Telex

The telex network is a switched network of teleprinters similar to a telephone network, for the purposes of sending text based messages. This network provided the first common media for international record communications using standard signalling techniques and operating criteria as specified by the International Telecommunications Union.

Development

T100 telex machine]] "Puma" telex machine of the 1980s]]

By 1935, message routing was the last great barrier to full automation. Large telegraphy providers began to develop systems that used telephone-like rotary dialing to connect teletypes. These machines were called "telex" (TELeprinter EXchange). Telex machines first performed rotary-telephone-style pulse dialing for circuit switching, and then sent data by Baudot code. This "type A" telex routing functionally automated message routing.

The first wide-coverage telex network was implemented in Germany during the 1930s as a network used to communicate within the government.

At the rate of 45.45 (±0.5%) baud — considered speedy at the time — up to 25 telex channels could share a single long-distance telephone channel by using voice frequency telegraphy multiplexing, making telex the least expensive method of reliable long-distance communication.

Canada-wide automatic teleprinter exchange service was introduced by the CPR Telegraph Company and CN Telegraph in July 1957 (the two companies, operated by rivals Canadian National Railway and Canadian Pacific Railway, would join to form CNCP Telecommunications in 1967). This service supplemented the existing international telex service that was put in place in November 1956. Canadian telex customers could connect with nineteen European countries in addition to eighteen Latin American, African, and trans-Pacific countries. The major exchanges were located in Montreal (01), Toronto (02), and Winnipeg (03).

In 1958, Western Union started to build a telex network in the United States. This telex network started as a satellite exchange located in New York City and expanded to a nationwide network. Western Union chose Siemens & Halske AG, now Siemens AG, and ITT to supply the exchange equipment, provisioned the exchange trunks via the Western Union national microwave system and leased the exchange to customer site facilities from the local telephone company. Teleprinter equipment was originally provided by Siemens & Halske AG and later by Teletype Corporation. Initial direct international telex service was offered by Western Union, via W.U. International, in the summer of 1960 with limited service to London and Paris.

In 1962, the major exchanges were located in New York City (1), Chicago (2), San Francisco (3), Kansas City (4) and Atlanta (5). The telex network expanded by adding the final parent exchanges cities of Los Angeles (6), Dallas (7), Philadelphia (8) and Boston (9) starting in 1966.

The telex numbering plan, usually a six-digit number in the United States, was based on the major exchange where the customer's telex machine terminated. For example, all telex customers that terminated in the New York City exchange were assigned a telex number that started with a first digit "1". Further, all Chicago based customers had telex numbers that started with a first digit of "2". This numbering plan was maintained by Western Union as the telex exchanges proliferated to smaller cities in the United States. The Western Union Telex network was built on three levels of exchanges. The highest level was made up of the nine exchange cities previously mentioned. Each of these cities had the dual capability of terminating both telex customer lines and setting up trunk connections to multiple distant telex exchanges. The second level of exchanges, located in large cities such as Buffalo, Cleveland, Miami, Newark, Pittsburgh and Seattle, were similar to the highest level of exchanges in capability of terminating telex customer lines and setting up trunk connections. However, these second level exchanges had a smaller customer line capacity and only had trunk circuits to regional cities. The third level of exchanges, located in small to medium sized cities, could terminate telex customer lines and had a single trunk group running to its parent exchange.

Loop signaling was offered in two different configurations for Western Union Telex in the United States. The first option, sometimes called local or loop service, provided a 60 milliampere loop circuit from the exchange to the customer teleprinter. The second option, sometimes called long distance or polar was used when a 60 milliampere connection could not be achieved, provided a ground return polar circuit using 35 milliamperes on separate send and receive wires. By the 1970s, and under pressure from the Bell operating companies wanting to modernize their cable plant and lower the adjacent circuit noise that these telex circuits sometimes caused, Western Union migrated customers to a third option called F1F2. This F1F2 option replaced the DC voltage of the local and long distance options with modems at the exchange and subscriber ends of the telex circuit.

Western Union offered connections from telex to the AT&T; Teletypewriter eXchange (TWX) system in May 1966 via its New York Information Services Computer Center. These connections were limited to those TWX machines that were equipped with automatic answerback capability per CCITT standard.

