From: varney@cbnewsd.cb.att.com (Al Varney)
Newsgroups: comp.dcom.isdn
Subject: Re: DTMF interaction on ISDN
Date: 13 Oct 92 22:05:23 GMT
Article-I.D.: cbnewsd.1992Oct13.220523.5786
Distribution: na
Organization: AT&T Network Systems, Lisle, IL

In article <1992Oct12.164424.15662@atlastele.com> bcapps@atlastele.com (Brent Capps) writes:
>In article <2927806136.1.p00136@psilink.com> "Bob Larribeau" <p00136@psilink.com> writes:
>>All of the ISDN telephone sets in the U.S. provide DTMF after the call 
>>cuts through.
>
>Does this mean that the ISDN set waits to receive answer supervision from
>the terminating office before it cuts over to DTMF?

   You'd think Bellcore's TR-TSY-000268 {ISDN Access Call Control
Switching and Signaling Requirements} would easily answer this question,
but it doesn't ... but then it mostly ignores SS7 trunking and assumes a
lot about other non-traditional interfaces.  "Cut-through" usually refers
to the cut-through of audio AT THE SWITCH in the "forward" direction on
an originating voice call; i.e., the far end can hear you.  This isn't
obvious to non-ISDN callers, since it occurs when outpulsing is complete
over a trunk or off-hook occurs on a line.  With SS7, it occurs either
after receipt of Address CoMplete or ANSwer (based on type of call, etc.).

   For ISDN callers, there are really 3 intervals:
                                           (equivalent interval for POTS)
SETUP sent to Switch,                      Off-hook, dial-tone, dialing
dialing any further                        digits, ....
digits, ....

CALL PROCeeding received                   Optional "clicks", provide
(no more digits wanted),                   audible ring (intra-switch)
ALERTing may be received                   or outpulse digits and
(audible is being provided                 cut-through (inter-switch).
at the terminating interface),             {Audible/tones from far end}
waiting for CONNect or
PROGress ...

CONNect received (call                     Optional "clicks", end of
answered) or PROGress                      audible ring, maybe some
received (inband info. ...                 other voice ....
available or not end-to-end
ISDN)

   In general, a CALL PROCeeding message is sent to the calling terminal
when the switch believes all the digits needed for routing have been provided.
Many terminals will indicate (visually) any extra digits dialed AFTER
receiving CALL PROCeeding and BEFORE the DTMF capability is available.

   For calls encountering a non-SS7 trunk circuit, a PROGress message with
either "call is not end-to-end ISDN; further call progress information may
be available inband" or "inband information or appropriate pattern now
available" should be sent to the caller at some point.  This would be
an appropriate time for the caller's CPE to allow DTMF on the B-channel.

>  Answer supervision
>isn't always returned to the originating party on interoffice signaling
>equipment, particularly over Feature Group A trunks which may be used for a
>lot of the smaller VRUs.  It seems to me the originating ISDN set ought to
>cut over to DTMF after it finishes dialing the last digit.

   But that would sacrifice some flexibility (future?) in permitting
"keypad" elements to be sent after 'CALL PROCeeding" and before and
'PROGress' message from the switch.

   TR-268 specifically states, in Section A.7.6 {Interswitch Call Over
Non-ISDN Outgoing Facility}, that "... no answer indication may be provided
over the non-ISDN facility."  Whether FG-A qualifies as a "trunk" might
be an issue (I think it's a line, others don't), and whether it's an
"interswitch" facility is also questionable (is the far end really a
Switch?).  But, so far, it looks like "non-answered facilities" calls
will still permit DTMF from ISDN sets.  Whether switches will really
cut through on such connections isn't an ISDN issue, but rather one
of what types of calls are permitted to such "facilities".

>Is DTMF cut over a mandatory feature or is it supplied at the discretion of
>the manufacturer?  My concern is that DTMF cut over that hasn't been
>through an official approval process may cause problems for voice mail/VRU
>systems in the areas of talk-off, dB twist, minimum duration and interdigit
>timing.  It would only take one ISDN set vendor doing it differently to cause a
>huge problem for a voice mail/VRU vendor.

   Wow! -- You worry about quality DTMF from ISDN sets, with the Fisher
Price telephones on the market today????  The wording in TR-268, Section
2.2 {Assumptions} is "If end-to-end, inband signaling is required by the
user, the user equipment is expected to send address information as DTMF
signals over the B channel. ... The CPE is responsible for determining
whether digits are sent as D-channel messages or DTMF signals on the
B channel."

   It would seem reasonable for VRU/voice mail vendors to be looking
at better interfaces, to insure "universal" access from all callers.
One such interface is ISDN itself (either BRI or PRI for volume users).
This provides both Answer and Disconnect indications on a line-type
interface -- and a lot of other advantages.

Al Varney - MY opinions, of course.