X21 » History » Revision 2
Revision 1 (laforge, 01/15/2023 06:03 PM) → Revision 2/3 (laforge, 01/30/2023 06:38 PM)
h1. X.21 X.21 is a specification for a differential, serial interface introduced in the mid-1970 by ITU-T. It was used as a digital signaling interface between (typically customer-owned) [[DTE]] and (typically provider-owned) [[DCE]]. X.21 was primarily used in the [[CSPDN__Datex-L|CSPDN]] networks such as DATEX-L. Key aspects of X.21: * all signals differential * not just data (Rx/Tx), but also bit-clock and optionally byte clock * state machines and signaling for call control ** outbound calls ** progress signals ** inbound calls (+accept/reject) h2. Types of Signals Note: Each of those signals actually has two physical circuits as the signals are all differential. |_.Name|_.Direction|_.Description| |Transmit Data|DTE -> DCE|Data transmitted by the DTE| |Control|DTE -> DCE|Indicates the meaning of the data sent on _Transmit Data_| |Receive Data|DTE <- DCE|Data received by the DTE| |Indication|DTE <- DCE|Indicates the type of data received on _Receive Data_| |Signal Element Timing|DTE <- DCE|The bit-clock| |Byte Timing|DTE <- DCE|The byte-clock (optional)| h2. Example sequence of events for outbound call !x21_figure_b1.png! h2. External Links * https://en.wikipedia.org/wiki/X.21 * http://www.arcelect.com/X21_interface.htm * http://old.pinouts.ru/SerialPorts/x21_pinout.shtml * https://www.itu.int/rec/T-REC-X.21/recommendation.asp?lang=en&parent=T-REC-X.21-199209-I
