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From: yarak@apple.com (Dennis Yarak)
Newsgroups: comp.dcom.lans
Subject: 802.3 AUI/MAU questions
Message-ID: <9505@goofy.Apple.COM>
Date: 31 Jul 90 18:44:41 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc
Lines: 31


After a careful reading of most of the standard, it appears that it would 
not be a violation for a powered-down host with a separately powered MAU, 
to interfere with the operation of the MAU.  Does anybody know, 
practically speaking, whether a problem exists with reliability or 
operation, for either the host or the MAU, when an MAU remains powered 
(and therefore driving AUI signal circuits) while its AUI host is 
completely shut down? Or perhaps someone can point to where the standard 
covers this situation?

Also, in analyzing worst case physical implementations over 185 meters of 
cable for 10BASE2, I just can't reproduce either threshold requirements 
for collisions, or where the 10 Ohm maximum DCR of the loop came from. 
Furthermore, a maximum of 30 nodes seems awfully conservative, as my 
simulations don't show anything near the breaking point.  If anyone has 
been on the committee from the early days, can they shed some light here?  
Are the thresholds, DCR, and node count ad hoc, compatibility having ruled 
the day for existing vendors rather than the actual worst case values 
expected from conformant transcievers and media?

Finally, do runt packets ever not get rejected at the physical layer?  
This is an issue when implementing shifted collision thresholds for long 
reach applications--Transmit mode CD allows a non-participating MAU to not 
recognize the collision, relying on the runt packet so generated to be 
discarded at the PHY.  I'm wondering if anyone has encountered problems 
identifiable to Transmit-mode thresholds on extra-long segments.

Thanks for any and all input.

Dennis Yarak
Now at Apple.