In telecommunication, foreign exchange service (FX) is a network-provided service in which a telephone in a given exchange area is connected, via a private line, to a central office in another foreign exchange, rather than the local exchange area where the device is located.
To call originators, it appears that the called party having the FX service is located in the foreign exchange area.
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In basic telephony there are two types of offices: local and foreign. A local office was assigned a specific area, and all telephone services provided to that area came from that central office. Each central office had its unique identifier. In the early days names were used, such as "Jackson" or "Newton". The office names were changed to three-digit numerical exchange codes (NNX), prefixed to the local phone number (not the area code).
Customers who want a telephone number provided by a neighboring telephone central office, lease a "foreign exchange" line. With the old two-wire loop technology, this would require an engineered circuit with increased costs. The practice is rare except in big cities.
In telecommunications, Foreign eXchange Office, or FXO, designates a telephone signaling interface that receives POTS, or "plain old telephone service". It generates the off-hook and on-hook indications (loop closure/non-closure) at the FXS's end of a telephone circuit. Analog telephone handsets, fax machines and (analogue) modems are FXO devices, though the term is rarely used except in connection with Foreign exchange service (FX).
FXO interfaces are also available for computers and networking equipment, to allow these to interact directly with POTS systems. These are commonly found in devices acting as gateways between local Voice over Internet Protocol (VoIP) systems and the public switched telephone network (PSTN).
An FXO device is any device that, from the point of view of a telephone exchange, appears to be a regular telephone. As such, it should be able to accept ringing signals, go on-hook and off-hook, and send and receive voice frequency signals. It may use loop start or ground start signaling. FXO channel units were invented and named in the middle 20th century for service at the "Office" end of an FX line via carrier system.
In telephony, a Foreign eXchange Station, or FXS, is a telephone interface that supplies battery power, provides dialtone, and generates ringing voltage. A device that connects to such an interface contains an Foreign exchange office (FXO) interface and could be a standard analog telephone or a private branch exchange (PBX) to receive telephone service.
Any telephone exchange is an example of an FXS, as is the telephone jack on the wall, though the term is rarely applied except in connection with foreign exchange service.
An FXS interface utilizes an FXO protocol to detect when the terminating device (telephone) goes on-hook or off-hook, and can send and receive voice signals.
An FXS interface provides service at the "station" end of a foreign exchange line.