Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!oliveb!olivey!jerry
From: jerry@olivey.olivetti.com (Jerry Aguirre)
Newsgroups: comp.dcom.lans
Subject: Re: Token Ring (was: Re: Info on LANs)
Message-ID: <35465@oliveb.olivetti.com>
Date: 3 Jan 89 20:42:52 GMT
References: <12786@cup.portal.com> <920001@hposdl.HP.COM> <10777@s.ms.uky.edu> <18659@agate.BERKELEY.EDU>
Sender: news@oliveb.olivetti.com
Reply-To: jerry@olivey.UUCP (Jerry Aguirre)
Organization: Olivetti ATC; Cupertino, Ca
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In article <18659@agate.BERKELEY.EDU> glass@tehran.berkeley.edu (Brett Glass) writes:
>A 4-Mbps Token Ring will outperform a 10-Mbps Ethernet under
>heavy loads because of the absence of collisions. And the

Absolutely true!  Except that it doesn't specify which ethernet we are
talking about.  I have heard from several sources that the theoretical
study that proved the above was not based on the latest 802.3 ethernet
standard.  No, it wasn't based on ethernet II either.  No, not ethernet
I either.

You have to remember that the study was done back when Xerox was
developing ethernet so it was based on an experimental version.  I
believe the key difference was in carrier sense, detection of
collisions, and random backoff.

Orriginal versions just transmitted whever they wanted to and depended
on luck to avoid collisions.  If there was a collision they didn't know
about it and had to wait for timeout before trying to retransmit.  It is
easy to predict how such an ethernet would perform under heavy load.
Talk to the average token ring guru and they will tell you this his how
ethernet performs today.  (While at the same time stating that ethernet
I, II, and 802.3 are incompatable and can't coexist on the same cable.)

Modern ethernets check to see if anyone is using the cable before trying
to transmit.  If two stations happen to start transmitting so close
together that that one doesn't see the other then a collision is
detected and both abort without wasting time sending the rest of the
packet.  So collisions don't happen as often and when they do they are
very short.  Finally, the colliding stations use a random backoff to
reduce the chances of subsequent collisions.

Finally, the performance figures for station to station and the various
problems with different ethernet boards and chips affect how much
bandwidth a single station can use.  They have little to do with the
aggrigate bandwidth that multiple stations can achieve.

			Jerry Aguirre