ATM Wide-Area Networks (was: sell shell accounts?)

Dave Siegel dave at
Tue Jul 23 17:22:23 UTC 1996

> > > Acceptable arguments are:
> > > o Switches can handle more throughput
> > 
> > That's difficult to quantify in theory *or* practice.
> > 	[...]
> I may misunderstand your assertion, but it doesn't seem all that
> difficult to quantify, at least to some coarse level.
> We have been using wide-area ATM switches at OC-3c for some time.
> It is pretty clear that the switches can handle OC-3c.  The
> early switches had relatively small output buffers, so they tended
> to loose cells before TCP could throttle back in congested circumstances.
> We are now using switches with much larger output buffers, and TCP appears
> able to throttle back fairly gracefully.

It's difficult to quantify wrt to how ATM plays a role in end-to-end 
performance on the Internet.  There is very little research to support how
ATM affects the overall performance.  Even Ameritech and PacBell restricted
the majority of their performance evaluation on performance in the switch,
and only extended their scope if they leased an ADSU to a customer.  Even
with the improved buffering, if you fill your pipe into the ATM switch,
your ATM switch still becomes a packet shredder, compared to the more
graceful packet drops seen on a clear channel line.

The performance of the switched technology is integral with the equipment
attached to it.  You cannot evaluate the performance of the system by
analyzing small chunks of it, and coming to conclusions on the whole system.

This is what I'm talking about.  It's easy to qualify that ATM switches
can switch cells much faster than routers can switch packets (with current
commercially available routers), but it's much more difficult to qualify
the scope of how ATM improves performance in the big picture.


Dave Siegel		     Sr. Network Engineer, RTD Systems & Networking
(520)623-9663 x130	     Network Consultant -- Regional/National NSPs
dsiegel at		     User Tracking & Acctg -- "Written by an ISP,					for an ISP."

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