Path: utzoo!mnetor!uunet!husc6!bbn!gatech!purdue!decwrl!mogul
From: mogul@decwrl.dec.com (Jeffrey Mogul)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet utilisation
Message-ID: <412@bacchus.DEC.COM>
Date: 2 May 88 22:08:48 GMT
References: <47@xenon.UUCP> <9110@g.ms.uky.edu> <5038@bloom-beacon.MIT.EDU>
Reply-To: mogul@decwrl.UUCP (Jeffrey Mogul)
Organization: DEC Western Research
Lines: 35
Keywords: ethernet csma/cd utilisation

In article <5038@bloom-beacon.MIT.EDU> mar@athena.mit.edu (Mark A. Rosenstein) writes:
>
>Yes, there are examples of a pair of hosts using 80% of an ethernet by
>themselves.  But that does not mean that 10 hosts, each doing
>different things and talking to different peers, could use that much
>of the net.  
> [...]
>If anyone has any simulations, or better yet, real world numbers to
>back this up, I'd like to hear them.
>					-Mark

Dave Boggs has been doing the real-world measurements (in the sense
that he's using real computers and networks; it's hard to find real
applications that can saturate an Ethernet).  We have written a paper,
together with Chris Kent, that describes these measurements, summarizes
the theory literature, and explains why numbers like "37%" are at best
a misunderstanding of the theory.

The paper has not yet been published; we will let people know when it is.
Some preliminary results: with 1536-byte packets, 1 of our hosts can
get 92% of the (10Mbit/sec) bandwidth; 2 hosts together get 99.8%;
22 hosts get 97%.  I.e., for large packets the ethernet really is
10 Mbits/sec.

Theory says that smaller packets lead to lower utilization, because
collisions occur more often.  For minimal-length (64-byte) packets,
the host we are using could not send them fast enough to get more than
26% utilization with 1 host on the air (that's 3700 packets per
second; this has nothing to do with collisions).  We get 75% with 3 hosts,
81% with 10 hosts, and it appears to stay at 81% through 22 hosts
(11500 packets/sec.)

Of course, the delays seen by each host get worse as the number
of transmitters go up, but the Ethernet does not suffer congestive
collapse even when an awful lot of iron is shouting really loud.