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As usual, the answer is, it depends......
First off, the DECHub 900 does not have an FDDI MAC, so it
does not contribute to the setting of TRT. It sounds like
there are only 3 nodes in this network, the DECconn 900 and the
two SGI machines.
SGI has long advocated the use of very high TRT values because
theoretically the ring's utilization under load increases from
80% to 90+ % . They ignore the fact that latency also will go up.
I'm again assuming that there are only 3 nodes in this ring. If
you raise T_Req of the concentrator to 168 ms, one of the SGI machines
will win the claim, and set the value of T_Neg. You will be able
to see this value via hubwatch, on the FDDI MAC Summary screen.
When the change is made, T_Neg will change from 7.99 to the value
determined by the SGI machine. That value will be, effectively,
the maximum time that they can transmit before having to give up the
token.
Now, if the station has the ring all to itself, all you do is increase
the number of back to back packets that got out of its transmitter
before it gives up the token. Since it is effectively the only station
that wants the token, it doesn't seem that you will get much of a
change in performance. If you do, it may be because of something
in their adapter besides just the setting of TTRT.
Its worth trying though, just to see what we can learn about these SGI
stations behaviors. Please post the results back here so that we can
see what happened.
If you have an FDDI analyzer, try to get before and after pictures
of the packets on the wire. I'd be curious to know how many back to
back packets get transmitted under each condition.
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