Teletypewriter eXchange

The telex network is a switched network of teleprinters similar to a telephone network, for the purposes of sending text-based messages. The term refers to the network, not the teleprinters; point-to-point teleprinter systems had been in use long before telex exchanges were formed starting in the 1930s. Teleprinters evolved from telegraph systems, and like the telegraph they used the presence or absence of a pre-defined level of current to represent the mark or space symbols. This is as opposed to the analog telephone system, which used differing voltages to encode frequency information. For this reason, telex exchanges were entirely separate from the telephone system, with their own signalling standards, exchanges and system of "telex numbers" (the counterpart of a telephone number). When telephone and telex exchange equipment was co-located, which was not uncommon, the different signalling systems would sometimes cause interference.

Telex provided the first common medium for international record communications using standard signalling techniques and operating criteria as specified by the International Telecommunication Union. Customers on any telex exchange could deliver messages to any other, around the world. To lower line usage, telex messages were normally first encoded onto paper tape and then read into the line as quickly as possible. The system normally delivered information at 50 baud or approximately 66 words per minute encoded using the International Telegraph Alphabet No. 2. In the late days of the telex networks, end-user equipment was often replaced by modems and phone lines as well, reducing the telex network to what was effectively a directory service running on the phone network.

Telex began in Germany as a research and development program in 1926 that became an operational teleprinter service in 1933. The service, operated by the Reichspost (Reich postal service) had a speed of 50 baud — approximately 66 words per minute.

Telex service spread within Europe and (particularly after 1945) around the world. By 1978, West Germany, including West Berlin, had 123,298 telex connections. Long before automatic telephony became available, most countries, even in central Africa and Asia, had at least a few high-frequency (shortwave) telex links. Often, government postal and telegraph services (PTTs) initiated these radio links. The most common radio standard, CCITT R.44 had error-corrected retransmitting time-division multiplexing of radio channels. Most impoverished PTTs operated their telex-on-radio (TOR) channels non-stop, to get the maximum value from them.

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