Path: utzoo!mnetor!uunet!seismo!sundc!hadron!inco!holt
From: holt@inco.UUCP (Mark Holt)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet utilisation
Message-ID: <2261@inco.UUCP>
Date: 4 May 88 13:19:16 GMT
References: <47@xenon.UUCP> <9110@g.ms.uky.edu> <5038@bloom-beacon.MIT.EDU>
Organization: McDonnell Douglas-INCO, McLean, VA
Lines: 62
Keywords: ethernet csma/cd utilisation
Summary: RE: ethernet utilization - a reference

In article <5038@bloom-beacon.MIT.EDU>, mar@athena.mit.edu (Mark A. Rosenstein) writes:
> 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
 
Also: (I tried to mail directly ... same old story)
News-Path: hadron!sundc!seismo!uunet!ingr!b11!xenon!goodloe

> 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.  
>  
Reference : Stallings, William, Local Networks - An Introduction, Macmillan,
1987, Chapter 9 - Network Performance.

The above is a readable, if inconsistant, discussion on some techniques
for approximating performance.  The book points out that the chief factors
affecting the performance of a LAN or a HSLN (Hi Speed Local Net) are:

quote
	Capacity
	Propagation Delay
	Number of bits per frame
	Local network protocols
	Offered load
	Number of stations
end quote

Stallings then goes into greater detail on how some of these parameters
affect performance.  For example, he shows (by way of derivation of 
formulas and use of graphs) how the number of bits per frame and the 
number of stations active can impact the capacity vs actual data rate 
(ideal : actual data = capacity) curve.  Also, he goes on to show
how much more efficient (maximum potential utilization) a token ring
topology is than CSMA/CD.

Now, before I get torched by people crying "real world, real world",
Stallings notes that the results he shows "...depend on the nature
of the assumptions made and do not reflect accurately the nature
of the real-world load."  However, I think he has built a pretty
good argument... :-) .

Hope this helps, my apologies if I have misrepresented Mr. Stallings.

mark
-- 
/*--------------------------------------------------------------*
*  Mark L Holt                                                  * 
*  McDonnell Douglas - Inco, Inc.                               *
*  ...!seismo!sundc!hadron!inco!holt                            *
*  "If you ain't the lead dog, the scenery never changes"       *
*  DISCLAIMER: The opinions expressed are my own and in no way  *
*  reflect the views of McDonnell Douglas or its subsidiaries.  *
*--------------------------------------------------------------*/