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How Datex-P works

Datex-P ( Dat a Ex change, p aketorientiert) is the product name of Deutsche Telekom for a communication network for data transmission, which on the X.25 interface protocol for packet switching based. It was introduced in Germany in August 1980 by the Deutsche Bundespost (DBP); initially in free trial operation, from August 1981 in regular operation. The data transfer rate could be selected from 50  baud ( acoustic coupler speed) up to 64,000 bit / s.

Existing connections were migrated to a new technical platform in 2010 and will continue to be operated; new connections can no longer be set up.


In the late 1980s, Datex-P represented the first access to a global data network that was available to everyone. The 16-bit home computers enabled inexpensive terminal emulation and all that was needed was a Network User ID (NUI) from the post office or a private one Provider to dial in via modem or acoustic coupler. Popular NUAs ( Network User Addresses ) were for example the CompuServe portal with numerous services or the chat on the servers of the Altos company .

Advantages of Datex-P

The Datex-P network brought a number of advantages:

  • Adaptation of different interface protocols of the end devices
  • Securing the order of the packages, flow control , speed adjustment
  • Secure transmission (repetition of faulty packets), thereby improving the bit error rate compared to the telephone network by a factor of approx. 100,000
  • Optional service features as in the telephone network: transfer of charges , closed user group , blocking a connection for incoming or outgoing calls


Development of Datex-P

After the number of network connections increased very quickly in the beginning, it was expanded with the technology of the Canadian manufacturer Nortel Networks , which had already equipped the Canadian DATAPAC network in 1976 . Its technology had the advantage of being duplicated: if a control processor failed, the replacement processor could immediately take over the tasks. As a result, the Datex-P network also achieves a high level of availability , which is very important in the financial sector , for example. In the meantime, however, the need for faster data transmission had increased significantly, so that frame relay was the preferred successor .

Packet-switched transmission

With the packet-switched transmission of data according to the X.25 protocol, which is used in the Datex-P network, there is no exclusively reserved line from the sender to the recipient . Instead, a virtual connection is established between the sender and receiver, which is given a connection identifier. This has the advantage that a sender can maintain a larger number of virtual connections to different receivers at different locations at the same time and only needs a single connection line. For example, the data center of a savings bank only needs a single connection line to give the computers in all Sparkasse branches access to its data stock via the Datex-P network.

The data to be transmitted is packed by the terminal in packets of a defined length and, depending on the recipient, provided with a connection identifier. The network nodes , the switching centers of the Datex-P network, forward these packets to the recipients using the connection identifier. If a terminal device (e.g. a "stupid" ASCII terminal) is not able to process data packets in accordance with the X.25 protocol, it can be connected to a PAD (packet assembler disassembler), which takes on this task for it.

In the X.25 terminology, the end devices are referred to as “DTE” (Data Terminal Equipment), the switching centers as “DCE” (Data Circuit Terminating Equipment). Transit exchanges are called "DSE" (Data Switching Exchange).

Connection to Datex-P

Originally, only transmission devices provided by the Deutsche Bundespost could be used to connect end devices to the Datex-P network. These were referred to as "data connection device (DAG)" or " data remote control device (DFG)".

  • A DAG can only transfer data from station A to station B; it knew no voting procedure.
  • A DFG knows the dialing procedure according to the international standard X.21 and can thus communicate its connection request to the exchange itself. The option was deactivated, however, because the X.25 protocol transfers the call number. This corresponds to an X.21 dedicated line operation .

Both device variants offer the well-known interfaces of the V-series ( V.24 , later also V.36) and the X-series (X.21), work in baseband , both in synchronous duplex and half-duplex operation , but have one shorter range than the analog modems for the telephone network that were common at that time (around 1980) . That was sufficient because they were not designed for the entire attenuation that can occur in the telephone network between two subscribers, but only for the relatively short distance to the next data converter point (DUST) of the integrated text and data network . A DUST was usually set up in the building of an exchange .

The future of Datex-P

The Datex-P services were provided by ITENOS GmbH until the beginning of 2019 . ITENOS is a subsidiary of T-Systems International GmbH. The service was discontinued on January 7, 2019.


Datex-P was also available in Austria. The service was discontinued in 2005, a reduced version for connecting point-of-sale systems was still operated. The connection was established via the D-channel of an ISDN basic connection and referred to as DX ISDN . This variant was also discontinued in 2013 as part of the conversion of the self-dialing remote service from SDH to VoIP . However, the Austrian Federal Railways still use an internal network to control electronic signal boxes.

See also

Individual evidence

  2. ^ Educational sheets of the Deutsche Bundespost , Issue B Telecommunications, 2/1983, p. 62
  3. Datex-P . Retrieved February 16, 2015.
  4. Press release on
  5. journal article Fa. Itenos the Federal Association of the security industry. ( Memento of December 10, 2015 in the Internet Archive ; PDF) accessed on December 8, 2015
  6. Datex-P: shutdown after almost 40 years: a data service is disappearing