From: deej@cbnewsf.cb.att.com (david.g.lewis)
Newsgroups: comp.dcom.isdn
Subject: Re: Network Connections over ISDN, and voice/data costs
Date: 19 Aug 92 12:57:55 GMT
Article-I.D.: cbfsb.1992Aug19.125755.13467
Organization: AT&T

In article <1992Aug17.161822.155@indyvax.iupui.edu> imhw400@indyvax.iupui.edu writes:
>In article <17079@ulysses.att.com>, lgm@ulysses.att.com (L. G. McKnight) writes:
>[much highly useful discussion deleted -- thanks for explaining!]
>> A call can be set up to route data to a packet switched network by any
>> number of routes.   Currently our CO switch routes D-channel data to
>> the packet handler section of the switch.  Higher data rates are
>> more economically sent as circuit switched data so our switch does
>> not have a link from the B-channel to the packet switched network.
>
>Um, what path do I take if I want to connect a service intended to be used
>concurrently by a significant number of subscribers, to the packet handler?  
>Do
>I put in a bucketful of BRIs and waste the B channels, accepting one call on
>each D channel?  I think I'd rather have the PH multiplex the packets onto a
>single higher-bandwidth channel (a B channel, for example, if the anticipated
>concurrent subscriber load is not *too* large).

A D-channel is capable of handling some number of X.25 VCs (I believe the
number is 8 or 16, and it may depend on which flavor BRI you're talking).
If the aggregate throughput is sufficiently low, VCs from multiple users can
indeed by 'multiplexed' onto a single D-channel.  If the aggregate
throughput is too high for a D-channel, a B-channel connection to the PH can
indeed be provided.

>> ...  But certainly B-channel connections are not expensive --
>> as every modem user knows.  Sending data over B-channels is no more
>> expensive than sending VBD using a modem.  

>> As I said above, all the long distance switches in the U.S. are digital
>> and digital switches are gradually relacing analog switches in the LEC's.
>> Digital switches are by their very nature designed to handle data.

But there's more to costs than switching - transmission costs money, too.
And just because a digital switch converts analog voice traffic to a 64kb/s
bitstream equivalent to switch it internally doesn't mean that all 64k is
created equal in transmission facilities.  For example:

* Significant amounts of interoffice trunking, both intra- and inter-LATA,
are riding on non-B8ZS, D4 (SF) framed DS1s.  While these are perfectly
adequate for voice and VBD, providing 64k clear channel trunks for 64k clear
bearer circuits requires B8ZS (which usually implies ESF framed) DS1s.
These cost money to install.

* 32kb/s ADPCM is used for voice in some places, particularly where traffic
capacity has grown faster than the facilities can keep up.  While ADPCM can
handle up to 9.6kb/s fairly reliably, you can (obviously) get twice as many
ADPCM trunks on a given facility than you can get 64k clear trunks.
Requiring 64k clear requires more facilities, which again cost money.

* In some special applications, particularly intercontinental facilities (be
they satellite or submarine), compression, statistical multiplexing, and a
whole lot of other things I don't understand are done to pack up to 5 voice
channels into a single DS0.  Again, 64k clear paths over these routes would
cost more money.

I'm not claiming that it's necessarily justified that telcos charge
significantly more for a 64k clear channel connection than they charge for a
voiceband connection - but on the other hand, it's inaccurate to claim that
the costs for 64k clear channels and for voice paths are the same.

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