From: johnb@kalpana.com (John Bartas)
Subject: Re: Please explain....
Organization: Kalpana Inc
Date: Sat, 14 Nov 92 01:52:08 GMT

In article <1992Nov4.221611.21276@megatek.com> randy@megatek.com writes:
>In article <1992Oct31.223152.27162@kalpana.com> johnb@kalpana.com (John Bartas) writes:
>[...]
>|	The Kalpana Etherswitch is designed to offer the bandwidth 
>|preservation of the router, but without the drawbacks. The
>|Etherswitch monitors all frames on all segments, and forwards only
>|the ones which need it. It does this based on the frame's MAC
>|destination address, which all ethernet & 802.3 frames have in the
>|first six bytes. The Switch keeps a table of which MAC addresses are
>|on which segment. If host A in the diagram sent a frame to host B,
>|the switch would look up host B's MAC address as soon as it had the
>|1st 6 bytes, realize it's on the "local" segment to A, and not
>|forward it. If A then sent a frame to C, the switch would look up
>|host C's MAC address, realize it's on segment 2, and switch in a
>|connection between segment 1 and segment 2. 
>[...]
>
>  Correct me if I'm wrong, but isn't this more commonly called just a
>"bridge", of which many are offered by a number of companies, and not just
>by Kalpana under the "Etherswitch" name?
>

I don't want to get into the internals of how other companies products
work, but I'll review a few points of the Etherswitch, and contrast it
with how (I think) some bridges work. I should stress that I'm a software 
person and faily new to this (Bridging) segment of the Industry. If I'm 
wrong I'm sure someone will chime in and correct me. :-)

First, the Kalpana Switch has no internal bus. Each port has direct access
to every other port, so several pairs of ports can be activly forwarding 
frames in parallel. Some bridge products (so I'm told) use internal busses
to move frames from port to port, and thus have to deal with with bus
contention in addition to everything else. This can hurt the aggragate
bandwidth of the bridged segments.

Another point is lantency. We measure our latency forwarding a 1514 byte
frame at less than 40 microseconds. Bridges we have measured run between
200-400 Microseconds. We get this speed via the method I described
previously. I *think* many bridges wait until they have received the
entire frame before they resend.

Lastly, The bridges I've seen generally have fewer than 15 ports. The
Kalpana asics can handle up to 15 ports without any performance
degradation. 

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.


>Randy Davis                            Email: randy@megatek.com
>Corporate Network and System Administrator    megatek!randy@uunet.uu.net
>Megatek Corporation, San Diego, California    ucsd!megatek.uucp!randy
>

cheers,

======================================================================
John Bartas                     | Disclaimer - I don't speak for Kalpana
Senior 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

Newsgroups: comp.dcom.lans.ethernet