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

From: [email-masked]
Date: Mon, 6 May 2002 13:04:56 +0530

Hi

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

Regards
Vivek

www.hssworld.com

Raphael Tryster <[email-masked]> on 05/05/2002 10:02:45 PM

To:   "[email-masked]'" <[email-masked]>
cc:    (bcc: Vivek Bansal/HSS)

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

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?

Raphael Tryster

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From: Al Varney <[email-masked]>
Date: Mon, 06 May 2002 10:46:40 -0500

At 01:04 PM 5/6/02 +0530, [email-masked] wrote:
>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 <[email-masked]> wrote on 05/05/2002 10:02:45 PM
>>

    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 
"[email-masked]" mailing list.  Two years ago (6/27/00) it was discussed 
on the "[email-masked]" 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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From: Al Varney <[email-masked]>
Date: Mon, 06 May 2002 10:46:40 -0500

Al Varney