VTAM's data compression facility enables VTAM® to compress the data on selected LU-LU sessions when you are using application programs in a multiple-domain network. The value specified on the COMPRES operand of the MODEENT macroinstruction determines whether data compression is allowed. If your application is the SLU, and you want to use compression, code COMPRES=REQD on the MODEENT macroinstruction in the logon mode table.

The CMPVTAM start option specifies the maximum compression level allowed for sessions involving the host's application programs. A VTAM host performs data compression only if the CMPVTAM start option has been specified with a value in the range 1-4. This level can be changed by the MODIFY COMPRESS command and displayed by the DISPLAY VTAMOPTS command.

If CMPVTAM has been specified with a value greater than 1, the CMPMIPS start option can be used to balance the number of machine cycles needed with the effectiveness of compression for outbound messages. Higher values for CMPMIPS will likely increase both compression effectiveness and cycle usage, while lower CMPMIPS values will likely lower both compression effectiveness and cycle usage.

Input and output compression levels for a specific application program are specified on the CMPAPPLI and CMPAPPLO operands on the APPL definition statement. The CMPAPPLI and CMPAPPLO operands specify the maximum compression levels for an application's input data (the data the PLU receives) and output data (the data the PLU sends), respectively. An application program's compression level can be modified by the MODIFY COMPRESS command and displayed by the DISPLAY ID command.

For more information about data compression, see the z/OS Communications Server: SNA Network Implementation Guide