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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.
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]
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.
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.
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 ...
The first version of the new hand telephone sets was marked by AT&T as D-76869, a specification type for small quantities of new equipment. The device was described by George K. Thompson of the American Telephone and Telegraph Company (AT&T) in United States patent No. 1508424 of 1924, without showing the handset that was still in development. [10]
In the early 1990s it was further upgraded to use the Motorola 88100 and 88110 Reduced Instruction Set Computing (RISC) CPUs. This RISC version of the SuperNode Computing Module was known as the BRISC (BNR Reduced Instruction Set Computing) CPU. With the BRISC CPU the DMS SuperNode had a processing capacity of 1,500,000 call attempts per hour. [5]
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.