From: deej@cbnewsf.cb.att.com (david.g.lewis)
Subject: Re: end-to-end digital connections on ISDN
Organization: AT&T
Date: Wed, 6 Jan 1993 21:33:55 GMT

In article <PICONE.93Jan6110826@csc000.csc.ti.com> picone@csc000.csc.ti.com (Joe Picone) writes:
>We currently have a T1 line installed in our lab that has been
>guaranteed by the long distance carrier (MCI) to provide fully digital
>connections once the signal has been converted from analog to digital
>at the nearest (digital) local office.  

I don't wish to denegrate the competition, but I believe your carrier is,
shall we say, overstating their case.

I don't doubt that they can guarantee that the connection is fully digital
from your lab, to the serving MCI switch, through the MCI network, to the
far-end MCI switch, and even as far as the facilities from the far-end MCI
switch to the LEC Point of Termination/Point of Interface.  However, it is
physically impossible for an IXC to guarantee *anything* with regard to
whether connections beyond the POI are digital or analog or how many D/A-A/D
conversions a signal will go through.

For instance, the IXC may be able to guarantee that all access facilities
(from the LEC network to the IXC network) are digital.  However, they can
not guarantee that all Access Tandems are digital.  The connection could go
from an MCI DMS-250 (digital) over digital facilities to a 1AESS(tm) switch
AT (analog SPC) (1 D/A-A/D), then over analog facilities to a DMS-100 EO
(digital) (another D/A-A/D) serving an analog loop (a third D/A-A/D).
Unless MCI is claiming that they have direct access trunks to all digital
EOs in North America, their claim rests on LEC network topology, which is
outside their control.  And I feel certain that somewhere in North America
there is at least one digital EO with trunks to an analog SPC AT with
connections to MCI...

>What are the chances that an ISDN phone will demonstrate the same
>characteristic (no reconversions)?

It depends on the density of deployment of digital switches and facilities,
but is no different than the probability of a fully digital end-office-to-
end-office connection when calling from an analog phone.  Voice calls from
ISDN sets and voice calls from analog sets are treated no differently by the
networks.

If the percentage of digital facilities and digital switches approaches
100%, the probability of an end-to-end digital connection approaches 100%.
I wouldn't hazard a guess as to what it is now.

>If there is a chance of a reconversion, can some carriers provide
>special feeds that are guaranteed to be digital all the way? If so,
>who?

You could always get a private line... :-)

> Some T1s,
>I am told, actually get converted to analog when they have to pass
>through a non-digital office (apparently, there are still some of
>these out there???). 

Lots.  The 1AESS(tm) switch was the most widely deployed switch in the Bell
System; given that they were still being deployed in the early 80's, it's
going to be a while before they're all fully depreciated and replaced by
digital switches.

>Some carriers claim to deliver end-to-end
>digital.

I think the big three IXCs all do; the catch, though, is that's end-to-end
*within the IXC's network*.  Once the call gets outside the IXC network,
it's dependent on what the LEC's deployed.

> Since ISDN is new, and point to point connectivity is
>probably not yet mature, people are speculating that reconversions can
>occur. I find this hard to believe.

Don't.  There's a lot of embedded base out there.


David G Lewis                              AT&T Bell Laboratories
david.g.lewis@att.com or !att!goofy!deej     Switching & ISDN Implementation