IMS TM Messages

A message, in the most general sense, is a sequence of transmitted symbols. The network inputs and outputs to IMS TM take the form of messages (plus control information) that are input to or output from IMS and the physical terminals, or application programs on the network (referred to as destinations).

Each unit of input to or output from IMS TM is called a transmission. A transmission can consist of a single message or multiple messages and the control information that is associated with those messages.

In general, IMS processes these messages asynchronously, which means that IMS does not always send a reply immediately, or ever, when it receives a message, and that unsolicited messages might also be sent from IMS. However, APPC/IMS and OTMA also support synchronous protocols, which means IMS always sends a reply when it receives a message.

There are three types of messages:

• Transactions. The data in these messages is passed to IMS application programs for processing.

• Messages to go to another logical destination (for example, network terminals).

• Commands for IMS to process.

If IMS is not able to process an input message immediately, or cannot send an output message immediately, the message is stored on a message queue that is external to the IMS system. IMS does not normally delete the message from the message queue until it has received confirmation that an application has processed the input message or that the output message has reached its destination.

Multiple and Single Segment Messages

A message can have one or more segments. A segment is defined by an end-of-segment (EOS) symbol, a message is defined by an end-of-message (EOM) symbol, and a transmission is defined by an end-of-data (EOD) symbol. Table 12-1 shows the valid combinations of the conditions represented by the EOS, EOM, and EOD symbols.

Table 12-1. Valid Combinations of the EOS, EOM, and EOD Symbols

Condition	Represents
EOS	End of segment
EOM	End of segment / end of message
EOD	End of segment / end of message / end of data

The relationships between transmission, message, and segment are shown in Figure 12-1.

Figure 12-1. Transmission, Message, and Segment Relationships

The character values or conditions that represent the end of segment and the end of the message, or both, depend on the terminal type.

For 3270 terminals, the physical terminal input is always a single segment message and transmission, but MFS can edit this into multiple IMS message segments. The EOS, EOM, and EOD conditions are all set after the Enter function key or the program function key is pressed and the data is transmitted.

On the output side, a message can be divided into multiple segments. Also, an application program can send different messages to different terminals, for example, a message to a printer terminal and a message to the display terminal. Each segment requires a separate insert call by the application program.

The format of a message segment, as presented to or received from an application program, is shown in Figure 12-2, and shows the following fields:

Figure 12-2. Format of a Message Segment

LL

Total length of the segment in bytes, including the LL and ZZ fields.

ZZ

IMS system field

Data

Application data, including the transaction code