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How Datex-L works

Datex-L was a public data network in Germany, which was installed in the early 1980s mainly for the Teletex service by the Deutsche Bundespost (DBP) and therefore also lost importance with this service, which was not able to establish itself. The Datex-L network was a circuit-switched network and offered data transmission rates of 200 bit / s up to 9,600 bit / s, and towards the end of operation also 64 kbit / s.

It was a protocol-transparent network: with the exception of the DATEX-L300 service, which offered 300 bit / s and required a code frame of 11 bit / character , any protocol with any character set could be transmitted. That wasn't a matter of course back then. At that time, data communication was dominated by byte-oriented protocols such as the BSC from IBM , the DDCMP from DEC or the well-known Arpanet protocol, while advanced bit-oriented protocols such as HDLC or SDLC were only just emerging.

As a circuit-switched network, the network set up and cleared the connections between the participants and ensured that only participants whose communication was coordinated and compatible were connected to one another. There was a separate numbering plan for this network, independent of the telephone numbers of the telephone network .

The Datex-L network offered the automatic selection of a data terminal equipment , speed dialing , direct call , subscriber service classes , connection identification and the transfer of charges for incoming calls as performance features . The connections in the DATEX-L network were charged on the basis of a high basic fee, the duration of the connection, the time of day and the distance zone, with the Federal Republic of Germany being divided into two distance zones. The high costs, which were significantly higher than those in the telephone network , were one of the reasons why the DATEX-L network did not prevail. A few years later, ISDN offered higher transmission rates at lower costs. The Datex-L network was no longer offered in the new federal states and was discontinued by Telekom in 1996.

Access to the Datex-L network was provided by so-called remote data switching devices , which were provided by the Post and could not be bypassed because of the modem monopoly that was still in force at the time (private modems were not allowed to be used, not even in the telephone network). There was even a remote switching device with a number switch for the DATEX-L200 service . The Datex-L network consisted of a number of special exchanges , some 20 of which were installed in Germany. The manufacturer was Siemens AG , which, because of the small number of units, did not develop excessive interest in this technology. Other countries developed similar data networks: the USA installed TELENET and TYMNET , to whose participants Datex-L also provided access via special gateways that connected the networks.

Relatively many countries were interested in circuit-switched data networks at that time, so that the ITU -T standardized its own access interface with the X.21 interface. It had fewer pins than the V.24 interface and improved electrical properties, so that it allows considerably longer connecting cables and higher data rates than these. This interface is still used in many ways, is a 15-pin D-Sub - Connector rarely according to ISO 4903. More today used features of the interface are a pin for the byte clock (optional usable) and the automatic dialing of a number according to a standardized procedure. Explicit data networks with X.21 access, which exist in many countries and which are still being expanded, are networks made up of leased lines or use direct calling .

See also


  • Conrads, Dieter; Telecommunication . Vieweg & Sohn Verlag, Wiesbaden 2004, ISBN 3-528-44589-0