Determining output protocols

Traffic on the ISC session is controlled by the VTAM® protocols associated with the message.

The primary bracket protocols used are:

Begin bracket (BB)
Signals the beginning of a bracket. Begin-bracket is an unconditional request when issued by the first speaker (secondary half session) and a conditional request when issued by the bidder (primary half session).
End bracket (EB)
Signals the end of a bracket, places the session in contention state, and allows either session partner to request a new bracket.
Change-direction (CD)
Turns control of the session over to the session partner and allows the session partner to send traffic on the session.

Traffic across an ISC session can be sent and received synchronously to the session. The processing of messages is performed either synchronously or asynchronously to the session. That is, the sending subsystem initiates a message and waits for a reply (synchronous) or does not wait (asynchronous).

Within ISC, the external synchronous mode of operation (as specified by FM headers) requires transmitted messages and associated replies, if applicable, to occur within a single bracket. The end of synchronous mode is signaled by EB. When in synchronous mode, IMS must be able to reply, if required, within the same bracket (before EB is received) to any outstanding message traffic with the exception of a message switch. If the input message protocol prohibits IMS from sending any required replies within the same bracket, the input message is rejected.

Within IMS, the protocols used for output that must be sent with ATTACH (because the other subsystem lacks SCHEDULER support) are predefined, regardless of whether the originating input transaction and resulting output replies are synchronous (ATTACH) or asynchronous (SCHEDULER). This output includes:

Nonlast chains (pages) of MFS paged output
The last (MFS paged) or only chain of response mode, conversation mode, test mode, and IMS command replies
Asynchronous output

You must define to IMS the protocols to be used with the last (MFS paged) or only chain of other asynchronous output. These are defined on the COMPTn keyword of the TERMINAL macro or on an ETO logon descriptor. Four parameters are supplied:

SINGLE1: Asynchronous output for this component is sent one message per bracket.
SINGLE2: Asynchronous output for this component is sent one message at a time with the VTAM begin-bracket (if necessary) and change-direction indicators to allow the receiving subsystem to optionally send its communication traffic.
MULT1: All asynchronous messages for a given LTERM are sent before the bracket is ended.
MULT2: All asynchronous messages for a given LTERM are sent before change-direction is sent.

For IMS-to-IMS sessions, the selection of SINGLE1, SINGLE2, MULT1, or MULT2 can be related to the transaction types and characteristics of the receiving IMS subsystem. The following considerations apply:

SINGLE1: This protocol is appropriate for sending message switches, nonresponse transactions, or nonconversational transactions to the receiving IMS.
SINGLE2: This protocol is appropriate for sending response mode (including Fast Path) transactions, commands, and test mode, because synchronous communication assumes that input generates corresponding output. Conversational mode is not supported on an IMS-to-IMS session.
MULT1 or MULT2: These protocols treat asynchronous traffic the same way as SINGLE1 or SINGLE2 treat synchronous traffic. They are useful in suppressing contention when large amounts of asynchronous traffic are queued for transmission on one (MULT1) or both (MULT2) subsystems.