How FXO and FXS Works?

Foreign eXchange Subscriber (FXS) and Foreign eXchange Office (FXO) are the names of the two most common interfaces (ports or plugs) found in analog telephony architecture.

FXO and FXS in General Terms

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 is an interface points to the subscriber end. An FXS interface provides the following primary services to a subscriber device:

Dial Tone
Battery Current
Ring Voltage

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:
on-hook/off-hook indication (loop closure)

** 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.

FXO and FXS Devices in PBX / IP-PBX

We connect the FXO interface on a phone to the FXS port supplied by a PBX, multiplexer, or Voice-over-IP gateway or router.

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 onhook/off-hook indication (loop closure) to the local Telco network.
FXS - FXO Call Procedures

An FXS device initiates a call by presenting ring voltage over the line to the
attached FXO device. (FXS devices cannot pass dialed digits.)

An FXS device receives a call by . . .
1) Detecting the line has been seized (the attached telephone–FXO device–has gone off hook)
2) Receiving Dual-Tone Multi-Frequency (DTMF) digits indicating how the call should be routed.
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.

An FXO device initiates a call by . . .
1) Going off-hook to seize the telephone line.
2) Dialing the Dual-Tone Multi-Frequency (DTMF) digits, which identify the destination to be called

An FXO device receives a call by . . .
1) Detecting the ring voltage supplied by the FXS device (VoIP Gateway, PBX, etc.).
2) Going off-hook to answer the call.

FXO and FXS in Hybrid Telephony Architecture -->>

By:
Ashvini Gaur
LinkedIn Connect

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