Input bracketing protocol
A workstation in session with IMS must indicate begin-bracket (BB) or both begin-bracket and end-bracket (EB) on both the first transmission of a chained message (multiple transmission) and each transmission of unchained messages.
The end-bracket indicator places both IMS and
the workstation between brackets and in a contention state. If only
the begin-bracket indicator is sent, the workstation should send a
change-direction (CD) indicator on each of the following:
- The last transmission of a chained message (multiple transmission)
- Each transmission of unchained messages
The CD indicator is necessary, because IMS cannot correlate multiple input messages. The CD indicator is implied if it is not specified.
If BB and CD are indicated, IMS indicates EB on its next output to the workstation. If IMS detects an error while receiving chained input on which only BB is specified, it returns an EB and an IMS error message.
Restriction: For SLU P terminals, a CD or EB indicator
must accompany each input chain; otherwise, the session terminates.