IST350I  DISPLAY TYPE = STATS,TYPE=COMPRESS
IST1435I LEVEL  INPUT                 OUTPUT
IST1176I                          BASIC    FROZEN
IST1177I   0    input             basic    **NA**
IST1177I   1    input             basic    **NA**
IST1177I   2    input             basic    frozen
IST1177I   3    input             basic    frozen
IST1177I   4    input             basic    frozen
IST314I  END

• This message identifies the type of information shown in the display. For this message group, type is always STATS,TYPE=COMPRESS.

• This message is a header message for message IST1177I.
• BASIC and FROZEN are OUTPUT values indicating the number of half-sessions using a given compression level for outgoing data.
• For more information, see the description of message IST1177I.

• This message is issued once for each possible data compression level.
• level is 0, 1, 2, 3, or 4.
  • Level 0 indicates that no data compression is used. This is the default compression level.
  • Level 1 indicates that VTAM uses run length encoding (RLE) compression. This type of compression simply replaces strings of identical characters with one or two bytes, without using a compression dictionary.
  • Levels 2, 3, and 4 indicate that VTAM uses an adaptive compression algorithm. This type of compression replaces strings of data with codes of 9, 10, and 12 bits for levels 2, 3, and 4, respectively. Codes identify entries in dictionaries of data strings.
    • In BASIC mode, which is always the initial mode, VTAM continuously updates the dictionaries so that they reflect the most recently compressed data.
    • In FROZEN mode, VTAM stops updating (freezes) the dictionaries to speed up compression processing. In this mode, VTAM can take advantage of the ESA/390 data compression facility, if it is available on the CPU.
    • Compression periodically switches from FROZEN mode to BASIC mode to resume updating of the dictionaries. It switches back to FROZEN when the dictionaries again reflect the most recently compressed data.
  • See the z/OS Communications Server: SNA Network Implementation Guide for more information on the RLE and adaptive compression algorithms.
• The INPUT value input represents the number of half-sessions using a given compression level for incoming data.
  • When a session is established, input is incremented by one at the input compression level used by the corresponding half-session in this host.
  • When a session ends, input is decremented by one at the input compression level used by the corresponding half-session in this host.
• The OUTPUT values basic and frozen represent the number of half-sessions using a given compression level for outgoing data.
  • When a session is established, basic is incremented by one at the output compression level used by the corresponding half-session in this host.
  • Each time VTAM freezes the compression dictionaries for a half-session on output, basic for that half-session is decremented by one, and frozen at the same level is incremented by one.

    Each time VTAM resumes updating the compression dictionaries for a half-session on output, frozen for that half-session is decremented by one, and basic at the same level is incremented by one.

  • When a session ends, basic or frozen (depending on the compression state at the time) is decremented by one at the output compression level used by the corresponding half-session in this host.
  Note: A session with both half-sessions in the same host is prevented from using compression. Each of its two half-sessions is counted separately for INPUT and OUTPUT on level 0.

• This message serves as a header for message IST1177I.
  • LEVEL indicates the data compression level.
  • INPUT indicates the number of half-sessions (one end of a session) using a given compression level for incoming data.
  • OUTPUT indicates the number of half-sessions using a given compression level for outgoing data. Values are basic and frozen.
• For more information, see the description of message IST1177I.