X.21 is a physical and electric interface recommendée and published by UIT-T in 1976 on the connection DTE/DCE.

It defines the alignment of the control characters of the calls and the checking of the errors, the elements of phase of control of call for the services to commutated circuit, the transfer of information up to 2 Mbit/s and them loops of test. The flow of 64 kbit/s is that more used.