ITEM: D5624L

Problem with slow transfer rate with SNA Services


I have a risc 580 with aix 3.2 installed. I have a device defined as
RJE for LU1. I have it connected to a 56kb line. The transfer rate is
very slow, how can I increase the response time  or performance on my


There are several places to check for performance problems.

1. The first thing is to ensure that there are not any errors that are
causing retransmissions or timeouts.  These errors may be in the SNA
Internal Error Log (/var/sna/snalog.1).

2.  If there are no errors in snalog.1, the next place to check for
errors is the attachment trace.  This can be turned on/off with the
traceson/tracesoff commands and then formated with the trcrpt command.
After formating the trace search for occurances of 'Timeout' either on
inactivity or acknowledgement.  If these occur, there is probably a
mismatch of the Transmit/Receive Window counts (specified in the
Logical Link profile) with the remote system.  In this case the remote
is a Token Ring attached 3174 (RS6K--TR--3174--56kb--NCP--Host).  To
eliminate the timeout errors try setting the Transmit Window Count to
127 and the Receive Window Count to 1.  If this does not help,
continue to step 3.

3.  In the Logical Link profile, the Transmit/Receive Window counts
and Maximum I-field size can greatly affect performance.  The Max I-
field corresponds to the MAXDATA VTAM parameter and to the I-field
size of the 3174.  The optimal size for the RISC is 'system-defined'
which is 4060.  However it is usually set lower on the 3174, the
actual value will be negociated down to the lowest value so, to
increase this you will need to check/modify the 3174 as well as the
RISC.  The Transmit and Receive Windows are also configurable on both
the 3174 and the RISC.  The best strategy is to have these parameters
match and to specify large values.  A good starting point is to have
the RISC at Transmit=127, Receive=1.

4.  The first 3 items describe improving performance at the Data Link
Control (DLC) level.  Performance is also tunable at the session
level.  The session level parameters are specified in the VTAM Logon
Mode Entry Table.  For the Mode you are using, check the RUSIZES
parameter in the Mode entry.  The RUSIZES should be set to the largest
value that does not exceed I-field Size - 9 (the 9 is the length of
the headers that are placed on the RU).  The RUSIZES can only be set
sizes which are defined in the VTAM customization manual.  If you have
set the I-field size at 4060, try an RUSIZE of 3840 by specifying
(RUSIZES=F8F8) in the Mode.

5.  The second set of parameters for tuning the session level is the
pacing.  Pacing indicates the number of RUs that can be transmitted
before the remote must indicate that it has recieved the last RUs
sent.  The highest value is no pacing which is set by setting
pacing=0.  However, it is possible to overrun buffers by setting no
pacing.  These are values that you will need to work with to get
optimal performance without overrunning buffers.  A good value to
start with is 10.

Support Line: Problem with slow transfer rate with SNA Services ITEM: D5624L
Dated: October 1993 Category: N/A
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