Re: IEEE 802.3 LAN Considerations vs. 802.5 Token Ring


Charles Hedrick (aramis.rutgers.edu!athos.rutgers.edu!hedrick@rutgers.edu)
7 Mar 88 04:53:26 GMT


The early Ethernet design studies claimed you could get at least
9Mbits out of Ethernet. However you normally don't want to configure
networks so that they are working that way. THis is true for any
network, whether Ethernet or token. In order to give people good
response on demand, you want peak capacity to be about a factor of 10
above what you expect typical useage to be. In fact people are now
getting transfers in excess of 1Mbit over Ethernet. (I've heard
claims of several Mbits with highly tuned software on fast machines,
but 1Mbit is fairly common these days.) In order to allow several
things to go on at once, this says you really do want a medium that's
at least 10Mbit. This is why the whole business about token
delivering predictable response is so wierd. Even if it is true that
a heavily loaded token system splits bandwidth evenly, nobody in his
right mind would configue a network so that it runs at full capacity
for anything more than a few milliseconds at a time. (And anecdotal
evidence is that token rings aren't really any more predictable
anyway.) The main argument for IBM token ring is when you have lots
of IBM equipment and want to do things like have your IBM 3270 cluster
controllers talk over the LAN. This is quite a reasonable thing to
do. On one of our campuses we expect to set up a token ring for
administrative use. However we expect to limit it to that. It will
handle 3270's and IBM PC's on administrative desks. For security
reasons, we don't want any student traffic on the same LAN. So we'll
use Ethernet and related technology for most of the general traffic.
Unless you have this sort of specific need for IBM-related networking,
Ethernet has the advantages of ubiquity and robustness. That is, you
can get Ethernet support for everything. Token ring hasn't been a
great success in the market. According to all the guys in Datamation
it's been about to wipe out Ethernet for the last 5 years now, but
somehow it has never taken off. Ethernet was also designed to be very
robust. You can install new taps without disrupting traffic. People
have violated the specs in all sorts of ways and still gotten it to
work. It is very conservative technology, and very well understood.
One of the chief designers for a well-known token ring company gave a
talk here a few years ago. He showed a bunch of performance
statistics and sort of surprised people by saying that as far as he
was concerned at 10Mbits and lower, token ring has only a minor
performance advantage. Certainly IBM's attempt to convince people
that 4 > 10 is simply absurd. It would be impossible to do Ethernet
at 100Mbits (cables would be restricted to 50 meters in length), so
all new protocols being designed are token ring. But that has led
some people to think that somehow token ring is a newer and better
technology. Not so. It's that we are beginning to move to faster
technology, and token rings make sense there. For 10Mb and slower,
the tradeoffs are different, and there are situations where each is
faster. In general Ethernet is better for a random mix, e.g. terminal
traffic and random FTP's, whereas token ring would be better for
certain kinds of graphics applications or real-time control work. For
most LAN's I'd say you're going to be better off to stick with
Ethernet unless you have good reason to do otherwise. The magic
initials IBM do carry some weight. When they first released the token
ring, I can see why people thought it was bound to succeed. But IBM
does sometimes fail to dominate an industry. Look at their attempt to
foist PL/I on us all. Token ring looks like one of these cases.



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