X.21 is a standard of the ITU-T interface for data transmission in public Datenwählnetzen (eg Datex- L). Due to the simple structure, it is often applied to direct ( point-to- point).

The recommendation X.21 includes the connection management and diagnostic capabilities via loopback. This defines the format of dialing characters, time frames and the other procedures in the form of state diagrams. In the data transmission phase through the switched line is transparent for all synchronous data formats. Clock lines (supplied by the network termination or generated by the terminal itself ) allow the terminal to the interface to synchronize to the transmission network.

Seen from the interface lines here is the X.21 easier run than the V.24 or EIA- 232 / RS-232 interface: it has typically five functions ( control / signaling line, transmit / receive data, clock ) and comes with a 15 - pin connector from ISO 4903.

The function of these lines X.24 defines the ITU -T recommendation.

Possible electric characteristics are described in ITU-T X.26, and X.27. They are defined so that a higher data transmission speed and greater range than is possible with an interface according to V.24.

  • Network protocol
  • ITU Recommendation