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X.21

The X.21 interface recommendations describe the use of both unbalanced signal lines according to X. 26 (V.10, similar to RS-423) and balanced signal lines according to X.27 (V.11, similar to RS-422).

X.21 is the ITU-T standard for access to digital networks. The X.21 interface uses the 15-pin connector according to ISO 4903 for mechanical coupling of the interface lines. The pin assignment depends on the interface configuration used according to X.26 or X.27.

Overview of the X.21 and X.21bis interface

Overview of the X.21 and X.21bis interface

X.21 applies to all networks with digital access, circuit-switched networks, data packet-switched networks and leased lines. In addition, X.21 defines the procedures for circuit-switched networks.

Connections and functions of the X

Connections and functions of the X

.21 interface Compared to the V.24 interface, the X.21 interface offers the following advantages: higher transmission speeds of up to 10 Mbit/s, greater bridgeable distances of up to 1,200 m, a considerably smaller number of interface lines, and the possible control of connection establishment and termination by the interface signals.

The functional characteristics are described in Recommendation X.24. Functionally, X.21 is equipped with a call collision: If another call arrives at the time of the establishment of an outgoing call, the incoming call is rejected and the outgoing call is switched on.

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Englisch: X.21
Updated at: 24.04.2013
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