[rescue] FDDI vs switched 100BaseTX, vs SPARC LX

Dave McGuire rescue at sunhelp.org
Wed Jun 20 22:10:41 CDT 2001

On June 20, Loomis, Rip wrote:
> Secondly, back when I wanted to play with FDDI
> I didn't have either the cash or the time...I
> still don't, but I'm curious.  Given that I've
> already got a fully-switched 100BaseTX backbone
> at home, is FDDI really worth playing with, perhaps
> for the LX and other older boxen that might come
> live in my basement?  Anyone have a favorite
> methodology/program to benchmark my existing
> setup to see if I can really get 200Mbps sustained
> throughput?

  I have several reasons for preferring FDDI, not all of which would be
considered "reasonable" by all people.

  My main reason may be called "puritanism" but it works for me.
100Mbps ethernet works pretty well, and if EVERYTHING is switched, you
get decent performance out of it.  But look at the design...switching
ethernet is a band-aid that's used to patch a limited-scalability
design after the fact.  Ethernet is SUPPOSED to have collisions...it's
a CSMA/CD medium.  But it's those collisions (in a multi-node network)
that wreak havoc on performance, so you have to eliminate them to get
decent performance.  An unswitched 100Mbps ethernet network with any
respectable number of nodes will go to hell in a handbasket at
55-60Mbps due to collisions and the very nature of CSMA/CD.  FDDI,
however, is a token-passing technology.  You can take it clear up to
95MBps without significant performance degradation...WITHOUT modifying
any of its basic design philosophies.

  So the basic gist of it is...to get reasonable performance out of
ethernet, you have to band-aid one of its basic design points.

  FDDI stuff is also cheap on the surplus market, because many people
consider it to be "obsolete".  Of course, "obsolete" pretty much means
"PeeCee vendors aren't pushing it".  Folks are invited to read Sun's
FDDI position paper at http://www.sun.com/products-n-solutions/hw/networking/connectivity/sunfddi/fddi_position.html.

  I also like FDDI's 4.5KB MTU.  Makes TCP transfers go ZOOM. :)
Further, FDDI is an extremely elegant design.  That, however, is
purely a matter of opinion.  Ethernet is popular because it's cheap
and familiar...not because it's technically superior.  I prefer
technically superior solutions and don't mind paying more or working
harder for them.

  As far as benchmarking...I've found TTCP to be a useful tool.  It is
good at quantifying the performance of an entire networking
_system_...the cards & medium, device drivers, and OS networking code
performance as a whole.

        -Dave McGuire

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