From: etxorst@eos.ericsson.se (Torsten Lif)
Subject: Re: DTMF interaction on ISDN
Reply-To: etxorst@eos.ericsson.se
Organization: Ericsson Telecom AB
Date: Wed, 14 Oct 1992 08:28:57 GMT

In article p00136@psilink.com, p00136@psilink.com (Bob Larribeau) writes:
>The ISDN set on the receiving side answers the call it sends a CONNect 
>message to the network.  The network responds to this message with a 
>CONNect ACKnowledge after cut through is completed.

This assumes that the receiving end is ISDN. But what if the call is
from an ISDN set, through a digital CO, to an older analog CO and to a
POTS line at the other end? Let's face it, it will be decades before
one can count on end-to-end ISDN for a random call.

And IF the call is ISDN end-to-end, then why do you need DTMF? The
terminals can use user-user-info for faster and non-B-channel
communication.

No, the way I see it, DTMF is needed only when an ISDN set calls
outside of the digital world and that's a whole new can o'worms.

Let's say you call your bank to read your account status. The bank sits
in a different part of town and has an analog PABX which connects to an
old analog CO. You open the call and send the address digits to your CO
(digitally). Then what?  Do you get the CONNect ACK when the other
party responds or when ringing starts at his phone? I've seen both in
field trials, depending on if the other CO was on SS7 or not.

So you can't rely on CONNect ACK. The user has to wait for the spoken
message from the other end. Then what? You switch the phone to send
DTMF. Do you do this for all calls? What about if you get a "call
waiting" indication or want to place the current call on "hold" and
call somebody else? *Normally* you want all signalling from your ISDN
phone to your CO to be digital (D-channel). You ONLY want DTMF when
you're calling certain special services. There are no 100 % reliable
signals or indicators that you can use to automatically cross-over
between DTMF mode and "normal" D-channel mode. You will have to provide
some kind of switch-over for the user to manually enter "DTMF mode".
Then, hopefully, you revert to "normal" mode whenever the call closes.
But you also need a way to revert to "normal" manually in case you want
to put the call on "hold".

You want the function to switch between D-channel dialling and DTMF
signalling to be intuitive and easy to do but at the same time so
foolproof that there's no way for the user to get stuck where he can't
answer a "call waiting" because he's in DTMF. Congratulations, whoever
got stuck with designing this function into a phone. You just won the
booby prize, fella'.

Then, when you're done, you can start thinking about how to implement a
similar functionality using user-user-info when the call actually is
end-to-end ISDN. How do you switch the keypad between "dialling" mode
and "u-u-info" mode? Do you send digits one by one (could be expensive
if billing is "per package") or in batches (how do you terminate a
batch)? If the other party provides responses by u-u-info, how do you
present them? Does your phone have a display? Can it handle non-numeric
info? How many characters? How many lines? What do you do when the CO
sends a "display" element and the other party sends a u-u-info at the
same time?

Sorry if I sound negative, but this is about the point where I got
disillusioned by ISDN. The ISDN "LEGO" blocks are great for building
the foundation of your house, but when you reach the upper floors you
find that some shape blocks are missing and others don't quite fit
together. It's going to be a long time and lots of trial-and-horror
before all equipment actually works together in the way it was intended
to.

---
 Torsten Lif                        !  These are my personal opinions only.
 Ericsson Telecom AB, EO/ETX/TX/AD  !  I do not speak for Ericsson.
 S-126 25  STOCKHOLM                !  Ericsson is perfectly capable of
 Phone: +46 8 719 4881              !  speaking for itself.