For output messages, CICS® provides user exits with access to the entire output data stream. User code can be written to remove redundant characters from the data stream before the data stream is sent to the terminal. This technique can produce a dramatic improvement in response times if the proportion of characters not needed is large, because telecommunication links are usually the slowest paths in the network.
Some additional processor cycles are required to process the exit code, and the coding of the exit logic also requires some effort. Use of a compression exit reduces the storage requirements of VTAM® and reduces line transmission time.
The simplest operation is to replace redundant characters, especially blanks, with a repeat-to-address sequence in the data stream for 3270-type devices.
Another technique for reducing the amount of data transmitted is to turn off any modified data tags on protected fields in an output data stream. This eliminates the need for those characters to be transmitted back to the processor on the next input message, but you should review application dependencies on those fields before you try this.
There may be other opportunities for data compression in individual systems, but you may need to investigate the design of those systems thoroughly before you can implement them.
For VTAM devices, the global user exit used to compress terminal messages is XZCOUT1. For programming information, see VTAM working-set module exits in the CICS Customization Guide.
The contents of output terminal data streams can be examined in a VTAM trace.