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Centrex is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises. [1] Centrex service was first installed in the early 1960s in New York's financial district by New York Telephone.
The voice switching fabric plan was similar to that of the earlier 5XB switch in being bidirectional and in using the call-back principle. [clarification needed] [citation needed] The largest full-access matrix switches (the 12A line grids had partial access) in the system, however, were 8x8 rather than 10x10 or 20x16.
DCO-SE (Line Switch 10,000 lines, software enhanced, class features, centrex, etc.) DCO-RLS (Remote Line Switch, controlled by hosts above) RLS 1000 (Pedestal mount cabinet 1000 lines in a subdivision - required multiple T-1's to host)
5ESS uses a time-space-time (TST) topology in which the Time-Slot-Interchangers (TSI) in the Switching Modules assign each phone call to a time slot for routing through the CM. CMs perform time-divided switching and are provided in pairs; each module (cabinet) belonging to Office Network and Timing Complex (ONTC) 0 or 1, roughly corresponding ...
As CLASS was an AT&T trademark, the term vertical service code was adopted by the North American Numbering Plan Administration. The use of vertical is a somewhat dated reference to older switching methods and the fact that these services can only be accessed by a telephone subscriber, going up ( vertically ) inside the local central office ...
Also taking advantage of the superior versatility of 5XB, Centrex was invented as a service package. Later stored program control exchanges allowed more extensive service features. Autovon originally used a four-wire version of 5XB, with a more complex marker to implement its nonhierarchical polygrid routing system.
A typical volume of Bell System Practices from the 1970s. The Bell System Practices (BSPs) is a compilation of technical publications which describes the best methods of engineering, constructing, installing, and maintaining the telephone plant of the Bell System under direction of AT&T and Bell Telephone Laboratories. [1]
From the 1960s, a simulated PBX, known as Centrex, provided similar features from the central telephone exchange. A PBX differs from a key telephone system (KTS) in that users of a key system manually select their own outgoing lines on special telephone sets that control buttons for this purpose, while PBXs select the outgoing line automatically.
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