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What is the difference between FXS and FXO ports

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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 environments. 

An FXS interface supplies ring, voltage, and dial tone for basic telephone equipment, keysets, and PBXs. An FXO interface is used for trunk, or tie line, connections to a PSTN CO or to a PBX that does not support E&M signaling (when local telecommunications authority permits). This interface is of value for off-premise station applications.

FXO and FXS interfaces indicate on-hook or off-hook status and the seizure of telephone lines by one of two access signaling methods: loop start or ground start. The type of signaling is determined by the type of service from the CO; standard home telephone lines use loop start, but business telephones can order ground start lines instead.

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 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 wisted pairs.

FXS - Foreign eXchange Subscriber interface (the plug on the wall) delivers POTS service from the local phone company 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:

Dial Tone, Battery Current and 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:

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

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.

Within the telephony industry, a device is often referred to by the type of interface it provides (our phone is an FXO device?, or even spoken of as being the interface (our wall plug is FXS?. Now, continuing our discussion in common usage . . .

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.

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.

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.

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 as 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.

FXS - FXO Call Clearing

Under normal circumstances an FXS device does not initiate call clearing. Instead, FXS devices rely on the two parties at each end of the call to recognize the call has ended (by saying goodbye or hearing the line go quiet); then the FXO device at each end clears its segment of the call.


You will come across the terms FXS and FXO when deciding to buy equipment that allows you to connect analog phones to a VoIP Phone system, or traditional PBXs to a VOIP service provider or to each other via the Internet.

An 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;

An FXS gateway is used to connect one or more lines of a traditional PBX to a VoIP phone system, or provider. Alternatively, you can use it to connect analog phones to it and re-use your analog phones with a VoIP phone system. 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.

Read 19737 times Last modified on Friday, 23 September 2011 09:41