Retronetworking

h1. 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.

h2. 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.

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h2. Network

Teletex is designed to use a [[CSPDN__Datex-L|CSPDN]] (circuit-switched public data network).  The Teletex user equipment interfaces to the network via a 2400 bps synchronous [[X21|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.

h2. 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

h2. Relevant Specifications

* ITU-T F.200 (Teletex service)

* ITU-T T.61 (Teletex character set)

* ITU-T X.21 (interface)