Summary of data compression

The following summarizes data compression.

Data compression is established between users of a session through the BIND and RSP(BIND) flows. VTAM® can handle compression settings at one end or both ends on behalf of VTAM applications.
Compression is disabled on sessions with both ends in the same VTAM.
VTAM defines compression levels 0 to 4 used in VTAM start option CMPVTAM and PLU-APPLication definition options CMPAPPLI (incoming flow) and CMPAPPLO (outgoing flow). The actual compression level settles on the lower of the CMPAPPLI value or the CMPAPPLO value, and the level acceptable for each of the hosts (CMPAPPL value for VTAM).
The CMPMIPS option can be used to balance CPU cycles versus LZ-like compression efficiency for outgoing data only. The value (0–100) can be changed dynamically using the command
```
F NET,VTAMOPTS,CMPMIPS=value
```
When CMPMIPS=0, data on all sessions is sent uncompressed. When CMPMIPS=100, most CPU cycles are used for best compression efficiency. A recommended value of 50 should save CPU cycles while maintaining high compression efficiency.
RLE compression is allowed with its normal CPU utilization anytime CMPMIPS is not 0.
Compression efficiency may be displayed on selected sessions using the command
```
D NET, SESSIONS,SID=sessionid
```
Efficiency is shown as a percentage reduction value on the original length of the uncompressed data.
Some VTAM compression level statistics may be displayed using the command
```
D NET,STATS,TYPE=COMPRESS
```
The logmode table option COMPRES= SYSTEM | PROHIB | REQD can be used for the logmode table entry for the sessions over which compression is requested.
- The logmode table applies to the SLU-controlling VTAM.
- The default value SYSTEM may be OK when the remote station is smart enough to accept, reject or update the compression levels requested by VTAM on the BIND CV66. This may be the case for a CM/2 acting as node 2.1.
- SYSTEM=REQD is required for VTAM APPL to VTAM APPL sessions. For a work station with compression capability but no compression negotiation capability on CV66, set COMPRES=REQD. The PLU VTAM compression levels (resolved from CCMPVTAM, CMPAPPLI, or CMPAPPLO) must be acceptable to the work station.