaptel hardware, and then the Zapata hardware. We’ll set up a very basic dialplan, and we’ll show you how to test the channel.

Zaptel Hardware Configuration

The zaptel.conf file located in /etc/ is used to configure your hardware. The following minimal configuration defines an FXO port with FXS signaling:

fxsks=2
loadzone=us
defaultzone=us

In the first line, in addition to indicating whether we are using FXO or FXS signaling, we specify one of the following protocols for channel 2:

• Loop start (ls)

• Ground start (gs)

• Kewlstart (ks)

The difference between loop start and ground start has to do with how the equipment requests a dial tone: a ground-start circuit signals the far end that it wants a dial tone by momentarily grounding one of the leads; a loop-start circuit uses a short to request a dial tone. Though not common for new installations, analog ground start lines still exist in many areas of the country.^[53] Ground start is really a rather strange thing, because it doesn’t exist in its analog form in Asterisk, so technically, there is no ground signal happening, but is rather a signaling bit that is intended for analog circuitry that historically would have been at the end of the T1. If this does not make much sense, don’t sweat it; chances are you won’t have to worry about ground-start signaling. All home lines (and analog telephones/modems/faxes) in North America use ...