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From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: 802.3 AUI/MAU questions
Message-ID: <2230104@hprnd.HP.COM>
Date: 3 Aug 90 22:33:08 GMT
References: <9505@goofy.Apple.COM>
Organization: HP Roseville Networks Division
Lines: 74

> 
> 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?

I don't think that there are any statements in the standard which cover
this specifically.  However, other requirements do result in the AUI
recievers having an input squelch.  The more recently developed MAU
definitions such as 10BASE-T explicitly include a requirement for input
squelch.  (For 10BASE-T the requirement is to squelch any signal less
than 160 mV.)  The normal state of lines from a powered down DTE would
be the same as that from a non-transmitting DTE -- 0 V, so I am not
sure what you are concerned about.
> 
> 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?

Without seeing your calculation, I can't tell what factors you might have
left out.  The numbers that went into the 10BASE2 standard were the
actual ones from the worst-case calculation.  Perhaps, you are leaving
out the effect of sending-end overshoot or impulse response of the 
collision detect filter.  Perhaps you are not calculating for the
worst case situation.  The thresholds, cable length etc were the result
of a worst case calculation.  The node count limitation was more
based on the potential for reflections from the nodes and from cable
impedence mismatches at each node adding and causing bit errors than
on the effect of the nodes on the collision threshold.  Nodes do
have a small effect on the collision threshold.
> 
> 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.

Packets shorter than minimum packet size get rejected at the physical
layer.  That is not the reason for receive mode collision detect.
Receive mode collision detect in coax transceivers (10BASE5
and 10BASE2) is necessary in order to maintain accurate carrier sense 
for the Carrier Sense Multiple Access/ Collision Detect (CSMA/CD) 
media access control.

During a collision, AC signals from the colliding nodes may cancel.
If the receiving nodes do not have receive mode collision detect, they
then fail to detect carrier during the cancelation.  Without accurate
carrier sense, the deferral algorithm does not work properly.  Lost
packets, CRC errors, and other such effects can occur.  I have seen 
signal cancelation occur on real networks.  It is not just a theoretical
idea.  It has a negative impact on network efficiency, though I doubt that
it would render a network inoperative.  The effects are worse on
a repeater, which is why 10BASE2 and 10BASE5 MAUs for repeaters
are required to implement receive mode colliison detect.
> 
> Thanks for any and all input.
> 
> Dennis Yarak
> Now at Apple.
> ----------
Pat Thaler
Opinions expressed are my own and not necessarily those
of IEEE 802.3