Foreign Exchange Office interface is the port that receives the analog line. It is the plug on the phone or fax machine, or the plug on analog phone system. Foreign exchange Office interface receives POTS service from Central Office of the Public Switched Telephone Network (PSTN) .An FXO interface provides the following primary service to the Telco network device:
- On-hook/off-hook indication (loop closure)
FXO and FXS interface go hand in hand to complete a connection and are always paired, i.e. similar to a male / female plug.
If we connect an FXS device to another FXS device, the connection will not work. Likewise, if FXO device is connected to another FXO it will not work. Hence, one cannot plug a standard analog telephone (FXO) directly into a standard analog telephone (FXO) and talk phone-to-phone. 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.
Detailed difference between FXS and FXO is available on this link – “FXS vs FXO”
FXO Call procedures –
Call Initiation—An FXO device initiates a call by:
- Going off-hook to seize the telephone line.
- Dialing the Dual-Tone Multi-Frequency (DTMF) digits, which identify the destination to be called.
Call Reception—An FXO device receives a call by:
- Detecting the ring voltage supplied by the FXS device
- Going off-hook to answer the call.
Line Power – FXS devices supply approximately 50 volts DC power to the line. During an emergency, FXO devices can use FXS line voltage for power in order to remain operable in the event of a local electrical power failure.