Req: Clarification on fast setup

Pete Cordell pete.cordell at BT-SYS.BT.CO.UK
Tue Sep 23 12:27:35 EDT 1997


Looking at the fast start stuff, I'm confused about the format of the
FastCap field and how it is used.  Is the intention to allow multiple
multiplexCapbility fields in the SETUP?  How would this be used as H.245
allows only one instance of the field?  Perhaps the encoding below would
describe the intended use better.

FastCap ::= SEQUENCE
        caps            SEQUENCE OF Capability,
        multiplexCap    multiplexCapability OPTIONAL,

        -- deleted fields
        cryptoTokens            SEQUENCE OF CryptoH323Token OPTIONAL,
        fastStart               SEQUENCE OF OpenLogicalChannel OPTIONAL,
        fastCap         FastCap OPTIONAL,
        canOverlapSend  BOOLEAN

Perhaps the semantics could describe this better if this is the case.

I also have a few questions:

In Glen's text it mentions that the OpenLogicalChannel parameters in the
SETUP message indicate parameters to be used in the callee to caller
direction, i.e. the reverse sense to an H.245 OpenLogicalChannel.  As
the text in H.323 says that these are the parameters "that would
normally be passed later in the H.245 OpenLogicalChannel" I'm wondering
if the statement is correct.  If the sense truly is reversed, then this
should be brought out more in the text in H.323.

Also, what I am not clear on is if a terminal can receive and transmit
G.723 and G.729 how it should indicate this in the fast channel mode.
Negotiation is important for this as G.723 might be the safer bet, but
G.729 might be better for delivery of in-band DTMF tones (or simply have
lower delay).  Also, future codecs that are better tailored to the
Internet may be developed, and you want to be able to use these as soon
as possible.  Could you supply an example of this?

How well do you see the scheme working when MCs are involved.  Is it the
case that MCs will have to use slow start, or have I missed something?
MCs won't be able to use communicationModeCommand so I guess they won't
be able to instruct endpoints what to do in any forcible way.  Is this
correct?  As we have much special text in H.323/H.225 on making MCs
work, perhaps some text on how MCs and fast start interwork should be

Perhaps some annotated ladder diagrams would be useful.  I appreciate
that you haven't had much time though.  As a question to the wider
community (Dale!), would it be possible to add extra clarifying text of
this nature in January if it was felt to be useful?

Best regards,

Pete Cordell
BT Labs
E-Mail: pete.cordell at
Tel: +44 1473 646436
Fax: +44 1473 645499

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