Path: utzoo!mnetor!uunet!husc6!mailrus!tut.cis.ohio-state.edu!bloom-beacon!athena.mit.edu!mar
From: mar@athena.mit.edu (Mark A. Rosenstein)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet utilisation
Message-ID: <5038@bloom-beacon.MIT.EDU>
Date: 2 May 88 18:59:54 GMT
References: <47@xenon.UUCP> <9110@g.ms.uky.edu>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: mar@athena.mit.edu (Mark A. Rosenstein)
Organization: Massachusetts Institute of Technology
Lines: 30
Keywords: ethernet csma/cd utilisation


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.  

When only 2 hosts try to use the entire network, the timing is very
tightly interleaved so that each sends another packet while the other
is listening.  But with several pairs of hosts attempting to use the
same net, they are likely to have collisions if each pair is
attempting to use 80% of the net.  In this case several hosts will be
waiting for the end of one packet so that they can start sending, and
may all start at the same time, causing a collision, then backoff and
retry.

I don't have any numbers to back this up, but I suspect that the
utilization will go down somewhat as more hosts are added, although it
would level off pretty quick.  I mean that one host could use the
entire bandwidth, all it ever needs to do is send.  Two cooperating
hosts could interleave packets and use nearly the entire network.
More pairs will collide a certain amount, but once you've added a
certain number of hosts, you will continue to get, say, 75% network
usage.

If anyone has any simulations, or better yet, real world numbers to
back this up, I'd like to hear them.
					-Mark

mar@athena.mit.edu			...!mit-eddie!athena.mit.edu!mar
		 Variables won't and Constants aren't