Interphase Tech iSPAN - Manual (Page 231)

iSPAN Serial WAN Adapter Users Guide 211
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) .