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.

Ooops. In my previous I had obviously referred to the wrong original posting.
I should have followed your advice.

> 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?)

Right, but the whole overhead issue is, in my opinion, not put in perspective.

Let's say you are really planning on using IP for voice. Compare that with
using ATM for voice, over AAL1. What are the overhead comparisons?

Using that IEEE Proceedings article I mentioned recently, the IP packets
you'll want to use if voice is in the network cannot exceed a small size;
something less than 200 bytes. This is over a large network, to obtain
latencies under 150 msec.

Since this sort of application would most likely use UDP/IP over Ethernet, the
overhead is 8 + 14 + 4 + 5*4 + 2*4 = 54 bytes for 200 bytes of payload,
optimistically, which translates to 27 percent overhead.

If you use AAL1 for voice, over SONET STS-3c, for a similar (but more
credible) QoS level, all you need at the 155 Mb/s trunk level is 14.6 percent
overhead.

So again, you can't fool mother nature. If you'll mix interactive voice into
your IP network, you will have to ensure that the data services don't get in
the way of the voice services. And to do that, the data service efficiency
will be reduced. In the actual voice part of the scheme, the small cell
overhead of ATM shows its worth. In ATM, the signaling evolution made it
possible to do without all the packet overhead IP requires.

Bert
manfredi@arl.bna.boeing.com

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