Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <864ssws3y9.fsf@ironbridgenetworks.com>,
  James Carlson <carlson@ironbridgenetworks.com> wrote:
> "ronald h. davis" <ronaldd@lucent.com> writes:
> > there is, of course, another option and that is to use aal-2 with
> > silence suppression.
>
> Which serves only to make the traffic more bursty, which is exactly
> what ATM excels in not handling correctly.
>
> > as you suggested, aal-1 for voice makes more sense than aal-5 for voice
> > because aal-1 has less overhead.  in fact, voice over atm is more
> > efficient than voice over ip even when using aal-5.
>
> Uh, I suggested no such thing.

Perhaps you didn't, but I agree with Ronald. Voice, when it is CBR voice, i.e.
like ISDN, is better over AAL1 than AAL5, obviously. The tradeoff with voice
over IP, assuming it ever becomes really useful, will be to trade off IP's
additional burden of things like RSVP + QoS IP routing, compared with ATM
signaling.

Or sure, you can just ignore all the problems, assume best effort packet
switching with no QoS is fine for packet voice over IP, and then ATM will seem
unnecessarily complicated.

> ATM, though, maintains state through each switch for the VPI+VCI
> mappings plus the PNNI routing.  This state grows linearly with the
> number of connections through the switch for the VC mappings and
> roughly logarithmically with the size of the network.

You said this twice, and I disagreed both times. In a decent sized ATM
network, many most of the intervening ATM switches should not care about most
of the VCIs they see going by. That's why you have VPs. Bundles of VCs would
be transferred through most of the ATM switches in a large mesh without those
switches caring about individual VCIs. Only for VPs which terminate at a
switch would the VCIs matter. In any event, telephone switching has been
going on for a good many decades, so this aspect of ATM is not completely
unchartered territory.

> IP has state for routing only, so it does NOT need per-flow storage,
> and employs more sophisticated summarization techniques on the routing
> as well.  (PNNI is like OSPF, but there's no ATM equivalent of BGP.)

IP will have state for flows as well. See RSVP. How successful RSVP will be in
a real network is still a bit of a question. I agree on PNNI being similar to
OSPF, but I fail to see why a BGP equivalent, when multiple ATM carrier
backbones become established, wouldn't be possible over ATM. I don't see PNNI
v1.0 as the end of that development process.

> Things are much better still for IP if you rip out the useless ATM
> layer and run IP over SONET, or, better still, IP directly on fiber.

Fiber? What does that have to do with anything? The physical layer is hardly
the big deal here.

> > voice really is the killer app for atm because it carries voice better
> > than ip does...the same doesn't hold true for data.
>
> I can agree with the data part, especially for very high speed
> (~1Tbps) networks where the typically small buffering in ATM switches
> interacts very poorly with the bursty nature of network traffic and
> the AAL-5 SAR.  Folks playing with data over "voice grade" switches
> are finding this a hassle.

Obviously, data buffers need to be large even over ATM, if the bursty nature
of data must be handled efficiently. Voice-sized buffers won't do.

> Whether voice carried in individual VCs (!) as suggested is a "killer"
> application for ATM remains to be seen.  I (and many others) still say
> that it doesn't scale well at all.
>
> About the only solid argument I've seen in favor of ATM is that,
> unlike IP, telcos know how to use it now.  It's integrated into TRKS
> and TMN.  Of course, there are some good integrated CMIP/SNMP
> management tools available today, and things are still improving, so I
> doubt this meager advantage will hold long.

The real question is just how successful IP can be for interactive, high
quality voice and video communications. You can either assume away all the
problems and see a rosy picture, or you can be realistic and add things like
QoS routing and RSVP to IP. At that point, you might want to reconsider which
system is simpler. There are a lot of people who know these things asking
themselves this question.

Bert
manfredi@arl.bna.boeing.com

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