If you choose to use data compression, you can adjust the use of compression for your system by using a combination of compression levels specified for VTAM® and for application programs. The following levels of data compression are available:

0

No compression

1

RLE

2

Adaptive small table

3

Adaptive medium table

4

Adaptive large table

• The overall compression limit for a VTAM host can be:
  • Initialized through the CMPVTAM start option. For more information about the CMPVTAM start option, see z/OS Communications Server: SNA Resource Definition Reference.
  • Changed by the MODIFY COMPRESS command. (The compression levels of active sessions are not affected.) For more information about this command, see z/OS Communications Server: SNA Operation.
  • Displayed through the DISPLAY VTAMOPTS command. For more information about this command, see z/OS Communications Server: SNA Operation.
• Input and output compression limits for an application program can be:
  • Initialized through the CMPAPPLI and CMPAPPLO operands on the APPL definition statement.
  • Changed by the MODIFY COMPRESS command. (The compression levels of active sessions are not affected.)
  • Displayed through the DISPLAY ID command.
• COMPRES is the compression-override operand on the MODEENT macroinstruction that specifies how the negotiation of compression should be handled. You can use COMPRES to override the BIND negotiation process and either require or prohibit compression. Using this operand data compression specifications, you can associate with a logon mode name. For more information about this operand, see z/OS Communications Server: SNA Resource Definition Reference.
• The DISPLAY SESSIONS command can indicate the compression levels in use for a given session and the average message length reductions achieved in both directions. For more information about this command, see z/OS Communications Server: SNA Operation.