RE: [Megaco] Sending tones via the other subscriber's ephemeral

["Tom-PT Taylor"<taylor@nortelnetworks.com>]

I'm probably making a mistake, answering your note as I come to it rather
than reading to the end of the thread, but I'll make the following general
points:

(1) At any point where the MG is at a trust boundary, the MGC will normally
set the termination representing the caller into receiveOnly mode to start
with to prevent fraud.  All other terminations along the call path can be
made sendReceive from the beginning.

(2) Al's general rule on which MG supplies ringback is right: it would be
one connected to the terminating line.  It gets a little more complicated if
the MG is connected to a SIP network through which the call is terminated.
In that case, I would differentiate between a 183 Session Progress
provisional response, which suggests that ringback will come from further
along the media path, and a 180 Ringing provisional response, which suggests
that no ringback is being provided.

-----Original Message-----
From: Raphael Tryster [mailto:raphael@tdsoft.com]
Sent: Tuesday, May 07, 2002 4:36 AM
To: Al Varney; 'megaco@ietf.org'
Subject: RE: [Megaco] Sending tones via the other subscriber's ephemeral


Thanks Al and Vivek. 
Your answers make sense to me, which leads me to ask if the examples in RFC
3015 and other places like draft-ietf-megaco-callflows-00.txt are wrong.
All the examples I have seen show ringback being provided by MGA.
Furthermore, some of the examples show the termination modes being recvonly
until the called party goes off-hook.  If this is the case, then the
half-duplex connection to which Vivek refers is in the wrong direction.  If
EphB in my example is recvonly, then it cannot send the tone to EphA.  Have
all the example writers been oversimplifying to the point of misleading?
Raphael Tryster 




-----Original Message----- 
From:   Al Varney [SMTP:varney@ieee.org] 
Sent:   Monday, 06 May, 2002 5:47 PM 
To:     'megaco@ietf.org' 
Subject:        Re: [Megaco] Sending tones via the other subscriber's
ephemeral 
At 01:04 PM 5/6/02 +0530, vibansal@hss.hns.com wrote: 
>It is recomanded that  ringback tone id be played by the terminating media 
>gateway. So in  this case, MGC controlling MGB tells MGB to send ringback 
>signal 
>on EphB,  MGB at australia would play the ringback tone on the ephemeral 
>termination EphB,  and subscriber A would hear Australian ringback tone 
>,as half 
>duplex connection is already open. 
    I'm not sure where this is "recommended" rather than "required", except 
in cases where MGA and MGB operate together as a single logical switching 
entity (a distributed PBX, perhaps). 
>>Raphael Tryster <raphael@tdsoft.com> wrote on 05/05/2002 10:02:45 PM 
>> 
>>Is the following a realistic scenario? 
>> 
>>Subscriber A in U.S.A. wishes to speak with subscriber B in Australia.
When 
>>the connection is established (MGA creates EphA in the same context as
PhyA, 
>>and MGB creates EphB in the same context as PhyB) and B's phone is
ringing, 
>>the MGC controlling MGA sends a ringback signal to PhyA.  But this would
be 
>>an American ringback signal, and A wants to know he is calling Australia. 
>>So instead, the MGC controlling MGB tells MGB to send ringback signal on 
>>EphB. 
>> 
>>Will this work, i.e. will subscriber A hear Australian ringback tone?  And

>>is it done in practice? 
    If by "in practice" you mean "in the existing telephone network", you 
will find ringback tone ALWAYS comes from the terminating switch 
(pre-MGC/MG).  There are several reasons for this: 
  - MGA would never know when to apply ringback, since it has no knowledge 
of the status of the terminating line.  In fact, it doesn't know if MGB is 
a terminating point or just an interworking point using traditional 
trunking to reach a third switching point (perhaps located in a third
country). 
  - MGA would not be aware (at ringback time) of cases where MGB is 
forwarding the call to another line somewhere in the world, or is 
interacting with an Intelligent Network service or is offering a voice 
announcement regarding the status of the called number.  (E.g., "this 
number is disconnected, calls are being taken on XXX-XXXX-XXX".) 
  - MGA (and any other MGs on the way to the final terminating MG) would 
have to be very quick in order for MGA to be able to remove ringback and 
cut through the connection before the called party's "Hello?" is
transmitted. 
    About a year ago (5/1/01), I discussed this same issue on the 
"megaco@fore.com" mailing list.  Two years ago (6/27/00) it was discussed 
on the "MEGACO@standards.nortelnetworks.com" mailing list.  And I've 
discussed it ad nauseam on several other standards lists (H.248, SIP, 
H.323, ISUP/BICC) over the past few years.  Here's the rationale in a 
single sentence: 
        "The PSTN does not, in general, provide a reliable 
         out-of-band indication of when Alerting of the 
         called party is occurring." 
            -  (quoting an ITU-T contribution) 
    I assume this will continue to come up until the H.323, H.248 and SIP 
folks finally get something published that shows real-world examples of the 
need to provide ringback from the terminating end of a connection.  (See 
the SIP "183 Session Progress" message for some examples.) 
    Thanks, Raphael, for providing this year's version of the perennial 
question.  Should you wish to examine other standards/requirements on this 
issue, note the ITU-T uses the term "Ring Tone" and Telcordia the term 
"audible ring tone" in many telephony documents.  "Ringback" seems to have 
originated in PBX/ISDN documents in North America in the 1980s. 
Al Varney 


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