Class 4 telephone switch
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A Class 4 or Tandem switch is U.S. telephone company (telco) central office telephone exchange used for long distance communications in the Public Switched Telephone Network (PSTN) to interconnect telephone company offices. In contrast, a Class 5 switch or end-office is primarily used to terminate local calls, in other words connect to the customer (also known as the subscriber). A tandem being a telephone switch to which no telephones are wired, its services were termed non-customer facing. Other terms are Toll Centers (TC), if operators are present, otherwise they are Toll Points (TP). Class 4 switches at that time often had an associated Traffic Service Position System (TSPS) to handle operator calls. After the Bell System Divestiture the class was called Access Tandems (AT).
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[edit] Overview
Tandem is a horse tack term referring to harnessing two beasts of burden in line rather than abreast. It is applied to telephone switching in the sense of, "to connect in series" because a tandem switch connects telecommunications trunks end-to-end, analogous to a series circuit. A tandem switch is an intermediate switch or connection between an originating telephone call or location, and the final destination of the call.
The majority of Bell System Class 4 switching in the 1950s and 60s was done by the CrossBar Tandem (XBT) variant of 1XB switch and the four-wire 4XB. Like other switches, modern Tandem switches are for Time-division multiplexing (TDM)-based, circuit-switched telephone calls. Tandems were more quickly converted to TDM than end-offices were, so in the mixed time division and space division environment of the 1980s and early 90s a tandem was sometimes called a "TDM switch."
In the past, most of the accounting, billing management and call recording was handled by tandem switches, also called "toll switches." During the last third of the 20th century such services were taken over by end-office switches.
[edit] Sector and Access Tandems
A sector tandem switch connects end offices for intra-Local access and transport area (LATA) traffic.
An access tandem switch provides the connection between end-offices and the POPs for IntereXchange Carriers (IXCs).
During the 1980s, Class 4 tandem switches were converted to deal only with high-speed, T1, T3, OC-3 and other four-wire circuit connections, in contrast to two-wire local lines on Class 5 end-office switches. By the turn of the century, almost all other switches also supported four-wire connections, so this contrast no longer applies.
Switches performing tandem services could be at the center of the LATAs they serve, providing both traditional sector tandem services (intra-LATA) between local end-offices and access tandem services to IXCs (inter-LATA).
[edit] 4ESS
One Tandem Switch is the 4ESS, a Lucent Technology switch designed for AT&T long distance communications. The DMS-250 variant of DMS-100 is similarly limited to tandem services only, but currently, most Class 5 switches can conduct both local and long distance phone services. In other words, the functions of a Tandem switch are integrated into a Class 5 switch, if the owners wish.
