X.21 (Packet-switched networks) International recommendation that defines a physical interface for X.25, using a 15-pin connector equivalent to V.11.

X.21 bis (Packet data networking) International recommendation that defines the most popular physical interface for X.25. Equivalent to RS-232 and V.24.

X.21 (bis) link Commonly used in European countries, this is a pay as you go link. When a signal is raised, the network automatically establishes a link to the remote unit.

X.25 Packet switching protocol that lets computers communicate via wide area packet switched data networks

XID See Exchange Identification (XID).

iSPAN Serial WAN Adapter Users Guide

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