value of co-location

Curtis Villamizar curtis at
Mon Jan 29 17:17:35 UTC 1996

In message <96Jan23.111533-0000_est.20608+185 at>, Sean Doran writes
> | If ATM is being used in VBR without the V mode, essentially providing
> | point to point connections between routers at above DS3 rate, then
> | there is no need for complex reassembly or any form of congestion
> | control.  That may turn out to be the way ATM is used by ISPs.  
> Ok, so in order to do VBR without the V mode, one effectively
> has to either *really* trust one's ATM network provider not
> to have any congestion which could lead to cell loss as we
> have seen pretty clearly that even very minimal, occasional
> cell loss is deadly -- or one has to do the UUPSI hack:
> purchase clear-channel L0 circuits and run ATM as a L1
> protocol on top of it.
> That is, the only ATM switches are the ones at the end points
> and owned by the NSP.

That's not true.  VBR without the V means setting PCR and SCR tot the
same value and reserving a VC and traffic shaping your output so as to
never exceed that value.  This would allow you to nail up a X Mb/s
pipe (where DS1 << X << DS3 might be cost effective, or DS3 < X << OC3
might also be cost effective).  The is no opportunity for multiplexing
gain if numerous telco customers do this (ie: are forced to do this if
ATM buffering and congestion mechanisms are not improved), but does
provide lossless service (unless something isn't working).

The UUPSI hack works just as well if you need more than DS3 and either
can't get ATM service from your provider or VBR minus V service is not
cost effective relative to OC3 SONET.  In that case PPP over SONET
would be a technically better choice if you can get it.  I won't
comment in public on the likelyhood of this.

> It is my prediction that by this spring, the only reasons why
> anybody would want to run ATM for connecting pieces of the
> Internet would involve unavailability of SONET/SDH from one's
> carrier(s), the desire to save money on point-to-point
> circuits by letting one's carrier(s) sell one VCs over some
> unknown fabric rather than real circuits over their
> transmissions infrastructure, an already-installed ATM
> fabric, or general insanity.

I mostly agree if you constrain "everybody" in the statement above to
mean the major ISPs.  I think you still may be missing the point that
VBR without the V is supposed to be lossless, not like UBR (currently
highly lossy under congestion by design :) or ABR (high vapor
preasure, as of yet unproven, but still might just work).

ATM doesn't solve everything, but it also isn't comkpletely useless.
As George Clapp points out, some people are using ATM successfully.
IMO just don't believe everything you read or hear about ATM without
thinking about how it will work in your application (not you Sean,
anyone's application, whether ISP or LAN or otherwise).


ps - Regarding Kent's comments.  There are other routers on the
horizon (beside Netstar and Vadim's deep secret and Ipsilon's "we're
not saying what it is").

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