From: bob1@cos.com (Bob Blackshaw)
Subject: Re: The ISDN network
Organization: Corporation for Open Systems
Distribution: na 
Date: Tue, 15 Dec 1992 14:24:24 GMT

In <1992Dec14.194114.23053@atlastele.com> bcapps@atlastele.com (Brent Capps) writes:

>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.

Using ISDN's compatibility checking features why would it default to
a modem when a voice call is indicated? Surely, you meant to say when
3.1 kHz audio is indicated. Also, there are low and high layer info
elements in the Setup message, which are able to denote which protocols
are expected at various layers.

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

I don't know about everyone else, but I'm not in the habit of making
data calls to unknown destinations. 


>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.

Again, why do you insist on closed user groups? Surely you have some
idea of who or what you are calling when using switched data.

>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.

Sorry, but I don't agree. The current gloom and doom attitude toward
ISDN make me think of the days when we started setting up packet-
switched networks. There were just as many nay-sayers back then, and
look what happened. As to the ubiquity of CSS7, well of course it
must be, ISDN does not work without it, however, CSS7 is perfectly
cost effective with or without ISDN and so will be deployed regardless.

>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                          | 

If you think ISDN is just a substitute for switched 56, then you might
be right. What you seem to be neglecting is the common channel signaling
between the user and the network. There is a great deal of power and
intelligence in a modern digital switch machine that is very nearly
inaccessible with POTS (a twelve key TT pad makes a lousy input device
for anything beside telephone numbers). There are application out there
that are not even a gleam in someone's eye today, but they will appear.

Bob.