Path: utzoo!mnetor!uunet!ccicpg!turnkey!stanton!donegan
From: donegan@stanton.TCC.COM (Steven P. Donegan)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet utilisation
Message-ID: <38@stanton.TCC.COM>
Date: 30 Apr 88 14:08:47 GMT
References: <47@xenon.UUCP>
Organization: Stanton Public Domain Systems, Stanton, Ca.
Lines: 34
Keywords: ethernet csma/cd utilisation
Summary: CSMA/CD is NON-DETERMINISTIC!

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.  
>  

I have seen these types of questions so many times that I feel it necessary to
respond. A CSMA/CD media protocol is non-deterministic. If only 2 stations are
connected via the cable and a transfer is started then the full bandwidth
available can be utilized. As the number of stations, distance between stations,
cable plant inconsistancies (I've never seen a perfect ethernet segment) and
many other factors change it becomes impossible to determine real throughput
by prediction. Only real world testing will give you valid results and due to
the nature of most networks (constant change) the results of any tests are
only valid for a limited time. With the exception of diskless workstation
nets I have seldom seen real network utilization exceed 20-30%. Diskless
workstation environments can eat up the bandwidth incredibly fast (6 - 10 SUN
stations can saturate a segment if you try :-))). Repeaters and bridges can
make this question even more impossible to answer. I find continuous performance
monitoring the only reliable way to determine the state of your network. The
items to watch for are an increasing (slowly, due to traffic; not quickly due
to hard problems) level of collisions, crc errors, too short or too long
packets. I have watched WD's ethernet grow from 1 building and <100 stations
to 16 buildings, 2800+ stations (station=end device, not node - nodes went
from 3 to roughly 200 as of today) and have run into many different performance
problems, all of which could be solved by segmentation and learning bridge
implementations. An ethernet is a synergy of all it's respective components,
only extremely careful design and monitoring can keep it running properly.

After all the above, it's still far easier than manageing a broadband net :-)

-- 
Steven P. Donegan
Sr. Telecommunications Analyst
Western Digital Corp.
donegan@stanton.TCC.COM