Path: utzoo!mnetor!uunet!husc6!mailrus!ames!ucsd!nosc!tetra!budden
From: budden@tetra.NOSC.MIL (Ray A. Buddenberg)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: maximum Ethernet throughput
Message-ID: <663@tetra.NOSC.MIL>
Date: 6 Mar 88 20:24:29 GMT
References: <8803012359.AA00957@acetes.dec.com>
Reply-To: budden@tetra.nosc.mil.UUCP (Rex A. Buddenberg)
Organization: USCG
Lines: 27

definition to give a straight answer.

Unless you know what reference model layer, you can't properly
reply.

Similarly, the number of nodes on the net makes an awful lot of difference.

OK, to the ethernet problem.  10 Mbit/sec at the physical layer turns
into about 4 Mbits at the network layer (neglecting trivial cases of
only two nodes on the network).  This decrease is due to the carrier
sense multiple access characteristics of ethernets.

At the transport layer, this tends to decrease further to around 1 Mbit
due to transport overhead.  The figure is plastic depending on whose
TCP you are using.  Unless you've got a lot of overhead in the
higher layers, this 1Mbit figure is pretty much the application-to
application value.

So what does this all mean?  At the network layer, a '4 Mbit' 802.5
token ring gets just as much data thru as a '10 Mbit' ethernet.  Caveat
emptor when the salesman comes to call.  As higher speed networks become
common -- FDDI starts out at 100 Mbit at the physical and network layers,
the Transport layer becomes an even more obvious bottleneck.  Indeed,
it appears that the bottleneck may be shifting from the LAN media toward
the DMA hardware.

Rex Buddenberg