{{Unreferenced, date=December 2009
Nortel
Nortel Networks Corporation (Nortel), formerly Northern Telecom Limited, was a Canadian multinational telecommunications and data networking equipment manufacturer headquartered in Ottawa, Ontario, Canada. It was founded in Montreal, Quebec, ...
's Switch56 was a
networking protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...
built on top of the telephone cabling hardware of their
Digital Multiplex System
Digital Multiplex System (DMS) is the name shared among several different telephony product lines from Nortel Networks for wireline and wireless operators. Among them are the DMS-1 (originally named the DMS-256) Rural/Urban digital loop carrier ...
and other
telephone switch
telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...
es.
The name comes from the fact that Switch56 carried 56 kbit/s of data on its 64 kbit/s lines, as opposed to most systems, including
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
, where the entire 64 kbit/s bandwidth was available for data. The speed was a side effect of Nortel using a 2-wire cable to carry both voice and switching commands, as opposed to other systems 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
The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls.
The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell Syste ...
supervision scheme. This data was split out at the "far end" as 56 kbit/s and 8 kbit/s subchannels.
Switch56 was built on top of the basic Nortel hardware to allow computers to put data into the existing telephony network. Although slow compared to even contemporary systems, Switch56 allowed network traffic to flow not only within an office like other
LAN systems, but between any branch offices that were connected using a Nortel
PBX like the
Meridian Norstar
The Nortel Norstar, previously the Meridian Norstar, was a small and medium-sized business digital key telephone system introduced by Nortel (formerly Northern Telecom) and later sold to Avaya. It featured automatic call distribution, and supp ...
. This was a much easier option to install than ISDN for most offices, requiring nothing more than a Switch56 bridge to their existing network. For the LAN role new telephone terminals were produced with a
RS-232C
In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such ...
port on the back, which were then plugged into the user's computer and used with custom software. Although interesting in theory, it appears Switch56 saw little use in this role.
Network protocols
Telephone exchange equipment