Retronetworking

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