X.21
acronymnetworkingISP & telecom
Stands for: ITU-T X.21 (circuit-switched serial interface)
ITU-T's interface for circuit-switched data networks - the serial physical-layer standard that often sat beneath X.25.
X.21 defines the physical and signaling interface between data terminal equipment and a circuit-switched public data network - the connector, voltages, and call-setup handshake. It was used directly on circuit-switched data networks and as the physical layer under X.25 in parts of Europe. Largely historical now, it is the companion 'X-dot' standard to X.25: X.25 the packet protocol, X.21 the serial interface it could ride on.
Also known as: x.21, x21, circuit-switched serial