RE: switch latency (was Command Queue Depth, Asymmetric/Symmetric)

["Douglas Otis" <dotis@sanlight.net>]

Oops, I can not read my own writing.  It is 8 micro-seconds per mile. Such
that 50 miles would be 402 micro-seconds.

Doug

> -----Original Message-----
> From: Douglas Otis [mailto:dotis@sanlight.net]
> Sent: Monday, September 18, 2000 10:49 AM
> To: Merhar, Milan; 'ips@ece.cmu.edu'
> Subject: RE: switch latency (was Command Queue Depth,
> Asymmetric/Symmetric)
>
>
> Merhar,
>
> FIFO depth comes into play as an additional factor.  If an
> average port includes somewhere from 16-64 packet depth for
> asynchronous traffic, these points of buffering become
> significant.  Expectation of delay should be in milli-seconds as
> few paths are limited to just a few such points of buffering.  3
> micro-seconds per mile in glass provides 150 micro-seconds to
> travel 50 miles.  If the lowest level switch is 1 Gb, then there
> is about 1.5 micro-seconds per 1.5k packet buffered.  Should a
> buffer depth be 10 at that point in time, then one switch may add
> an additional 15 micro-seconds.  Once data is delivered into the
> NIC, even more significant buffering takes place.  Although it
> may be possible to consider micro-second delivery within a
> Metro-Area-Network, drop just one packet and this will more than
> triple transit time.  If you expect a level of performance, you
> should consider both distance, buffering and head of queue blocking.
>
> Doug