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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¶
Updated by laforge 9 months ago · 3 revisions