Questions on H.GCP applied on text telephony

Gunnar Hellstrom Gunnar.Hellstrom at OMNITOR.SE
Thu Jun 10 07:40:49 EDT 1999

The structure you described for bearers, terminations, multiplexors,
contexts etc. described seems reasonable.
For a PSTN termination in text telephone mode of the highest level, where
the PSTN way of alternating between voice and text is supported, I see it
best described as a kind of user-driven multiplexor.

When there is a carrier from the text phone, the multiplexor conveys the
text stream.

When the carrier ceases, the multiplexor conveys the audio stream.

Text arriving from an IP termination may cause the state of the PSTN
multiplexor to change from audio to text and then transmit the text in the
prevailing text telephone mode.

Audio coming from an IP termination while the multiplexor is in the text
mode can not go through. The multiplexor stays in the text mode until the
user makes the alteration. ( usually by lifting the handset and "stealing"
the line from the textphone.)

On the IP side, H.323 Annex G is supposed to support simultaneous audio and
text. There is no need for this kind of user driven multiplexor there.

I understand that the requirements behind this description should go into
section 11.2.4 of megaco requirements (D-286 of Santiago). But where in
H.gcp do you want the description of parameters and functions needed in the
gateway components for this functionality?

Gunnar Hellström

At 10:09 AM 1999-06-10 +0200, John Segers wrote:
>In the current MEGACO model as described in draft-01, a Termination
>describes the media and in-band signaling for one user, and how they are
>transported etc.  A Context associates all Terminations in a session and
>implies audio mixing if there are more than two.
>This idea is retained in the Santiago work.  Going to multiple media, a
>Termination still describes the media and in-band signaling for one
>user.  A Context still associates all Terminations in a session.  For
>multimedia sessions, the mixing/switching can no longer be implied, but
>will be described by means of Context properties.  If there is only
>audio, nothing changes in the Context.
>In order to allow H.320 and H.324 where multiple media are multiplexed
>on one or more bearers, there needs to be a distinction between
>TerminationID and BearerID in order to retain the concept that a
>Termination describes the media, in-band signaling and transport.
>You see, the goal was to stay close to the basic monomedia connection
>model for multimedia, always keeping in mind that there should be as
>little overhead as possible for controlling voice calls (setup,
>teardown, ...).  The places where you see overhead occurring in the
>Santiago proposal are indeed when dealing with multiple bearers for
>multimedia, and, as Brian Rosen pointed out, in Audits.  It seems to me
>that this is acceptable because these operations will occur much less
>frequently than the voice/fax/NAS setups.
>John Segers
>HaoHou wrote:
>> Jonh,
>> Thanks for your reply,
>> Please see comments below.
>> >
>> > Second, the use of Add/Subtract/Modify commands.  You are proposing to
>> > use Add/Subtract to add media streams to a session or delete them from
>> > it.  This changes the semantics of the commands drastically.  They are
>> > intended to perform operations on Contexts (add or remove
>> > Terminations).  In the Santiago proposal this semantics is maintained.
>> > Operations to change Termination parameters are performed using the
>> > Modify command.
>> >
>> > Regards,
>> >
>> > John Segers
>> >
>> That’s why the Santiago proposal makes the Modify command awkward. ?
>> Suppose we have 3 bearer channels already in a Termination.
>> In order to release one bearer channel, we have to list both the other two
>> bearer channels in the MODIFY command.
>> In order to add one more bearer channel, we have to list the new
>> bearer channel and all the existed bearer channels in the MODIFY
>> command.
>> I don’t think the change of semantics will affect the protocol and the
>> implementation  too much. Instead, it will make the protocol easier to
>> be understood:
>> As long as you want to add some entities into the context or
termination, you
>> use ADD. As long as you want to sbustract some entities from the context
>> or termination, you use Subtract. It is a more natural way to express the
>> intention
>> of the commands.
>> Could you tell me why Santiago proposal wants to keep the old semantics
>> unchanged,
>> after the big change of the Context and Termination concept? What does
it gain?
>> I’d rather change the semantics to meet the change of the concept.
>> Best Regards,
>> Hao
>John Segers                                  email: jsegers at
>Lucent Technologies                                        Room HE 306
>Dept. Forward Looking Work                      phone: +31 35 687 4724
>P.O. Box 18, 1270 AA  Huizen                      fax: +31 35 687 5954
Gunnar Hellström

Ericsson Home Systems

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