Foreign exchange service (telecommunications)
Foreign exchange service (FX) is a telecommunications network service in which a telephone in a given exchange area is connected, via a private line, to a telephone exchange or central office in another foreign exchange, rather than the local exchange area where the device is located.
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 wanted a telephone number provided by a neighbouring telephone central office leased 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.
A subscriber located just outside the exchange boundary of a large city, or just outside the flat-rate local calling area for the city, would find that many numbers which would have been local from the city itself became long-distance. In many areas, local flat-rate service was subsidised by artificially-expensive long-distance toll service for much of the 20th century. As an "FX line" is a number from the neighbouring city, it has the full big-city calling area for both incoming and outbound calls.
While a cost of hundreds of dollars monthly for the leased line was not uncommon, to a business handling large volumes of calls from the larger city the cost may have been justified by long-distance toll savings at a time when long-distance was pricey and alternatives were limited.
Conventional "foreign exchange" leased lines have become less common due to newer alternatives:
- An outbound "extender" is an automated local number at a service bureau in the larger city. A suburban subscriber (who can call the city itself locally but is long distance to suburbs on the other side) could call the extender locally, get a city dial tone and dial back out locally to the larger area.
- Remote call forwarding served a similar function for inbound calls only. A suburban business could get a downtown big-city number; clients anywhere in the larger city's local area could call locally, only to be silently redirected via a second local call to the destination.
- Interactive voice response systems have been hosted at answering service bureaux for clients such as suburban radio stations accepting calls from listeners in the larger city. As the machine is on a city number, it is reachable from the full metropolitan calling area.
- Mobile telephone exchanges normally are issued from the larger city and have that city's full calling area.
- Voice over IP numbers may be obtained from most cities and used almost anywhere in the world. VoIP renders the subscriber's physical location meaningless, as long as broadband Internet is available at the site.
Some terminology from the original "FX line" service is retained on voice over Internet equipment, such as "FXS" and "FXO" (foreign exchange station and foreign exchange office) to indicate whether VoIP equipment is designed for connection to telephones (stations) or telephone lines (office).
Foreign exchange office
In telecommunications, foreign exchange office (FXO) designates a telephone signaling interface that generates the off-hook and on-hook indications (loop closure/non-closure) at the Foreign exchange station's (FXS) 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).
In a nutshell
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.
Foreign exchange station
In telephony, a Foreign exchange station (FXS), is a telephone interface that supplies battery power, provides dial tone, and generates ringing voltage. A device that connects to such an interface contains a foreign exchange office (FXO) interface and could be a standard analog telephone or a private branch exchange (PBX) to receive telephone service.
An FXS interface provides service at the "station" end of a foreign exchange line.