ATM Wide-Area Networks

Tim Salo salo at
Tue Jul 30 21:03:32 UTC 1996

>From: Sean Doran <smd at>
>To: pferguso at, salo at
>Subject: Re: sell shell accounts?
>Cc: nanog at
>Date: 	Wed, 24 Jul 1996 01:43:30 +0100
>| o	Given that each link is rather expensive, it behooves the
>| 	NSP to try to avoid needlessly moving a packet back and
>| 	forth across a DS-3 local loop just because my ATM 
>| 	provider can't provide me with any more PVCs.
>| 	(Which was my major point, by the way.)
>Interestingly enough, the NSPs I observe moving large amounts
>of traffic around are set up in such a way that their routers
>are generally colocated with carrier transmission gear, and
>so there *is* no local loop to speak of, as an added cost to
>an outside supplier.
>	[...]

Hopefully, my thinking is clearer than my writing...

What I originally called "local loop B," now relabeled "This Thing Here,"
is probably comprised of:

o	a port on the ATM switch/service

o	a connection between the router and the switch, (usually a
	wide-area link, given that most wide-area ATM services 
	backhaul traffic to a few ATM switches)

o	a port on the router

A short-cut VC would avoid needlessly passing packets back and forth
across "This Thing Here."  That would seem to be a good thing in
most circumstances, inasmuch as some or all of the components of
"This Thing Here" cost something and may potentially become a bottleneck.


|    ________           _______________________          ________
|   | Router | loop A  |                       | loop C | Router |
|   |   A    |=========| Wide-Area ATM Service |========|   C    |
|   |________|         |_______________________|        |________|
|                                  |
|                                  | <=== This Thing Here
|                                  |
|                              ________
|                             | Router |
|                             |   B    |
|                             |________|

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