Data communication computer

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The Data Communication Computer ( DCC ) is an X.25 - router from the German manufacturer TDT AG. Based on a development in the 1970s, the 4th generation of these devices already exists, which is purely a contract development and is no longer sold by the actual manufacturer.

Originally, a DCC was only used to transmit asynchronous serial data from end devices (e.g. point heating systems in the Deutsche Bahn track network ) over long distances using a modem and to provide communication with additional connection security using the X.25 protocol. For this purpose, the pure user data is packaged in an X.25 frame and transmitted from one X.25 component to the next using various transmission technologies (e.g. synchronous modems on copper lines).

In addition to the DCCs, which are mainly used as network nodes in X.25 networks, PADs or their successors HS-TA ( high-speed terminal adapter ) are used to connect serial terminals in the Deutsche Bahn hazard reporting networks .

X.25 networks from Deutsche Bahn still make up large parts of the transmission networks for the MAS90 or DBMAS signaling system technologies . A migration to pure TCP / IP structures has been sought for a long time, but has not yet been implemented.

Advantages of X.25 connection security using DCC

The connection monitoring available in the X.25 protocol, which goes beyond the functionalities in pure TCP / IP, is a decisive argument for using X.25 for communication processes that do not offer any corresponding monitoring mechanisms in the application layer of the OSI reference model .

Due to the standard value of 2 hours, for which most systems maintain a TCP session even without communication, connection errors on WAN routes are only detected after this time without further measures in the application layer.

X.25, however, permanently exchanges control packets between two network nodes involved - even without any payload. In the event of corresponding faults on the connection paths, this means that they are recognized after approx. 20-30 seconds.

Classification of a DCC in the network structure

A DCC is a network router that connects

  • unsecured, serial data streams via RS232-based interfaces ( X.3 / X.21bis )
  • unsecured, serial data streams based on TCP / IP
  • secured, serial data streams based on X.25 (X.25 / X.21bis, X.25 / G.703 or X.25 over TCP / IP )

mediate or routes.

technology

Connectivity

A DCC provides the following physical interfaces:

  • X.3 / X.21bis: asynchronous serial interface RS232 basis
  • X.25 / X.21bis: Synchronous, serial interface for the transmission of X.25 user data via synchronous modems
  • X.25 / G.703: Synchronous interface for synchronous transmission of X.25 user data via G.703 / 64kBit.
    G.703 as the transmission protocol in OSI layer 2 has only been available since the 2nd generation of the DCC.
  • Ethernet: Network interface for the transmission of serial data streams via TCP / IP (unsecured) or X.25 over TCP / IP (secured).
    Ethernet as a transmission medium has only been available since the 3rd generation of the DCC.

Protocol conversions and routing options

A DCC can perform the following protocol conversions for routing purposes:

  • S2X / X2S (Serial-to-X.25 / X.25-to-serial):
    Asynchronous, serial data can be packed in an X.25 frame and forwarded to neighboring X.25 components according to routing rules (or . vice versa).
  • X2X (X.25-to-X.25):
    X.25 data packets can be forwarded to other, neighboring X.25 components based on configurable criteria.
  • S2T / T2S (Serial-to-TCP / IP / TCP / IP-to-Serial):
    Asynchronous, serial data can be forwarded to other components via Ethernet (TCP / IP) (or vice versa).
    Please note that in this case no additional connection security (such as provided by X.25) is used.
  • T2X / X2T (TCP / IP-to-X.25 / X.25-to-TCP / IP):
    Serial data streams that arrive at the DCC via TCP / IP can be enriched with X.25 frames to secure the connection Follow-up via X.25 to be forwarded to neighboring X.25 components. In the opposite direction, the additional information available in the X.25 protocol to secure the connection is removed and the data is forwarded via pure TCP / IP without this additional information. In this case, the message target is always the DBMAS message system.
  • XOT (X.25-over-TCP / IP):
    It is possible to distribute the data packet in X.25 format via a network interface (TCP / IP) and in turn from the respective destination (also a DCC4 / HS-TA) to be processed as a pure X-25 data packet. This makes it possible to network old existing systems or new systems with one another and also to use different transmission paths.

Individual evidence

  1. ↑ History of the TDT company with milestones in X.25 router development in Germany