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 |
