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From: morgan@jessica.stanford.edu (RL "Bob" Morgan)
Newsgroups: comp.dcom.lans
Subject: Re: EtherSwitch
Message-ID: <1990Aug13.225710.16904@portia.Stanford.EDU>
Date: 13 Aug 90 22:57:10 GMT
References: <1990Aug11.185430.500@hellgate.utah.edu>
Sender: news@portia.Stanford.EDU (USENET News System)
Organization: Academic Information Resources
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> 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."

Walt mentions one problem with such a device, that the net to be
forwarded to may be busy at the time a frame is to be forwarded.  This
is even more likely in the case of a multicast frame.  I can only
guess that such a beast would have to be able to buffer at least as
many frames as an ordinary bridge to deal with this situation.  In
fact, since it has 6 ports, its buffer would have to be a lot bigger.
The "no-delay" switching would really just be an optimization in the
case of finding the destination network free.

Another problem with this gizmo is collisions.  Since an Ethernet
frame is still vulnerable to collision up to its 42nd (or so) data
byte, this switch would often propagate collision fragments from one
net to another, which a proper bridge would not do.  This would result
in an over-busy net spewing collision fragments to others, which seems
undesirable.  Maybe the switch waits until after the minimum frame
size time (ie, max round-trip time) to start forwarding?

Of course, frames with bad CRCs get forwarded too.  Again, this would
spread the effects of a marginal net to others.  I guess it's still
true that you don't get sup'm fer nut'n.

 - RL "Bob" Morgan
   Networking Systems
   Stanford