Teletex¶

Teletex is a mostly-forgotten technology somewhere between Telex and Telefax:

• like Telex, it transmitted data as characters
  • however, Teletex supports a more complete alphabet with lowercase characters and special characters (Telex: Only 5-bit uppercase)
• like Telefax, it transmitted formatted entire pages/documents
  • however, Teletex transmits the text characters and their formatting (Telefax: A raster-pixel-image).

Teletex was invented aroud 1981. German operation was from 1981 through 1993.

Terminal Devices¶

Early/Simple Teletex devices are typically built from pre-existing daisy-wheel typewriters with single-line memory/editing/display capability. Those typewriters were extended with additional communications hardware and software in order to receive and/or transmit in Teletex format.

Later, more complex Teletex devices were text/word processing systems, either dedicated or in the form of personal computers. Those were then likewise extended with a Teletex communications hardware + software.

• Olympia Disque with IKBS
• Wandel Goltermann DTX-1

Network¶

Teletex is designed to use a CSPDN (circuit-switched public data network). The Teletex user equipment interfaces to the network via a 2400 bps synchronous X.21 interface.

ITU specifications also specify operation of Teletex over PSPDN, PSTN or ISDN networks, but it is not known if any of this was ever deployed anywhere.

Communications protocol (stack)¶

• X.21 is used as physical layer (synchronous 2400 bps)
• X.21 is used for CSPDN signaling, i.e. for dialing the call
• once the transparente bit-synchronous 2400bps channel is established via X.21, the following protocols are spoken (from bottom to top):
  • X.75 SLP in HDLC framing with parameters as per T.70 section 3.3.2.2
  • T.62 session layer
  • T.62 document layer

Relevant Specifications¶

• ITU-T F.200 (Teletex service)
• ITU-T T.61 (Teletex character set)
• ITU-T X.21 (interface)