In 1970, Cuba and Pakistan were still running 45.5 baud type A telex. Telex is still widely used in some developing countries' bureaucracies, probably because of its reliability and low cost. The UN asserted at one time that more political entities were reliably available by telex than by any other single method.

Around 1960[?], some nations began to use the "figures" Baudot codes to perform "Type B" telex routing.

Telex grew around the world very rapidly. Long before automatic telephony was available, most countries, even in central Africa and Asia, had at least a few high-frequency (shortwave) telex links. Often these radio links were first established by government postal and telegraph services (PTTs). 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 initially specialised equipment was required, many amateur radio operators now operate TOR (also known as RTTY) with special software and inexpensive hardware to adapt computer sound cards to short-wave radios.

Modern "cablegrams" or "telegrams" actually operate over dedicated telex networks, using TOR whenever required.

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 (in this case Telefonaktiebolaget L.M. Ericsson in Sweden) and S is the country 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".

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". At the beginning of the message, the sender would transmit a WRU (Who aRe yoU) 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 less verifiable forms of communications such as telephone and fax.

The usual method of operation was that the message would be prepared off-line, using paper tape. All common telex machines incorporated a 5-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 (e.g., an inventory list) 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-One and Microsoft Mail systems. In addition to permitting email to be sent to telex addresses, formal coding conventions adopted in the composition of telex messages enabled automatic routing of telexes to email recipients.

Teletypewriter eXchange

The Teletypewriter eXchange (TWX) was developed by the Bell System in the United States and originally ran at 45.45 baud or 60 words per minute, using five level Baudot code. Bell later developed a second generation of TWX called "four row" that ran at 110 baud, using eight level ASCII code. The Bell System offered both "3-row" Baudot and "4-row" ASCII TWX service up to the late 1970s.

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

Western Union purchased the TWX system from AT&T; in January 1969. The TWX system and the special area codes (510, 610, 710 and 810) 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.

International Record Carriers

Bell's original consent agreement limited it to international dial telephony. The Western Union Telegraph Company had given up its international telegraphic operation in a 1939 bid to monopolize U.S. telegraphy by taking over ITT's PTT business. The result was a de-emphasis on telex in the U.S. and a "cat's cradle" of international telex and telegraphy companies. The Federal Communications Commission referred to these companies as "International Record Carriers" (IRCs).

Western Union Telegraph Company developed a subsidiary named Western Union Cable System. This company later was renamed as Western Union International (WUI) when it was spun off by Western Union as an independent company. WUI was purchased by MCI Communications (MCI) in 1983 and operated as a subsidiary of MCI International.

ITT's "World Communications" division (later known as ITT World Communications) was amalgamated from many smaller companies: "Federal Telegraph", "All American Cables and Radio", "Globe Wireless", and the common carrier division of Mackay Marine. ITT World Communications was purchased by Western Union in 1987.

RCA Communications (later known as RCA Global Communications) had specialized in global radiotelegraphic connections. In 1986 it was purchased by MCI International.

Before World War I, the Tropical Radiotelegraph Company (later known as Tropical Radio Telecommunications, or TRT) put radio telegraphs on ships for its owner, the United Fruit Company (UFC), to enable them to deliver bananas to the best-paying markets. Communications expanded to UFC's plantations, and were eventually provided to local governments. TRT eventually became the national carrier for many small Central American nations.

The French Telegraph Cable Company (later known as FTC Communications, or just FTCC), which was owned by French investors, had always been in the U.S. It laid undersea cable from the U.S. to France. It was formed by Monsieur Puyer-Quartier. International telegrams routed via FTCC were routed using the telegraphic routing ID "PQ", which are the initials of the founder of the company.

Firestone Rubber developed its own IRC, the "Trans-Liberia Radiotelegraph Company". It operated shortwave from Akron, Ohio to the rubber plantations in Liberia. TL is still based in Akron.

Bell Telex users had to select which IRC to use, and then append the necessary routing digits. The IRCs converted between TWX and Western Union Telegraph Co. standards.

Demise

Telex is still in operation, but has been mostly superseded by fax, email, and SWIFT.

References

Category:Telecommunications equipment Category:Telecommunications systems