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 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)

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)

Example sequence of events for outbound call¶

Electical Signals / Coding¶

Data lines (R/T)¶

B > A	1
B < A	0

Control lines (I/C)¶

B > A	OFF
B < A	ON

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