Switch56

Article summary

Nortel's Switch56 was a networking protocol built on top of the telephone cabling hardware of their Digital Multiplex System and other telephone switches. It was also offered by most other digital telephony suppliers, despite there being no official standards. Service was widely resold under a number of names by telecommunications companies that had large investments in digital backbones. Bell Atlantic called their version "Switched 56", while Southwestern Bell called theirs "MicroLink 1", for instance. The term switched 56 became common to refer to the concept in general. The name is in two parts. The "switched" refers to the fact that the system uses on-demand dial-up connections, which are routed through the public telephone switching system. The 56 refers to the fact that it carried 56 kbit/s of data on its 64 kbit/s lines, as opposed to most systems, including ISDN, where the entire 64 kbit/s bandwidth was available for data. This particular speed was a side effect of Nortel using an early 2-wire protocol to carry both voice and switching commands, as opposed to other systems like AT&T's Signalling System No. 7 where the command data was carried on a separate set of low-speed lines. Switch56 "folded" the two sources of data into one, placing a single bit from the command channel onto the end of every 7 bits of data, similar to the original T-carrier supervision scheme.…

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