In data communications, a specification of the International Telegraph and Telephone Consultative Committee (CCITT) that defines the connection of data terminal equipment to an X.21 (public data) network.
The iSeries™ system supports short—hold mode (SHM) operation for use with X.21 circuit—switched networks. X.21 short—hold mode is characterized by a series of connections and disconnections with a remote controller or system on an X.21 circuit—switched line. When there is no data traffic, the connection is broken, but the SNA sessions remain active. When either side has data to send, the connection is established again.