Conflicting text in H.323 concerning the requirement for establishing a H.245 control channel??
Chris Wayman Purvis
cwp at ISDN-COMMS.CO.UK
Tue Mar 20 11:13:41 EST 2001
That point was clear. What was not clear was why you consider it to be a
conflict (for which the GROUP would probably consider a FIX to be REQUIRED)
rather than just something for which you can't see the logic (for which YOU
PERSONALLY consider a change to be DESIRABLE).
There are three reasons that I can think of off the top of my head for
1. If either end of the call wants to send DTMF during the call, turn the
call into a conference, or maybe alter audio or video codecs, an H.245
session will be required at that time. There is a potential delay in these
things happening if the H.245 session is started on demand rather than
automatically as of right at the beginning of the call.
2. Backwards compatibility. A device written to the current spec is
entitled to require some sort of H.245 channel to be open during an ongoing
call, and may have no way of opening one (it may not know the other end's
H.245 address). Just removing the requirement to which you refer would
leave one endpoint in many calls UNABLE to open an H.245 channel, initiate a
capabilities exchange, determine master/slave, send flow control commands
etc. Maybe acceptable in a SET but, I think, not in anything more.
3. Forwards compatibility. Who knows what other features might be
negotiated through capabilities exchange in the future?
My vote is for leaving well alone!
frank.derks at philips.com wrote:
> I thought I was being clear enough, so let me try again. 6.2.8/H.323 states
> that an enpoint must open one (and exactly one) H.245 control channel. When
> Fast Connect is being used, I assume that the intention is that no such control
> channel is opened.
> To be compliant with 6.2.8/H,323 I would have to open a H.245 control channel
> irrespective of which type of H.245 procedures I will be using. So if I intend
> to use Fast Start (and assuming that the other party also supports this), I
> still have to open a H.245 control channel.
> cwp at isdn-comms.co.uk on 20-03-2001 15:17:14
> To: Frank Derks/HVS/BE/PHILIPS at EMEA2
> cc: ITU-SG16 at mailbag.cps.INTEL.COM@SMTP
> Subject: Re: Conflicting text in H.323 concerning the requirement for establishing a H.245 control channel??
> Why do you consider this text to be "conflicting"?
> Specifically, with what does it conflict?
>> 6.2.8/H.323 states: "The endpoint shall establish exactly one H.245
>> Control Channel for each call that the endpoint is participating in."
>> 8.1.7/H.323 never states that when Fast Connect is being used such a
>> control channel should be established. As far as I understand the
>> mechanism this is only required to switch to "normal" H.245 procedures.
>> It would seem that section 6.2.8 should be rephrased to make clear that
>> the H.245 control channel shall only be established when "normal" H.245
>> procedures are being followed and not in the fast connect case.
> Dr Chris Purvis -- Development Manager
> ISDN Communications Ltd, The Stable Block, Ronans, Chavey Down Road
> Winkfield Row, Berkshire. RG42 6LY ENGLAND
> Phone: +44 1344 899 007
> Fax: +44 1344 899 001
> MEMO 03/20/01 16:37:40
Dr Chris Purvis -- Development Manager
ISDN Communications Ltd, The Stable Block, Ronans, Chavey Down Road
Winkfield Row, Berkshire. RG42 6LY ENGLAND
Phone: +44 1344 899 007
Fax: +44 1344 899 001
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