Foreign exchange service (telecommunications)


Foreign exchange service is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another exchange area, called the foreign exchange, rather than the local exchange area where the subscriber station equipment is located.
To call originators, it appears that the called party having the FX service is located in the foreign exchange area. It is assigned a telephone number of the foreign exchange.
The telecommunication circuit between central offices that implements foreign exchange service has complementary interface types at each end. At the foreign central office that provides the service, the interface is called the foreign exchange office end, and at the end where the subscriber station is connected, it provides the foreign exchange station interface.

Purpose

Basic telephony terminology distinguishes two types of offices: local and foreign. A local office is assigned a specific area, and all telephone services provided to that area originate from that central office. Each central office has a unique identifier. Until the 1960s, central offices usually had names, derived from locally distinct geographic or historical contexts. The office names were changed to three-digit numerical central office codes, prefixed to the local telephone number.
Customers who wanted a telephone number provided by a neighboring or remote telephone central office leased a "foreign exchange" line. With two-wire loop technology, this typically required an engineered circuit with increased costs. The practice, rare except in big cities, is in decline.
Foreign central office or foreign zone services were, from a technological standpoint, deployed with the same methods as foreign exchange. They differ only in that the remote office is in exactly the same rate centre or merely in a different zone of the same US metropolitan city. Much like FX service rates depend on the distance between rate centers, FCO service prices depend on the distance between exchanges.

Function

An FX line has the local calling area of the foreign exchange in which it is numbered.
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 subsidized by long-distance toll service for much of the 20th century. As an "FX line" has a number from the neighboring city, it has the city calling area for both incoming and outbound calls.
For instance, a suburban business may want to market extensively to Toronto, a large city with flat-rate local calling:
The "FX line" is usually treated as part of the distant city when originating calls to N11-style numbers, such as information or emergency telephone numbers.
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.
Originally, the FX line was a physical copper pair of telephone wires from the foreign exchange which were connected to the local subscriber loop at the local exchange, without passing through the local switch. This dedicated circuit is often replaced with a virtual circuit, where the local switch sends the FX calls to the foreign exchange on existing trunks.
In rare instances, the supposed "foreign" exchange actually resided on the same physical telephone exchange at the same location, but clients were billed based on nominal centre-to-centre distance between different rate centres.
A similar "FCO" service provided no difference in local calling area. Historically, it was a means to obtain features not available on the local exchange or keep an existing business telephone number operational after a cross-town move.
Conventional "foreign exchange" leased lines and their variants have become less common due to newer alternatives:

Circuit interfaces

The circuit that implements the foreign exchange service has two ends, one at the central office that provides the service in the foreign exchange, where the telephone number is assigned, and one at the central office that services the subscriber station. The former is called a foreign exchange office interface, and the latter a foreign exchange station interface. These two interface types perform complementary functions in signaling.

Foreign exchange office

The foreign exchange office interface is a telephone signaling interface that terminates the foreign exchange line at the central office that provides the telephone number and the call switching for the service. It generates the off-hook and on-hook indications through loop closure and non-closure of a direct current circuit powered by the serving central office switch.

Foreign exchange station

The foreign exchange station interface is located at the wire center of the subscriber equipment, and supplies battery power and dial tone, and generates ringing voltage toward the subscriber station of the foreign exchange service.

Use in voice-over-IP systems

Some of the terminology of the foreign exchange service is retained in modern digital packet telephony to indicate whether VoIP equipment is designed to be connected to telephone lines from a central office or to telephone stations.
FXO and FXS interfaces are available for computers and networking equipment to interface these directly with plain old telephone service systems.
An FXO device is any device that, from the point of view of a telephone exchange, appears to be a 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.
An FXS interface is any interface that functions like a standard telephone line jack on the wall. An FXS interface utilizes a line protocol, most commonly loop start, to detect when the terminating device goes on-hook or off-hook, and can send and receive voice signals.