Newsgroups: comp.dcom.lans
Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!hellgate.utah.edu!cs.utah.edu!haas
From: haas@cs.utah.edu (Walt Haas)
Subject: EtherSwitch
Date: 11 Aug 90 18:54:29 MDT
Message-ID: <1990Aug11.185430.500@hellgate.utah.edu>
Organization: University of Utah CS Dept

I just finished reading an article in the August issue of Data Communications
about a thing called an EtherSwitch, made by Kalpana Inc. in Campbell, CA.
The article was written by the inventor of the device and President of
Kalpana, so perhaps it wasn't totally objective about the pros and cons of
this approach :-).

The EtherSwitch functions like an Ethernet bridge with 6 ports.  It is not
a store-and-forward device, but instead looks at the address bits of the
packet coming in from a port and decides which port to send it out.  The
article says (p 84) "...switched packets are delayed an average of only
40 microseconds; packets handled by a bridge are delayed an average of
800 microseconds, the time it takes to read and forward an entire packet."

That's all fine if the destination Ethernet port does not have traffic at
the time the packet appears.  However, let's suppose that the destination
port is busy at the time the packet arrives there.  It seems to me as I
think about this device, that a collision will be detected 40 microseconds
into the packet.  Assuming the EtherSwitch propagates a jam back into the
source port, there remains only 6 microseconds for that jam to propagate
all the way to the far end of the source Ethernet.  According to my
quackulations :-) that limits the Ethernet to less than the usual length...
maybe a couple of 500 meter segments and one repeater.  And he said that
40 microseconds was "average", implying that half the time the delay is
greater...

Does anybody in netland understand these things?  I'd appreciate any
enlightenment!

-- Walt Haas    haas@cs.utah.edu
