From: mwr@eng.tridom.com (Mark Reardon)
Subject: Re: Please explain....
Reply-To: mwr@eng.tridom.com (Mark Reardon)
Organization: AT&T Tridom; Marietta, Georgia
Date: Thu, 19 Nov 1992 16:30:29 GMT

In article <1992Nov19.045954.7586@kalpana.com>, johnb@kalpana.com (John Bartas) writes:
|> In article <1992Nov16.235124.1097@netcom.com> seifert@netcom.com (Rich Seifert) writes:
|> >In article <1992Nov14.015208.17875@kalpana.com>, johnb@kalpana.com (John Bartas) writes:
|> >> I would be very interested if someone who knows more about conventional
|> >> bridges than I do could try to define exacltly what makes a given
|> >> device a hub|bridge|switch. I'm aware of other companies building
|> >> switches; I'd like to hear what they think.

My area in AT&T currently makes none of these products, we instead 
offer a router that works with our VSAT network.  I have worked at
a bridge company before though and think I have a good understanding 
of the technology.  A HUB in ethernet parlance is a 10BaseT HUB.
It is simply a repeater.  

In the context of this posting a bridge or switch works at the 
next layer above a repeater.  They terminate the collision 
domain and insert the data into another.  The old bridges that 
predate such things as spanning tree fit the description you
have offered of the EtherSwitch except that they did the 
decision processes in software instead of an ASIC.  Now more 
vendors of bridges are adding hardware assist to their ports
to do filtering but this is really unimportant to the consumer.

What is important to the consumer is that the product clearly
state what it does in terms that are understandable.  It is
also important that it adheres to any standards applicable
or explain where it doesn't and what advantages the consumers
gain form it.  An example:

Our bridge allows spanning tree blocking links to be used in 
the following circumstances:
        1. Only routes that have close cost factors are used.
        2. Traffic between any two ethernet addresses will 
           will always use the same path.
        3. ......
This option may be disabled by .....

Given the choice I would always want the conforming bridges 
since I have watched networks grow beyond the designers vision 
and I like the idea of being able to have backup paths.

|> >> 
|> >>
|> >
|> >Fromn a "black box" perspective, there is really no difference between
|> >a switch and a fast bridge. Except that some switches don't implement
|> >the Spanning Tree protocol and can cause sever network disruption in
|> >certain configurations.
|> > 
|> Perhaps what we should be measuring is how much usable bandwidth the various 
|> switch/bridge products provide. If two comparably priced products 
|> provide bandwidths that are significantly different, say a factor of 
|> two apart, the faster product clearly has applications that the slower
|> one does not. We do some testing of that sort here, Our current top of 
|> the line box claims 70 Mbits/sec under ideal conditions. (The hardware
|> guys tell me :-}. I'll let our competitors speak for themselves.

This number is a little different than what most vendors spec.  They
usually rate their product in packets per second and the packet sizes
used in testing.  This takes out of the specs game the question of is
a product really packet size independent.  I will do a little converting 
to packet numbers.

70Mbit/sec is 8,750,000 bytes/sec.
If packet size is 1514 this is 5779 pps.
If packet size is 64 this is 136718 pps.

A full ethernet at these packet sizes are about 880 and 14880 respectively.
Thus the switch can handle over 9 links full of min. size packets and
6.5 for max. size packets.  In my opinion these are very respectable
numbers.  Understand though that there are 50,000 pps bridges available
that are packet size independent, ie their numbers are the same for
each packet size.  I beleive Cisco quotes this number now.

|> 
|> >
|> >
|> >-- 
|> >Rich Seifert                    Networks and Communications Consulting
|> >seifert@netcom.com              (408) 996-0922
|> >                                (408) 996-2860 FAX
|> >"... specialists in Local Area Networks and Data Communications systems"
|> 
|> ======================================================================
|> John Bartas                     | Disclaimer - I don't speak for Kalpana
|> Pricipal Software Engineer      | in any official capacity. I'm just
|> Kalpana, Inc. (408)988-1600x141 | an employee who reads the net.
|> 
|> "We have met the enemy, and he is us." -Pogo

-- 
Mark

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| Mark Reardon           |  AT&T Tridom                             |
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