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From: glass@tehran.berkeley.edu (Brett Glass)
Newsgroups: comp.dcom.lans
Subject: Re: Token Ring (was: Re: Info on LANs)
Message-ID: <18672@agate.BERKELEY.EDU>
Date: 2 Jan 89 19:34:30 GMT
References: <12786@cup.portal.com> <920001@hposdl.HP.COM> <10777@s.ms.uky.edu> <18659@agate.BERKELEY.EDU> <13096@bellcore.bellcore.com>
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In article <13096@bellcore.bellcore.com> karn@ka9q.bellcore.com (Phil Karn) writes:

>His results show that a properly constructed Ethernet is quite capable of
>carrying a sustained load at a level considerably greater than 40% of its
>capacity.  Therefore it is impossible for a 4 Mbps Token Ring to outperform
>a 10 Mbps Ethernet, regardless of the Token Ring's efficiency.

Boggs' argument may ignore certain key practical considerations.  While
doing the research for my article, I spoke to many consultants who'd
designed large networks for campuses and businesses. All recommended that
the Token Ring be used as the backbone (at least) -- even before the 16 Mbps
ring was announced -- and claimed that the throughput was similar. With all
due respect to Dave Boggs (who seems to be quite a brilliant fellow), I'd
credit these people with less partiality than one of the inventors of
Ethernet. In this case, it's probable that Boggs was looking only at the
number of bits travelling down the cable, rather than overall system
throughput.  There's a big difference, as I'll explain below.

One of the reasons a 4 Mbps Token Ring can outperform a 10 Mbps Ethernet
(other than the lack of collisions I mentioned earlier) is the built-in
acknowledgement provided by the trailer at the end of a returning frame.  On
an Ethernet, one must send a packet to acknowledge receipt of a message --
with all the delays inherent in setting up a buffer, waiting for the cable
to clear, etc. This overhead can cut the net throughput of an Ethernet by
more than 75% under any protocol requiring reliable data transport.

I have direct experience with this phenomenon. The Token Ring isn't the only
network that implements built-in acknowledgements; ARCnet does, too.  When I
first set up a 2.5 Mbps ARCnet in my apartment for a consulting project, I
was surprised to find that it ran faster than my 10 Mbps Cheapernet under
the same network software! I thought this was just a fluke until I confirmed
my observations with a few gurus who designed low-level network software for
a living.  The writers of QNX, a UNIX-like operating system based on message
passing, tell me that a 2.5 Mbps ARCnet outperforms a 10 Mbps Ethernet under
their OS for the same reason. Two implementors of IBM's NetBIOS protocol
(CBIS and Performance Technology) reported similar results. (I don't have a
Token Ring right now, but I expect that it would perform even better than
ARCnet due to the higher bit rate, larger frame sizes, lower latency, and
even more efficient acknowledgement scheme.)

Moral: Like CPU clock speeds, network bit rates tell only part of the story
about real-world performance. It's not just what you've got, it's how you use
it.

>He also observed that with the progress being made in "porting" Ethernet to
>media other than coaxial cable, "it is now possible to run Ethernet on
>wiring plants originally installed for IBM Token Ring". 

That's true; most networking magazines carry ads for baluns which take
Ethernet coax at one end and Token Ring cabling at the other. But the
motivation for using such kluges is usually logistics.  Too many hardware
vendors support only one network standard, and customers are forced to adapt
or rewire. At least one thing can be said for the Token Ring wiring system
in such situations: You can run Ethernet over Token Ring cabling, but not
the other way 'round!

>"Ethernet works in practice, but not in theory".

By this, does Boggs mean that the IEEE and others are standardizing
equipment that can't be proven to work? Gee, maybe I should give up EE
and change my major to voodoo.... ;-)

-- Brett Glass

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Copyright (C) L. Brett Glass 1989. All rights reserved.
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