From: deej@cbnewsf.cb.att.com (david.g.lewis)
Newsgroups: comp.dcom.isdn
Subject: Re: The D channel: how is it used & billed?
Date: 23 Jul 92 21:50:07 GMT
Article-I.D.: cbfsb.1992Jul23.215007.21963
Organization: AT&T

In article <nkrqajs@rhyolite.wpd.sgi.com>, vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
> In article <1992Jul22.195218.9871@nntpd.lkg.dec.com>, goldstein@carafe.enet.dec.com (Fred R. Goldstein) writes:
> > (quoting)
> > >Q. How is D channel traffic billed?  Is it per-byte or per minute?
> > 
> > In the Massachusetts tariff, which applies to residence and business
> > customers, packet traffic is charged on a usage basis.  While there
> > is a trivial (2 cents, I think) charge for call setup, there is no
> > connection time element.  Packets are charged at 60 cents per 
> > kilosegment (the usual X.25 segment:  Small packets are segments,
> > large packets are a segment per -- I think -- 128 bytes or fraction
> > thereof) during busy hour, and 50 cents during off hours. ...
> 
> Do I understand this right? $0.60/(1000*128Bytes)  Or $8*.6 = $4.80/MByte?
> 
> A v.32bis modem will move about 1400*3600 Bytes/hr or 5MB/hr of
> uncompressible (e.g. pre-compressed) data, and perhaps 20MB/hr of ASCII
> text.

As has been said before, don't include compression in your comparisons,
because if you can compress it to go over one channel, you can compress it
to go over any other channel.  Stick with the pre-compressed numbers for
comparison purposes - 5MB/hr.  (If you figure a consistent 14.4kb/s, it
works out to ~6.5MB/hr; 9.6kb/s works out to ~4.3MB/hr.)
 
> A single B-channel running at 56Kbit/sec (instead of 64K because of the
> inter-CO problems) will move about 25MByte/hour with no compression.
> 
> I've been told you can get switched-56 to connect your B channels at
> the same rates as long distance.  

Depends where you go.  Some carriers price on a par, others price at a
premium of anywhere from 10 to 50 percent per minute.

> Long distance costs about
> $0.11/minute anywhere in the U.S. if you're willing to pay a front-end
> fee of about $10/month.

In general, LD doesn't cost "$0.xx/minute", it costs "$0.yy for the first
minute, $0.xx for each additional minute".  This is relevant later on.
 
> Thus, for $4.80 you should be able to move about 17.6MBytes through a
> B-channel to anywhere in the U.S., after you get the call set up, and the
> computers logged-in or whatever.

"After you get the call set up and the computers logged-in or whatever..." -
remember this item as well.

> You can move only about 8.7MByte for $4.80 with a v.32bis/v.42/v.42bis
> modem.
> 
> So a long distance B channel is about 17 times as cheap as a D-channel.
> A v.32bis modem is about 8 times as cheap as a D-channel.
> 
> What have I computed wrong?  Why is anyone even slightly interested in
> buying D-channel bits?

You haven't computed wrong, you've just defined the problem very narrowly.
If all you're interested in doing is an application that requires a single
long-holding-time connection on a relatively infrequent basis (e.g. a
nightly bulk file transfer or something similar), you're absolutely right -
a B-channel connection or a high-speed modem or some such is certainly
cheaper.

However, as the holding times decrease and the frequency of call
setup/teardown increases, the initial period (minute or six seconds) rate
starts to dominate.  At an extreme, if you're making sixty one-minute
calls a day (where the first minute is at a premium of anywhere from 25% to
100% to cover the cost of call setup) instead of one sixty-minute call a
day, your cost increases by that first-period premium.  If you have an
application that requires sending 256 bytes of data and getting back 1000
bytes in response, and you're doing that on average 60 times an hour for
eight hours a day, your total data movement is just over 4.8Mb - and your
aggregate data rate is about 83bps.  Using a 56k channel held up for 8
hours, even at $0.11/minute, costs $52.80.  Setting up and tearing down 240
calls a day, assuming a 50% premium for the first minute and one-minute
minimum billing periods, would cost $79.20 - the first-minute cost
dominates.  Using the per-segment cost Fred cited, the total would be $11.52.

So if you assume that the purpose of the channel is to pump as much data
through in as little time as possible, you're right - D-channel packet isn't
appropriate.  However, if you assume a different application - particularly
anything that operates in a query-response mode, especially if it's a
multipoint interface with, say, a half-dozen terminals hanging off it - the
parameters change and things crossover.

David G Lewis                              AT&T Bell Laboratories
david.g.lewis@att.com or !att!houxa!deej     Switching & ISDN Implementation
Newsgroups: comp.dcom.isdn