ckollars@east.sun.com (Chuck Kollars - Sun ECD Marketing Support) (05/09/89)
*Someone recently asked about problems on some cable segments but not
*others. Some kind of repeater problem was suspected. The poster
*reported that reducing the NFS write buffer size to 512 made it sort
*of work. The poster may have been from BBN.
To fix the problem, disable heartbeat on the transceivers that connect
the repeater to the thick cable. ("heartbeat disabled" == "no SQE"
== "Ethernet Type I")
Most repeaters match the standard in that they do not forward collision
signals from one side to the other. (Whether you agree or disagree with
the standard is a separate issue.) For hardware implementation reasons,
many repeaters furthermore _require_ transceivers with_out_ heartbeat...
and misbehave very badly if you don't give them what they want.
Check the documentation that came with your repeater to see if it mentions
transceiver requirements. Or, if you can't find the documentation, just
try fix and see if it works.
A "heartbeat" signal sets a latch inside these repeaters. If the very
next packet is going the opposite direction, that latch being set will
cause the packet to be truncated and jammed as though it had been
collided. Yet this fact is never relayed back to the original sending
station. Net result: lots of lost packets/timeouts/retransmissions. You
can see this in TCP retransmission statistics, or with "ping".
This applies to the 15-pin connectors on _all_ repeaters, both "multiport"
models and 2-port models. It obviously doesn't apply to the
thinnet/cheapernet/coax connectors since the transceivers for those
connectors are inside the repeater.
The repeater will probably pass all diagnostic tests, either because all
the test packets were going in the same direction, or because the repeater
was removed to a bench for testing.
Any one of these methods will get you a transceiver without heartbeat: a)
use a VERY old transceiver [ex: TCL rev A or B]; b) for transceivers that
are a couple years old, pry open the housing, find the internal jumper,
and move it; c) find one that was specifically ordered with no heartbeat
when it was purchased; or d) for some very new transceivers, find the
switch or jumper on the outside of the case and move it.
-chuck kollars ...!sun.COM!suneast!ckollars