From: oberman@ptavv.llnl.gov
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Ping Reports: 31% Packet Loss...Possible Causes?
Date: 15 Oct 92 11:47:58 PST

In article <m291Hg915a@atlantis.psu.edu>, barr@pop.psu.edu (David Barr) writes:
> In article <1992Oct14.075613.1@ptavv.llnl.gov> oberman@ptavv.llnl.gov writes:
>>In article <-071H6tx4a@atlantis.psu.edu>, barr@pop.psu.edu (David Barr) writes:
>>Not again! It seems that SQE is about the top area of total confusion in LANs.
>>This advice is simply WRONG! If in doubt, get a book on the subject. But PLEASE
>>don't post this sort of advice!
> 
> How about "Keeping the Link" by Martin Nemzow?
> 
>>SQE is a signal passed between the transceiver and the host. It does not send
>>out any signal on the backbone media.
> 
> p 55. "When the network is not busy, and idle signal is transmitted by all
> source/destination nodes, which is the 0.7-volt carrier sense.  This base
> voltage is sometimes called the 'heartbeat.'"
> 
> Hm.   This sure sounds like a signal to me.
> 

Boy, is this ever garbled! While I've read some silliness about LANs, if this
quotation is accurate, this book sets a new low. Carrier sense has NOTHING to
do with SQE. I retract my recommendation to read a book on the subject.
I guess I'll have to say "read 802.3". But that is VERY tough sledding for
someone who is not really familiar with the principles involved and even then
is pretty tough. The DIX Ethernet V2 spec is far more readable, but is not
quite the same. The single biggest difference between Ethernet V2 and 802.3 is
the replacement of CPT by SQE. Both are often called "heartbeat".

(Side note: Vernon, you are right about my advice to read a book. Guess I need
to find a good, accurate book to recommend.)

Any time a signal on the network generates a DC current which produces a
voltage exceeding 0.7 volt into 25 ohms (28 ma), any MAU attempting to access
the cable will detect carrier and defer transmission. If each idle MAU
generated such a signal, the whole thing would not work. N.B. I believe the .7
volt level is correct, but I have NOT confirmed it. It is at least close.
Someone snarffed my copy of the spec and I'm too lazy to run down to the
library to look it up.

SQE is a pulse generated by the MAU to the host after each write on the
collision line to confirm the operation of the collision detection circuitry.
IT is the only way that a system can tell if the collision detection is
working.

>>802.3 mandates SQE for all terminal
>>devices. That means anything EXCEPT a repeater.
> 
> Yep.  It is for exactly this reason that I've had to turn off SQE
> on my one network.  It seems the repeater (a CISCO AGS) that I get doesn't
> handle SQE, and if I have it enabled on my machines, the network quickly
> grinds to a halt as the repeater dumps collision notices onto the wire.

A cisco AGS is a router, NOT a repeater. And I have previously heard reports
that it did not properly meet specs in this regard. I have queried cisco and
received a reply that they did handle SQE correctly. A later message then said
that they think it does. Since it uses cisco's own Ethernet interface instead
of a chip like the AMD Lance  or a SEEQ, I can't confirm it. I can confirm that
my AGS (7 Ethernets and over 500 local nodes) has no trouble routing with SQE
on. This may be dependent on the microcode on the MEC or MCI card, since that
is where the Ethernet operations are really handled.

>>SQE exists because there is no
>>other way for a system to know that its collision detection circuitry has
>>failed. And the failure of this circuitry can result in major network failure.
> 
> Fine.  There are also cases when leaving it on results in major network
> failure.  Solution?  If it hurts, don't do it.

If leaving it on, when the equipment is not a repeater (or the dread fanout),
causes problems, get the offending equipment fixed! 

R. Kevin Oberman			Lawrence Livermore National Laboratory
Internet: koberman@llnl.gov		(510) 422-6955

Disclaimer: Don't take this too seriously. I just like to improve my typing
and probably don't really know anything useful about anything.
Newsgroups: comp.protocols.tcp-ip