Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!husc6!uwvax!umn-d-ub!umn-cs!wsmith
From: wsmith@umn-cs.cs.umn.edu (Warren Smith [Randy])
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet utilisation
Keywords: ethernet csma/cd utilisation
Message-ID: <5344@umn-cs.cs.umn.edu>
Date: 11 May 88 19:52:17 GMT
References: <47@xenon.UUCP> <9110@g.ms.uky.edu> <5038@bloom-beacon.MIT.EDU> <2261@inco.UUCP>
Reply-To: wsmith@umn-cs.UUCP (Warren Smith [Randy])
Organization: University of Minnesota
Lines: 69
Posted: Wed May 11 14:52:17 1988


Sorry for the delay, I'm just catching up on my news here...

In article <47@xenon.UUCP> goodloe@xenon.UUCP (Tony Goodloe) writes:
>I have heard (from some forgotten source ) that the access mechanism used on
>Ethernet will not allow a REAL csma/cd network to achieve more than about
>50%-60% network utilisation. Can someone point me in the direction of a paper,
>article, personal experience, etc. that supports or refutes this claim?
>Many thanks.  
 

And then added the following (not all on one line this time):
>I feared that my posting didi make it to the net in its entirety, and I can
>see now that it must not have. My question was "what maximum utilization
>can one expect to see on an Ethernet?" I know that a csma/cd net is
>non-deterministic, that's why the question exists, otherwise I could
>determine it :) I was looking for more sources like "Performance of an
>Ethernet Local Netwotk: A Preliminary Report" by Shoch and Hupp  or
>"Performance Analysis of CSMA/CDD" by Tobagi and Hunt. I haven't made it
>through Tobagi and Hunt, but according to Shoch and Hupp they have seen
>a maximum of ~98% utilization down to maximum ~60% utilization depending on the
>test conditions. Read the article for more info. I was looking for more
>info like this, or real live experience. Our net gets up to about 35%
>and we do have individual nodes capable of more. What I am curious about
>is what people have seen or caculated (using statistical means (no pun
>intended)) on other nets with lots of these beasts on them. Maybe this
>posting will make it out in one piece.

There are numerous articles and books that review the basic analysis
of Ethernet (see Boggs and Metcalfe from CACM sometime in 1976 for
the first analysis).  Almost every computer networking text includes some
portion of a CSMA analysis.  Aside from Shoch and Hupp there is very
little published on measured Ethernet performance.  There was a paper in
1986 by Raj Jain and Shawn Routhier (IEEE Selected Areas of Comm) on some
measurements of a 10Mbps token ring at MIT - you may want to track down
some of the papers they reference.  If you know of other papers
in this area please tell me.

Here are some of my observations (assuming a network with a large number
of stations that aren't synchronizing to avoid collisions):
  1) Ethernet can utilize up to about 96% of the 10Mbps bandwidth
  2) Higher utilization results in higher delays (due to collisions
     and deference).
  3) Larger (physically) Ethernets waste more time detecting collisions,
     which are also more likely.
  4) Just because the Ethernet itself can reach ~96% utilization doesn't
     mean your stations or higher level protocols will survive it.
  5) At about 50-60% load the delay shoots up dramatically
  6) I have monitored an Ethernet with a sustained load of 48%
     averaged over an hour.  This was a network with about 50 nodes,
     many diskless.  Performance was poor but bearable (debatable).
     This network usually averaged around 25% load during the busy
     period of the day.
  7) Token rings (in general, assuming the same bandwidth) do much better
     in terms of delay at the higher loads.  However delay becomes a
     significant factor beyond 70-90% load.  Of course the token ring
     gets a slight penalty in terms of delay at low loads.

After going back and looking at your question again I see that what I
said in (5) resembles what you heard from your forgotten source.
The numbers are fuzzy since there are so many other factors involved.
For some Ethernets 40% load may be unbearable, on others 70% load
no problem.

Randy Smith
wsmith@umn-cs.cs.umn.edu
...!ihnp4!umn-cs!wsmith
-- 
Randy Smith