From: bcapps@atlastele.com (Brent Capps)
Subject: Re: The ISDN network
Organization: Atlas Telecom Inc.
Date: Mon, 14 Dec 1992 19:41:14 GMT

In a previous article, "Bob Larribeau" <p00136@psilink.com> writes:
>Couple of different ways.  The CCITT standards specify that a Bearer 
>Service Information Element will be included in the incoming SETUP 
>message that specifies either Circuit Mode Data or Voice.  The terminal 
>adapter selects the modem based on that information.

Scenario: automated dialing equipment delivering ASCII or fax data.
"Known" means the type of terminating equipment used is known to originator,
ie., it's a closed user group.  We can't make any assumptions about unknown
terminators, including the use of DNs to distinguish between incoming ISDN 
and POTS calls.  This is the real world.

case        terminator is:     end-to-end?     originator uses:
----      ----------------     -----------     ----------------
   1      known modem only      don't-care                modem
   2      known ISDN+modem              no                modem    note 1
   3      known ISDN+modem             yes               either
   4       known ISDN only      don't-care                 ISDN 
   5               unknown              no                modem    note 2
   6   unknown (ISDN only)              no              blocked    note 3
   7               unknown             yes                 both    note 4

note 1: the terminator will default to modem when the incoming SETUP 
	indicates a circuit mode voice call.  This requires the originator
	to use modem as well.

note 2: from cases 1 and 2, all non end-to-end calls to unknown destinations 
        must be via modem.

note 3: from case 3, the originator must assume modem on non end-to-end
        calls, and if the terminator doesn't have a modem no connection can
	be established.  The difficulties involved in detection of POTS call 
	progress signals preclude automatically switching to ISDN if the modem
	connection can't be established.

note 4: the originator will try ISDN first then switch to modem if an
        end-to-end connection can't be obtained.

Conclusions:

As a practical matter, for switched data either SS7 must be ubiquitous or the
receiving party must be known (closed user groups).  Case 6 dictates the
continued use of ISDN and modems for both sending and receiving from unknown 
types of terminal equipment.

The problem is that there's no way out of this trap.  Even after SS7 is 
universally deployed, there will still be modems out in the PSTN, so you
will have to maintain your modem ad infinitum.  There is no strategy for
phasing them out except among closed user groups.  As the trend is toward
more modems in the PSTN (mostly G3 fax machines), this does not bode well 
for ISDN.

ISDN is only practical for closed user groups.  It will never replace 
POTS telephony except in niche markets like point-to-point dialup LAN 
access a la Switched 56.  The size of this market will determine whether 
it will ever become cost-effective. 
-- 
Brent Capps                          | 
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