X.21bis - Computer Definition
The Physical Layer specification within the X.25 protocol suite. X.21bis defines call control procedures, synchronization mechanisms, and the mechanical and electrical parameters for cables and connectors, which are similar to RS-232. X.21bis supports full-duplex (FDX) transmission at speeds from 9600 bps to 64 kbps. See also FDX, Physical Layer, synchronization, and X.25.