X.21(redirected from X.21bis)
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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X.21An ITU standard protocol for a circuit switching network.
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