Telex - Operation and Applications

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-1 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.