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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.
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 ...
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]
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 ...
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.
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Multiline hunting (sometimes MLH, line hunting or MHG, multiline hunting group) is a feature that allows multiple telephone lines going into a business to act as a single group, called a hunt group. This type of fallback is a somewhat more complex form of call forwarding. If the line called is busy, the call goes to the next available line.