Telex
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 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.
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.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 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 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, many amateur radio operators operate TOR, also known as radioteletype, 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 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 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.
Operation and applications
Telex messages are routed by addressing them to a telex address, e.g., "14910 ERIC S", where 14910 is the subscriber number, ERIC is an abbreviation for the subscriber's name and S is the country code or location code. Solutions also exist for the automatic routing of messages to different telex terminals within a subscriber organization, by using different terminal identities, e.g., "+T148". The country codes for the first countries to adopt telex are single letters, while other countries have two-letter codes. Some specialty services and American cities have three-letter network or location codes, and a few towns have four-letter codes.A major advantage of telex is that the receipt of the message by the recipient could be confirmed with a high degree of certainty by the "answerback", which is a transmission-control enquiry character. At the beginning of the message, the sender would transmit a WRU code, and the recipient machine would automatically initiate a response which was usually encoded in a rotating drum with pegs, much like a music box. The position of the pegs sent an unambiguous identifying code to the sender, so the sender could verify connection to the correct recipient. The WRU code would also be sent at the end of the message, so a correct response would confirm that the connection had remained unbroken during the message transmission. This gave telex a major advantage over group 2 fax, which had no inherent error-checking capability.
The usual method of operation was that the message would be prepared off-line, using paper tape. All common telex machines incorporated a five-hole paper-tape punch and reader. Once the paper tape had been prepared, the message could be transmitted in minimum time. Telex billing was always by connected duration, so minimizing the connected time saved money. However, it was also possible to connect in "real-time", where the sender and the recipient could both type on the keyboard and these characters would be immediately printed on the distant machine.
Telex could also be used as a rudimentary but functional carrier of information from one IT system to another, in effect a primitive forerunner of electronic data interchange. The sending IT system would create an output on paper tape using a mutually agreed format. The tape would be sent by telex and collected on a corresponding paper tape by the receiver and this tape could then be read into the receiving IT system.
One use of telex circuits, in use until the widescale adoption of X.400 and Internet email, was to facilitate a message handling system, allowing local email systems to exchange messages with other email and telex systems via a central routing operation, or switch. One of the largest such switches was operated by Royal Dutch Shell as recently as 1994, permitting the exchange of messages between a number of IBM Officevision, Digital Equipment Corporation ALL-IN-1 and Microsoft Mail systems. In addition to permitting email to be sent to telex, formal coding conventions adopted in the composition of telex messages enabled automatic routing of telexes to email recipients.
United States
Teletypewriter Exchange Service
The Teletypewriter Exchange Service was developed by the American Telephone and Telegraph Company in the United States, commencing service on November 21, 1931.From 1942 to 1952, AT&T published progress with the system in the trade magazine TWX. It published articles that touched upon many aspects of the technology.
From inception to 1962, access to the service was provided by operator-assisted, manual switching. By 1962, the network had grown to 100 switchboard locations to handle the traffic, causing considerable delay in the speed of connections of up to 2 minutes on average. On August 31, 1962, the service was integrated into the Direct Distance Dialing network, which improved connection times to about thirty seconds. For the new dial technology, each station was assigned a ten-digit telephone number from a reserved set of N10 area codes, designated as Service Access Codes. Area code 510 was assigned for the United States and Area Code 610 in Canada. Sixteen operating centers were established across the United States.
Later in the decade, the United States was subdivided into three service regions. and assigned codes from the remaining set of SACs. SAC 710 covered the Northeast of the United States. 810 was assigned from Michigan southward and east of the Mississippi River to Ohio, Indiana, and Kentucky, and the entire South and 910 served west of the Mississippi to the Southwest and West Coast.
TWX lines were configured with a special Class of Service to prevent interconnections with voice services.
Western Union purchased the TWX system from AT&T in January 1969. The TWX system and the use of the special US area codes continued until 1981, when Western Union completed the conversion to the Western Union Telex II system.
Canada moved its TWX-numbers, as well as Datalink services, to the non-geographic area code 600, effective October 1, 1993, in exchange for returning 610.
The network originally transmitted at a speed of 45.45 baud, or approximately 60 words per minute, using five-bit Baudot code, often referred to as 3-row coding with 32 characters arranged in three key rows of the keyboard.
In 1963, AT&T implemented a new coding technology for TWX, called 4-row based on the new Teletype Model 33 teleprinter using a 110-baud modem and a subset of the seven-bit ASCII code without lower-case letters. TWX was offered in both 3-row Baudot and 4-row ASCII versions up to the late 1970s.
The modem for the 4-row ASCII service was the Bell 101 dataset, developed by 1958 for military applications. It is the direct ancestor of the Bell 103 modem that launched computer time-sharing. The 101 was revolutionary because it allowed the Bell System to run TWX on its regular voice telephone lines.
The code and 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 and speed conversion was facilitated by a special service unit made by Western Electric. Multiple code and speed conversion units were placed at each operator position.
During the conversion to Telex II, the remaining 3-row Baudot customers were converted to the new service during the period 1979 to 1981.
In February 1969, AT&T installed the first electronic switching system for TWX service. It was a version of the No. 1ESS switch, arranged for data features in the Long Lines Department of AT&T. It had a capacity of handling 1,250 4-row teletypewriters. However, due to the purchase of TWX by Western Union, further installations were canceled.
Western Union's Telex II system was re-acquired by AT&T in 1990 in the purchase of the Western Union assets that became AT&T EasyLink Services.