Porte FXS e FXO

Analog telephony, also known as Plain Old Telephone Service (POTS), is the service the local phone company typically delivers to your home. Local phone companies deliver POTS from their Central Office (CO) to the subscriber’s premises over a circuit consisting of two copper wires. To increase the distance over which the signal can be transmitted the two wires are twisted together,
which also reduces electromagnetic interference. So these two-wire copper cables are commonly known as “twisted pairs.”

Definitions
FXS - Foreign eXchange Subscriber interface (the plug on the wall) delivers POTS service from the local phone company’s Central Office (CO) and must be connected to subscriber equipment (telephones, modems, and fax machines). In other words an FXS interface points to the subscriber. An FXS interface provides the following primary services to a subscriber device:
l Dial Tone
l Battery Current
l Ring Voltage
You may also see the FXS acronym rendered as Foreign eXchange System.

FXO - Foreign eXchange Office interface (the plug on the phone) receives POTS service, typically from a Central Office of the Public Switched Telephone Network (PSTN). In other words an FXO interface points to the Telco office. An FXO interface provides the following primary service to the Telco network device:
l on-hook/off-hook indication (loop closure)

How it Works
Because of the characteristics described above, a telecommunications line from an FXO port must connect to an FXS port in order for the connection to work. Similarly, a line from an FXS port must connect to an FXO port in order for the connection to work. When the FXO port on your analog telephone is connected to the FXS port in the wall, you receive (FXS) service from the telephone company – and you hear a dial tone when you pick up the phone.
Common Usage
Within the telephony industry, a device is often referred to by the type of interface it provides (“your phone is an FXO device”), or even spoken of as being the interface (“your wall plug is FXS”). Now, continuing our discussion in common usage . . .
What Doesn’t Work
If you connect an FXS device to another FXS device, the connection will not work. Likewise, if you connect an FXO device to another FXO it will not work. So, for example, you can NOT plug a standard analog telephone (FXO) directly into a standard analog telephone (FXO) and talk phone-to-phone.

Networking
The FXS/FXO scenario becomes a bit more “interesting” when we introduce additional network
elements, such as a Private Branch Exchange (PBX) or a Voice-over-IP gateway or router. For
example, you can connect the FXO interface on a phone to the FXS port supplied by a PBX,
multiplexer, or Voice-over-IP gateway or router.
PBX Connections
A PBX provides both FXS and FXO interfaces.

FXS - When you connect a PBX to analog phones, you plug phone cables into FXS ports on the PBX. The FXS ports on the PBX provide POTS service, including battery current, ring voltage, and dial tone to the phones.
FXO - When you connect a PBX to the Telco Central Office, you plug the (FXS) lines from the phone company into FXO ports on the PBX. The FXO ports on the PBX provide on-hook/ off-hook indication (loop closure) to the local Telco network.