fxofxs

There are two type of daughter cards that represent the type of device or line you are connecting to, this is a meager attempt to deobfuscate the acronym's:

Ok Mr. Peabody fire up the way back machine, these terms have been around for 75 years and are not Asterisk specific. I will not attempt to discuss the subtleties of non-US variants.

Both of these circuits work on a single pair of wires. These wires are called Tip and Ring. Sometimes abbreviated T&R.

Foreign Exchange Office:

This is a Line or a Trunk type port. It connects to a standard POTS (Plain Old Telephone Service) facility and allows trixbox to use that line as a trunk for originating (placing) or terminating (receiving) calls.

The acronym stands for Foreign Exchange Office. Foreign (as in outside the PBX) Exchange (a telephone switch) Office. Office is the telephone switching facility, it's slang "which office is your line served out of?"

Key points of a FXO line, it expects ringing and battery (DC telephone voltage -48vdc) to be present on the line. Normal signaling is loop start. A loop is simply what happens when you pick up the phone it makes an electrical loop telling the exchange you are off hook and want to make a call. Other types of start supported include ground start a format used in PBX trunks to indicate off hook condition. Ground Start trunks will not have a dial tone when you pick them up until you provide a momentary ground on the Tip side. The resistance tells the exchange that you want to make a call. Ground start circuits are more expensive than loop start and are usually measured service (pay as you talk) PBX rate.

Foreign Exchange Station:

A station is a standard phone that you would use with a POTS line. The FXO card allows you to use analog phones with trixbox. You can access all of the features of trix with * codes and hook flashes (momentary interruptions of loop generated with the flash button or your finger pressing the hook button). It's called a hook because early phones hung on the wall and the receiver was placed on the hook. This gives you two conditions "on hook" ready to receive a call and off hook which is busy. You will see these terms used often. To come off hook is to "seize" a circuit (yet another term).

When you are in a hotel room or a hospital and you pick up the phone, dial 9 for an outside line or 3 or 4 digit dial another extension that is an FXS or PBX station line. trixbox will provide dial tone, ringing, call waiting stutter (the interruption of dial tone to indicate you have a voice mail), call waiting tone in other words everything you expect a phone to do.

FXS ports are assigned to extensions in Asterisk just like SIP (IP) telephones. Each FXS will have it's own extension. You can call between the extensions.

Electrically an FXS port is a called a "wet" line because it has battery (voltage -48vDC on it). It is called battery because in the phone companies office the power on your phone lines is provided by large banks of batteries that keep the network supplied with power. Asterisk also rings the phone by providing "ring voltage" ~90vac @ 20hz. If you ever come in contact with a ringing phone circuit you will get a nice little zap.

Summary:

FXO: Connects trunk line to trixbox

FXS: Connects telephone set to trixbox

Now that you have the terms down strap on a butt set, put a punch tool in your back pocket and show some butt crack.