X.21




<communications, standard> A digital signaling interface recommended by ITU-T that includes specifications for DTE/DCE physical interface elements, alignment of call control characters and error checking, elements of the call control phase for circuit switched services, data transfer at up to 2 Mbps, and test loops.

64 kbps is the most commonly used transfer rate.



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X11R6
X.12
X2
X.208
X.209
 Association Control Service Element
presentation layer
Remote Operations Service Element
session layer
transport layer
 X.214
X.215
X.216
X.217
X.219