Path: utzoo!utgpu!watmath!clyde!att!ucbvax!decwrl!mogul
From: mogul@decwrl.dec.com (Jeffrey Mogul)
Newsgroups: comp.dcom.lans
Subject: Re: Token Ring vs. Ethernet
Message-ID: <28@jove.dec.com>
Date: 9 Jan 89 18:55:27 GMT
References: <5786@medusa.cs.purdue.edu>
Organization: DEC Western Research
Lines: 37

In article <5786@medusa.cs.purdue.edu> narten@cs.purdue.EDU (Thomas Narten) writes:
>Look, go read the paper by Boggs et al.  They look only at the
>architecture.  They don't say "Ethernets are better than token rings";
>they *do* put to rest the myth that token rings are inherently
>superior to CSMA/CD by a *wide* margin.  In other words, statements
>like "token rings are better than ethernets" are just plain nonsense
>(but then, so are statements like "ethernets are better than rings").

Alas, it's not so simple.  Because the efficiency of CSMA/CD depends on
the ratio of packet length (expressed in time, or packet-bits/bit-rate)
to the propagation delay for the entire cable, it doesn't scale well
to high-delay (i.e., high bandwidth*length) networks.  For example,
a 100 Mbit CSMA/CD net would only perform as efficiently as the 10Mbit
Ethernet if the faster net were one tenth the length of the shorter
net, or if the minimum packet size were ten times longer.  Since it
doesn't make sense to require 600-byte packets, a 100Mbit CSMA/CD net
would probably only work for cables < 500 meters.

There might be some tricks to get around this (see the paper by
Maly et al, "Dynamic Bandwidth Allocation in a Network", SIGCOMM 88,
for something that might work) but basically it's a fact of life.

In our paper, we said
	At higher bit rates or for longer networks, ring topologies
	may be the only acceptable approach, but experience with
	Ethernet proves that at 10 Mbits/second, over a kilometer
	or so of cable, CSMA/CD is quite successful.
(Experiments conducted after that was written suggest that even
5 Km of cable works well.  Dave Boggs believes that CSMA/CD is
"viable for any combination of bandwidth under 20 Mbits/sec and
distance under 10 Km.")  One should also not write off point-to-point
or star technologies.

Bottom line: you need to pick the parameters before you can make
a comparison between two LAN technologies.

-Jeff