Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <864ssprgkq.fsf@ironbridgenetworks.com>,
  James Carlson <carlson@ironbridgenetworks.com> wrote:

> Ok, let's do that, then.  G.723.1 encodes 30ms (240 samples) of voice
> into ten 16-bit (20 bytes) code words in 5.3Kbps "toll-quality" mode.
> If this is transmitted over UDP/IP (28 bytes of header) as
> packet-over-SONET (8 more bytes of overhead), the result is a lousy
> 36% efficiency (20/56) before considering silence suppression.

Compared with 85+ percent efficiency for voice over ATM/SONET.

> But so what?

I agree, in general, so what? Efficiency can be measured in many ways, but it
isn't me raising the "cell tax" red herring all the time, right?

> Efficiency of low-speed applications like voice just
> isn't an issue.  Efficiency shows up as an issue only when you're
> looking at data in the aggregate.

I think you might have missed the point that in a mixed network, ALL packets
must be of restricted size. This might not be necessary in the fast backbones,
but is necessary in the slower access networks. So all apps pay a price for
multimedia (high quality) support, in terms of "efficiency."

> Non-voice data transmission already
> surpasses voice in public networks, and data is growing in volume at
> anywhere from 100% to 900% per year while voice is essentially flat.
> By the time CLECs deploy voice over IP, voice will be just a niche
> data type.

Maybe, although the future of interactive applications in general is not going
to remain flat. The availability of PCS and similar devices will likely change
that trend you think is going to be persistent. Curves that rise sharply are
almost always S curves, in the historical perspective, not exponentials.

> (If you choose to compare AAL1 against voice over IP, things are much
> better for IP.  AAL1 requires six cells at 318 bytes to transmit that
> same voice data, making voice over IP five and a half times more
> efficient.  If you try to compress to make the comparison fairer, then
> AAL1 has at best a marginal efficiency improvement, since it is forced
> to use 48 byte cells whether or not it needs to; which means that the
> same 30ms sample that takes 56 bytes on an IP link takes 53 bytes on
> an ATM link.)

Voice is a serial bit stream. You fill up 47 bytes per cell and transmit the
cell. No other considerations are relevant in this comparison. You wouldn't
send voice over ATM in some weird packet size.

> > Since this sort of application would most likely use UDP/IP over Ethernet,
the
>
> Ethernet?  How did that crop up?

I'm comparing voice over SONET against voice over an Ethernet/IP LAN. Seems
pretty fair to me. But in any event, your comparison was even less favorable,
so I guess I shouldn't complain.

> And, once again, ATM insists that you pay this performance tax,
> whether or not the "quality of service" features are important to you.

Again, you first need to look at the performance tax you'll have to impose in
an IP network that carries multimedia before you worry about any "cell tax."
That was the whole point of this discussion.

Bert
manfredi@arl.bna.boeing.com

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