Path: utzoo!news-server.csri.toronto.edu!rutgers!usc!rpi!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!nucsrl!tellab5!laidbak!mcdchg!att!drutx!druco!moleres
From: moleres@druco.ATT.COM (Rick Moleres)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet performance degradation?
Keywords: PC, NFS, Novell, Netware, DOS
Message-ID: <7804@drutx.ATT.COM>
Date: 12 Mar 91 17:11:32 GMT
References: <1991Mar7.220351.9761@uhura1.uucp> <1991Mar7.221028.9883@uhura1.uucp>
Sender: news@drutx.ATT.COM
Distribution: usa
Lines: 30

> From: bryan@uhura1.uucp (Bryan Curnutt)
> Newsgroups: comp.dcom.lans
> Subject: Ethernet performance degradation?
> Message-ID: <1991Mar7.221028.9883@uhura1.uucp>
> Date: 7 Mar 91 22:10:28 GMT
> Reply-To: bryan@uhura1.UUCP (Bryan Curnutt)

> I've also heard (though I don't know how true it is) that Ethernet
> network performance degrades drastically if too many nodes are added
> to the network, and that the number of nodes to do this is relatively
> small.  Is there any truth to this?
> -- 
> Bryan Curnutt            bryan%uhura1@uunet.uu.net

As I understand it, it is the amount of traffic that degrades
performance, not the number of nodes.  One could argue that
increasing the number of nodes increases the amount of traffic
on the ethernet, but that is not necessarily true.

An ethernet uses CSMA/CD (Carrier Sense Mutliple Access/
Collision Detection) to gain access to the bus.  When a collision
is detected, an exponential backoff algorithm is used by a node
to determine when to try again.  If the amount of traffic on the
ethernet is heavy, then collisions occur often and the interval
before retries becomes exponentially long.  Pretty soon you have
an under-utilized bus because the nodes are waiting for their
next retry time.  Thus, performance on the ethernet has degraded
due to heavy traffic.

-Rick