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From: krowitz%richter@UMIX.CC.UMICH.EDU (David Krowitz)
Newsgroups: comp.sys.apollo
Subject: Re: "Ethernet or Apollo Token Ring?"
Message-ID: <9004091504.AA07810@richter.mit.edu>
Date: 9 Apr 90 15:04:49 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
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MIT telecommunications office estimates that in a *REAL* "real world" situation
(ie. several dozen Sun-3's, DEC Vaxstations of various sorts, and IBM PC's running
project Athena software) they never get more than 30% of the specified ethernet
performance. I can believe that you'd get 85% of the spec with only two nodes
talking -- you would not get vary many collisions -- but try putting half a dozen
diskless Sun-3's talking to 4 or 5 file servers all doing geophysics, oceanography,
or meteorology applications. It's a whole different ballgame.

A single node reading a remote data file, or doing diskless paging, can easily
generate 100 1kbyte packets per second. This is ~1/10th the capacity of an ethernet.
If three nodes (out of a few dozen) were running like this, and if they were
somehow avoiding any collisions (which would eat up extra net bandwidth), they
would be consumming 30% of the network's capacity. Any addition node would have
at least a 30% chance of causng a collision ON EVERY SINGLE TRANSMIT IT ATTEMPTS!
Each collision with a 1Kb packet consumes 0.1% of the network capacity. At 100
packets/sec, this one additional node is eating up a minimum of 3% of the network
capacity in collisions by itself. Add routing daemons, rlogin/telnet sessions (which use
a TCP/IP packet for every single character you type!), and twenty to thirty nodes all
running NFS with a 50-50 split between diskless nodes and severs and the network
capacity eaten up by collisions is substaintial. 


 -- David Krowitz

krowitz@richter.mit.edu   (18.83.0.109)
krowitz%richter.mit.edu@eddie.mit.edu
krowitz%richter.mit.edu@mitvma.bitnet
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