What is FXO and FXS ?

FXO and FXS are the ports used by the analog phone or known as POTS ( Plain Old Telephone Service )

FXS, known as Foreign Exchange Subscriber interface, is the port which actually provide analog line to the subscriber. It is the "socket on the wall" which delivers a dialtone, battery current and ring voltage.

FXO, known as Foreign Exchange Office interface, that deliver the analog line. It is the port on your analog telephone/fax machine incoming port. It delivers an on-hook/off-hook indication (loop closure).

How does it works, technically ?

The FXO and FXS is always working on a pair. A FXO port is always connected to a FXS port or vice versa.

Technically, when you wish to make an outbound call :

1. You pick up the phone from FXO devices. Then FXS port will detect that it is OFF-HOOK.
2. When you dial a number, a DTMF ( Dual-Tone-Multi-Frequency ) digits will be passed to FXS port.

When there is an inbound call :

1. When FXS received an incoming call, it will generate a DC ring voltage to the FXO device.
2. The FXO device ( phone ) will rings.
3. You pick up the phone, and line is established.

Note: The analog phone line passes approximately 50 volts DC power to the FXS port. That’s why you get a faint ‘shock’ when you touch a connected phone line. This allows a call to be made in the event of a power cut.

Type of connections

Without PBX system ( i.e. home land line )

A phone is connected directly to the FXS port ( on the wall jack ) which provided by your landline company.

With PBX system

If you have a PBX, then you connect the lines provided by the landline company to the PBX and then the phones to the PBX. Therefore, the PBX must have both FXO ports (to connect to the FXS ports provided by the telephone company) and FXS ports (to connect the phone or fax devices to).

With FXO Gateway

To connect analog phone lines to an IP phone system, you need an FXO gateway. This allows you to connect the FXS port to the FXO port of the gateway, which then translates the analog phone line to a VOIP call.

With FXS Gateway

An FXS gateway is used to connect one or more lines of a traditional PBX to a VOIP phone system or provider. You need an FXS gateway because you want to connect the FXO ports (which normally are connected to the telephone company) to the Internet or a VOIP system.

With ATA adaptor ( FXS adaptor )

  An FXS adapter is used to connect an analog phone or fax machine to a VOIP phone system or to a VOIP provider. You need this because you need to connect the FXO port of the phone/fax machine to the adapter.

Source from 3CX