Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <86vhl7o6ox.fsf@ironbridgenetworks.com>,
  James Carlson <carlson@ironbridgenetworks.com> wrote:

[ ... ]

> Please reread what the original poster wrote.  He asked for a
> comparison of the efficiency of IP over SONET versus IP over ATM.
>
> This has nothing whatsoever to do with the question of whether or not
> ATM actually does provide QoS or if it's worth the expense.  He wasn't
> asking about any such thing.
>
> > Newer ATM encapsulation techniques with QOS in mind use TCP over ATM,
> > thereby getting rid of the IP overhead of TCP/IP. Is bandwidth really
> > the issue ?
>
> As the original poster noted, it certainly is an issue for the folks
> paying for the links.  If you're not footing the bill, then I suppose
> it is less important.  (TCP over ATM without IP?  References?)

The original poster (assuming I have the cortrect one) was assuming an ISDN-
like use of ATM: give everyone what appeared to be a wide CBR channel and let
him sit on it all day long. This way of using ATM is probably not realistic,
in that it is exactly this sort of problem that ATM was designed to solve.

To solve it, however, and in fact to provide QoS guarantees in any system, you
_have_ to introduce the concept of cost. One way or another, the negotiation
for QoS has to include in it a tradeoff: more functionality (which eats up
network resources) comes at a price. This is one of the problems diff-serv is
having to address, by the way. QoS without associated cost tradeoffs is just
plain vapor.

One relatively easy but inflexible tradeoff might be bandwidth vs QoS. For
example, just give the end users two options which mirror what they have
today, but over the same network. Either you get controlled latency and
jitter, but at reduced bandwidth, or you get whatever bandwidth is available,
but with no guarantees. ATM could implement such a scheme pretty easily.

Bert
manfredi@arl.bna.boeing.com

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