ITEM: BY5459L

SNA: MAX RU Size versus MAX I-Field (BTU) Size

QUESTION:
   I am using IBM Communications Server to provide SNA communications over
   tokenring to an IBM mainframe.  I have set MAX RU Size = 4096 in my LU6.2
   mode profile.  I am finding that my data is being segmented.  Why is this,
   and how can I avoid data segmentation?

ANSWER:
   The MAX I-Field Size (also known as MAX BTU) for IBM Communications Server
   over tokenring is 4046.  To avoid data segmentation, you should set MAX RU
   Size to 9 bytes LESS than this value (ie. MAX RU Size = 4037).  In general,
   your MAX RU Size should equal 9 bytes less than your MAX I-Field Size.  You
   define the MAX RU Size in the LU6.2 mode profile and the MAX I-Filed Size
   in the tokenring DLC profile.  However, you can only specify a user defined
   Max I-Field Size up to the 4046 limit.  Any I-Field size specification larger
   than this will be ignored and the size of 4046 will be used.

   Here is what the Communications Server Planning and Performance Guide has to
   say about this:

      Generally, throughput improves with larger RU
      size, with the exception of crossing 3840. Since this test was run on
      a token ring, the largest BTU currently supported by Communications
      Server for AIX over a token ring is size 3968 for SNA Services/6000
      running on AIX 3.2 (and 4046 for Communications Server for AIX running
      on AIX 4.1). Consequently, the largest RU to fit in a BTU is size
      3840. In this case, when RU size is larger than 3840, the BIU
      containing this RU is segmented into multiple PIUs (and BTUs). This
      segmentation causes some loss of throughput. If you keep increasing
      the RU size, it eventually catches up and more than compensates for
      the penalty of BIU segmentation.  It is important to bear in mind that
      the point at which segmentation begins is determined by the frame
      (BTU) size, which can be modified by the user up to 4046 bytes. If the
      frame is large enough to contain the BIU, no segmentation takes place.



Support Line: SNA: MAX RU Size versus MAX I-Field (BTU) Size ITEM: BY5459L

Dated: May 1997 Category: N/A

This HTML file was generated 99/06/24~13:30:19

Comments or suggestions?
Contact us