Telex - Teletypewriter EXchange

Teletypewriter EXchange

The Teletypewriter eXchange Service (TWX) was developed by the AT&T Corporation in the United States and originally ran at 45.45 baud or 60 words per minute, using five level Baudot code. AT&T began TWX on November 21, 1931. AT&T later developed a second generation of TWX called "four row" that ran at 110 baud, using eight level ASCII code. TWX was offered in both "3-row" Baudot and "4-row" ASCII versions up to the late 1970s.

TWX used the public switched telephone network. In addition to having separate Area Codes (510, 610, 710, 810 and 910) for the TWX service, the TWX lines were also set up with a special Class of Service to prevent connections from POTS to TWX and vice versa.

The code/speed conversion between "3-row" Baudot and "4-row" ASCII TWX service was accomplished using a special Bell "10A/B board" via a live operator. A TWX customer would place a call to the 10A/B board operator for Baudot – ASCII calls, ASCII – Baudot calls and also TWX Conference calls. The code / speed conversion was done by a Western Electric unit that provided this capability. There were multiple code / speed conversion units at each operator position.

AT&T operated a trade magazine related to the Teletypewriter eXchange service called TWX Magazine, from 1944 to 1952, and published articles which touched upon many aspects of the technology.

Western Union purchased the TWX system from AT&T in January 1969. The TWX system and the special area codes (510, 610, 710, 810 and 910) continued right up to 1981 when Western Union completed the conversion to the Western Union Telex II system. Any remaining "3-row" Baudot customers were converted to Western Union Telex service during the period 1979 to 1981.

The modem for this service was the Bell 101 dataset, which is the direct ancestor of the Bell 103 modem that launched computer time-sharing. The 101 was revolutionary, because it ran on ordinary unconditioned telephone subscriber lines, allowing the Bell System to run TWX along with POTS on a single public switched telephone network.