Foreign exchange service (FX) is an access service in a
telecommunications network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, message ...
in which a
telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
in a given exchange area is connected, via a
private line
In telecommunication, a private line is typically a telephone company service that uses a dedicated, usually unswitched point-to-point circuit, but it may involve private switching arrangements, or predefined transmission physical or virtual path ...
, as opposed to a switched line, to a
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
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
Call or Calls may refer to:
Arts, entertainment, and media Games
* Call, a type of betting in poker
* Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage
Music and dance
* Call (band), from Lahore, Paki ...
originators, it appears that the
called party
The called party (in some contexts called the "B-Number") is a person who (or device that) answers a telephone call. The person who (or device that) initiates a telephone call is the calling party.
In some situations, the called party may number m ...
having the FX service is located in the foreign exchange area. It is assigned a
telephone number
A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...
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 (FXO) end, and at the end where the subscriber station is connected, it provides the foreign exchange station (FXS) 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. The
Bell System
The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundr ...
established a unified set of
central offices prefixes after World War II. The
central offices usually had
names
A name is a term used for identification by an external observer. They can identify a class or category of things, or a single thing, either uniquely, or within a given context. The entity identified by a name is called its referent. A persona ...
, derived from locally-distinct geographic or historical contexts. Under the standardized number plan, each central office was assigned a three-digit number unique within each
area code
A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, rea ...
that was prefixed to the local telephone number.
The prefixes often still reflected the geography and had value in user's perception of the number, beyond the pure technical function of uniquely identifying the central office. Calls with a different prefix might incur additional charges, so businesses on one central office might want a number that is local call for customers of a different central office. Prefixes, since they related to geography, often carried the cachet of their neighborhoods; some central office prefixes were
immortalized in popular culture for that reason.
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 (FCO) or foreign zone (FZ) services were, from a technological standpoint, deployed with the same methods as foreign exchange (FX). They differ only in that the remote office is in exactly the same rate centre (FCO) or merely in a different zone of the same US metropolitan city (FZ). 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
Toronto ( ; or ) is the capital city of the Canadian province of Ontario. With a recorded population of 2,794,356 in 2021, it is the most populous city in Canada and the fourth most populous city in North America. The city is the ancho ...
, a large city with flat-rate local calling:
* If the business is in an adjacent suburb (such as
Mississauga
Mississauga ( ), historically known as Toronto Township, is a city in the Canadian province of Ontario. It is situated on the shores of Lake Ontario in the Regional Municipality of Peel, adjoining the western border of Toronto. With a popul ...
or
Markham Markham may refer to:
It may also refer to brand of of clothing which originates from South Africa which saw it's establishment in 1873.
Biology
* Markham's storm-petrel (''Oceanodroma markhami''), a seabird species found in Chile and Colombia
* ...
) a local number will reach the city but not the suburbs on the other side. Adding an FX line with a Toronto
+1-416 number would provide full coverage.
* If the business is located just outside the larger city's local calling area, an FX number in the next-closer suburb would provide a limited coverage of the city. An
Oshawa
Oshawa ( , also ; 2021 population 175,383; CMA 415,311) is a city in Ontario, Canada, on the Lake Ontario shoreline. It lies in Southern Ontario, approximately east of Downtown Toronto. It is commonly viewed as the eastern anchor of the G ...
business may lease an FX line from suburban
Ajax
Ajax may refer to:
Greek mythology and tragedy
* Ajax the Great, a Greek mythological hero, son of King Telamon and Periboea
* Ajax the Lesser, a Greek mythological hero, son of Oileus, the king of Locris
* ''Ajax'' (play), by the ancient Greek ...
as that community is local to both Toronto and Oshawa, even though Ajax does not have the full Toronto calling area.
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 number
Most public switched telephone networks have a single emergency telephone number (sometimes known as the universal emergency telephone number or the emergency services number) that allows a caller to contact local emergency services for assista ...
s.
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 (which handles all billing) on existing trunks.
In rare instances, the supposed "foreign" exchange actually resided on the same physical
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
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 (the distant exchange is in the same rate centre). Historically, it was a means to obtain features not available on the local exchange (such as
DTMF
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
tone dialling when first introduced in 1963)
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:
* 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
In telecommunications, remote call forwarding is a service feature that allows incoming calls to be forwarded to a remote call forwarding number, such as a cell phone or another office location and is designated by the call receiver.
Customers ma ...
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 coverage area could call locally, only to be silently redirected via a second local call to the destination.
*
Interactive voice response
Interactive voice response (IVR) is a technology that allows telephone users to interact with a computer-operated telephone system through the use of voice and DTMF tones input with a keypad. In telecommunications, IVR allows customers to interact ...
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
A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...
exchanges (in countries which use geographic numbers) normally are issued from the larger city and have that city's full calling area.
*
Voice over IP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of speech, voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms In ...
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 unrestricted
broadband Internet
In telecommunications, broadband is wide bandwidth data transmission which transports multiple signals at a wide range of frequencies and Internet traffic types, that enables messages to be sent simultaneously, used in fast internet connections. ...
is available at the site.
Local number portability
Local number portability (LNP) for fixed lines, and full mobile number portability (FMNP) for mobile phone lines, refers to the ability of a "customer of record" of an existing fixed-line or mobile telephone number assigned by a local exchange ca ...
allows an existing number to be moved to VoIP (or, in some countries, a
mobile telephone
A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...
) which can then be freely moved out of the original geographic location while keeping the directory listing and service area unchanged.
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 (FXO) interface, and the latter a foreign exchange station (FXS) interface. These two interface types perform complementary functions in signaling.
Foreign exchange office
The foreign exchange office (FXO) interface is a
telephone signaling interface
In telecommunications, an interface standard is a standard that describes one or more functional characteristics (such as code conversion, line assignments, or protocol compliance) or physical characteristics (such as electrical, mechanical, or op ...
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 In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
and
on-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
indications through loop closure and non-closure of a direct current (DC) circuit powered by the serving central office switch.
Foreign exchange station
The foreign exchange station (FXS) interface is located at the wire center of the subscriber equipment, supplying battery power and
dial tone
A dial tone is a telephony Signaling (telecommunication), signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is ...
, and generating
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
Plain old telephone service (POTS), or plain ordinary telephone system, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1 ...
(POTS) systems.
An FXO device is any device that, from the point of view of a
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
, appears to be a telephone. As such, it should be able to accept
ringing signal
A ringtone, ring tone or ring is the sound made by a telephone to indicate an incoming call. Originally referring to and made by the electromechanical striking of bells, the term now refers to any sound on any device alerting of a new incoming ...
s, go on-hook and off-hook, and send and receive
voice frequency
A voice frequency (VF) or voice band is the range of audio frequencies used for the transmission of speech.
Frequency band
In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. It is for this reason tha ...
signals. It may use
loop start
Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the ...
or
ground start {{refimprove, date=January 2008
In telephony, ground start is a method of signaling from a terminal of a subscriber local loop to a telephone exchange, where one side of a cable pair is temporarily grounded to request dial tone. Most middle 20th-ce ...
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
Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the ...
, to detect when the terminating device (telephone) goes
on-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
or
off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
, and can send and receive voice signals.
See also
*
Telephone line
A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...
*
Telephone signaling interfaces
References
*
External links
Differences between FXS and FXO
{{DEFAULTSORT:Foreign Exchange Service (Telecommunications)
Telephone exchanges
Communication circuits
Telephony signals