Telex is a telecommunication system that allows text-based messages to be sent and received by teleprinter over telephone lines. The term "telex" may refer to the service, the network, the devices, or a message sent using these. Telex emerged in Germany in the 1930s and became a major method of sending text messages electronically between businesses in the post–World War II period. Its usage declined as the fax machine grew in popularity in the 1980s. The technology operates over the circuits of the public switched telephone network or by private lines, using binary signals encoded with the International Telegraph Alphabet No. 2 at a standard speed of 50 baud. Telex provided the first common medium for international record communications using standard signalling techniques as specified by the International Telecommunication Union (ITU). A key feature of telex was the "answerback" system, which allowed senders to confirm with a high degree of certainty that their message had been received by the correct recipient. Telex networks were established in many countries. In the United States, the American Telephone and Telegraph Company developed the Teletypewriter Exchange Service (TWX) beginning in 1931, while Western Union built a separate telex network starting in 1958. The United Kingdom developed its own telex network from the 1930s, with full automatic switching completed by 1961. Canada, New Zealand, Kenya, and many other nations also established telex services, often connecting local subscribers to an international network spanning Europe, Africa, Asia, and the Americas. In the 1980s, the successor to CCITT, ITU-T, developed an enhanced system, Teletex, which only saw limited adoption, although some of the associated standards influenced computer networking protocols such as LDAP. Telex has been largely superseded by fax, email, and systems such as SWIFT, though radiotelex via HF radio remains in use in the maritime industry as part of the Global Maritime Distress and Safety System.
Technology The technology operates on switched station-to-station basis with teleprinter devices at the receiving and sending locations. It operates over the circuits of the public switched telephone network or by private lines. Point-to-point teleprinter systems had been in use long before telex exchanges were built in the 1930s. Teleprinters evolved from telegraph systems, and, like the telegraph, use binary signals, with mark and space logic represented by the presence or absence of a certain level of electric current. This differs from the analog telephone system, which used varying voltage to represent sound. 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 telephone numbers). 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 reduce connecting line usage, telex messages were 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 last days of the traditional telex networks, end-user equipment was often replaced by modems and phone lines, reducing the telex network to what was effectively a directory service running on the telephone network.
Development
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 German Reichspost had a speed of 50 baud, which is approximately 66 words per minute. Soon after, telex services were developed by other nations. Telex spread within Europe and 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. The cost of TOR equipment has continued to fall. Although the system initially required specialised equipment, as of 2016 many amateur radio operators operate TOR, also known as radioteletype (RTTY), with special software and inexpensive hardware to connect computer sound cards to short-wave radios. Modern cablegrams or telegrams actually operate over dedicated telex networks, using TOR whenever required. Telex served as the forerunner of modern fax, email, and text messaging – both technically and stylistically. Abbreviated English (like "CU L8R" for "see you later") as used in texting originated with telex operators exchanging informal messages in real time – they became the first "texters" long before the introduction of mobile phones. Telex users could send the same message to several places around the world at the same time, like email today, using the Western Union InfoMaster Computer. This involved transmitting the message via paper tape to the InfoMaster Computer (dial code 6111) and specifying the destination addresses for the single text. In this way, a single message could be sent to multiple distant telex and TWX machines as well as delivering the same message to non-telex and non-TWX subscribers via Western Union Mailgram.
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