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laforge, 01/15/2023 06:03 PM

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.

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)

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