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 ON

External Links

Updated by laforge over 1 year ago · 3 revisions

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