A class-4, or tandem, telephone switch is a U.S.

telephone company A telephone company, also known as a telco, telephone service provider, or telecommunications operator, is a kind of communications service provider (CSP), more precisely a telecommunications service provider (TSP), that provides telecommunicat ...

central office

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 ...

used to interconnect local exchange carrier offices for long distance communications in the

public switched telephone network The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public Telecommunications, telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that ...

. A class-4 switch does not connect directly to telephones; instead, it connects to other class-4 switches and to

class-5 telephone switch {{No footnotes, date=August 2008 A class-5 telephone switch is a telephone exchange in the public switched telephone network (PSTN) that directly serves subscribers and manages subscriber calling features. Class-5 services include basic dial-tone, ...

es. The telephones of service subscribers are wired to class-5 switches. When a call is placed to a telephone that is not on the same class-5 switch as the originating subscriber, the call may be routed through one or more class-4 switches to reach its destination.

Etymology

Tandem Tandem, or in tandem, is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction. The original use of the term in English was in ''tandem harness'', which is used for two ...

'' derives from the Latin adverb ''tandem'' meaning ''at length'', and is used in English to mean a group of two people or machines working together, usually in series. A tandem switch is used to interconnect other switches via trunks. Thus, tandem switches are always part of a series of switches and lines that connect telephone callers to each other.

Sector and access tandems

A sector tandem switch connects local telephone exchanges (class-5 switches) and carries traffic within the

local access and transport area Local access and transport area (LATA) is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the Modification of Final Judgment (MFJ) entered by the United States District ...

(LATA). An access tandem switch connects local telephone exchanges to long-distance telephone companies (or ''

interexchange carrier An interexchange carrier (IXC), in U.S. legal and regulatory terminology, is a type of telecommunication company, commonly called a long-distance telephone company. It is defined as any carrier that provides services across multiple local access ...

s'', "IXCs"). The point at which an access tandem connects to the IXC's switch is called the ''

point of presence A point of presence (PoP) is an artificial demarcation point or network interface point between communicating entities. A common example is an ISP point of presence, the local access point that allows users to connect to the Internet with their ...

'', or POP. Modern tandem switches are often located at the center of the areas they serve, and may act as both sector tandems and access tandems.

History

Before the

Bell System divestiture The breakup of the Bell System was mandated on January 8, 1982, by an agreed consent decree providing that AT&T Corporation would, as had been initially proposed by AT&T, relinquish control of the Bell Operating Companies, which had provided loc ...

, class-4 switches in a telephone office that had

operators Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...

present were called "toll centers." If no operators were present, they were called "toll points." Either type of class-4 switch might be referred to as a "toll switch." These terms were used because long-distance, or "toll," calls had to pass through class-4 switches, where the billing for the calls would be handled. Class-4 switches at that time often had an associated

Traffic Service Position System The Traffic Service Position System (TSPS) was developed by Bell Labs in Columbus, Ohio to replace traditional cord switchboards. The first TSPS was deployed in 1969 and used the Stored Program Control-1A CPU, "Piggyback" twistor memory (a prop ...

(TSPS) to handle operator-assisted calls. TSPS automated many functions previously handled by the local operator with a "cordboard" telephone switch, such as certain aspects of coin-operated telephone calls. It also allowed the telephone company to route operator calls to remote locations, rather than requiring operators at each switch. After the divestiture, as human operators became less common, the terms changed. Today, a class-4 switch that connects class-5 switches to the long-distance network is called an "access tandem." A class-4 switch that connects class-5 switches to each other, but not to the long-distance network, is called a "local tandem." The majority of class-4 switches in the Bell System during the 1950s and 1960s used

crossbar switch In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of int ...

es, such as the Crossbar Tandem (XBT) variant of the

Number One Crossbar Switching System The Number One Crossbar Switching System (1XB), was the primary technology for urban telephone exchanges served by the Bell System in the mid-20th century. Its switch fabric used the electromechanical crossbar switch to implement the topology of ...

, or ''1XB switch''. The Number 4 Crossbar ("4XB") tandem switch was used in the North American toll network from 1943 until the 1990s, when it was replaced by more modern digital switching equipment, such as the Lucent 4ESS switch or the Nortel DMS-200. The last 4XB switch in the United States was installed in 1976. During the 1980s, class-4 tandem switches were converted to deal only with high-speed digital

four-wire circuit In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its n ...

connections: T1, T3,

OC-3 Optical Carrier transmission rates are a standardized set of specifications of transmission bandwidth for digital signals that can be carried on Synchronous Optical Networking (SONET) fiber optic networks. Transmission rates are defined by rate of t ...

, etc. The two-wire local line connections to individual telephones were relegated to the class-5 switches. By the dawn of the 21st century, almost all other switches also supported four-wire connections. Modern tandem switches, like other classes of telephone switch, are digital, and use

time-division multiplexing Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...

(TDM) to carry circuit-switched telephone calls. Tandems were more quickly converted to TDM than the class-5 end-offices were. During the transition to digital switching in the 1980s and 1990s, when both TDM and traditional "space division""Space division" is a

retronym A retronym is a newer name for an existing thing that helps differentiate the original form/version from a more recent one. It is thus a word or phrase created to avoid confusion between older and newer types, whereas previously (before there were ...

used to distinguish traditional telephone trunk lines—where a call would fully occupy a set of wires within a "trunk," or bundle of wires, between switches—from the new TDM trunks, where more than one call could be placed on a pair of wires by digitizing the call and sending the data for each call in pre-defined "timeslots" assigned to the call. switches were in use, American phone company employees often referred tandems as "TDM switches" as a result. In the past, most of the

accounting Accounting, also known as accountancy, is the measurement, processing, and communication of financial and non financial information about economic entities such as businesses and corporations. Accounting, which has been called the "languag ...

, billing management, and call record-keeping was handled by the tandem switches. During the last third of the 20th century, these tasks were performed by the class-5 end-office switches.

Switching equipment

* The Lucent 4ESS is a digital switch widely used as a class-4 switch in the United States. It was developed by AT&T's

Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...

division, before that division was spun off as Lucent. The first 4ESS was installed in

Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interactive Map of Chicago , coordinates = , coordinates_footnotes = , subdivision_type = Country , subdivision_name ...

in 1976. The last 4ESS in the AT&T Long Lines network was installed in 1999. In the late 2010s and early 2020s, the traditional 4ESS switch is slowly being replaced by the Nokia N4E (New 4ESS) switch in the AT&T Long Distance network. * The Lucent 5ESS, a class-5 switching system, is sometimes used as a class-4 switch (or as a mixed class-4/5 switch) in markets that are too small to justify a 4ESS switch. * The Nortel DMS-250, a larger variant of the

DMS-100 The DMS-100 is a member of the Digital Multiplex System (DMS) product line of telephone exchange switches manufactured by Northern Telecom. Designed during the 1970s and released in 1979, it can control 100,000 telephone lines. The purpose of th ...

, is a popular competitor to Lucent's 4ESS, especially among telephone companies that were not previously a part of AT&T. Other DMS switches can also be used as tandems. * The Nortel SP1 4-Wire was an early electronic switch used as a class-4 switch. Other class-5 digital switches are often used as class-4 switches for smaller applications.

Footnotes

References

External links

Definition of "tandem switch" in PC Magazine Encyclopedia
Telephone exchange equipment