Capability Exchange procedures

Derks, Frank F.Derks at PBC.BE.PHILIPS.COM
Wed Oct 6 13:18:42 EDT 1999

Dear All,

I just went through the descriptions of the Capability Exchange in both the
H.323 and the H.245 documents and it would seem that they are not quite

The first two sentences in the first paragraph in H.245, "5.2 Capability
Exchange", seem to mean that a terminal should not send any media that "can
not be received" by the receive terminal. The only way to achieve this is
that the receive terminal send a TCS Reject for all those media types that
it can not receive. However, the third sentence contradicts this by stating
that those capabilities that are not understood (or can be used) should be
ignored. Furthermore, the fourth sentence states that not understood
extensions to a capability that is understood (and can be used) should be
ignored, which could result in unsuccessful communication. So how should
this paragraph be read.

The second paragraph in H.245, "5.2 Capability Exchange", states: "The total
capability of a terminal to receive and decode various signals is made known
to the other terminal by transmission of its capability set."

I would say that the "total capability" includes both the receive and the
transmit capabilities and not just its "receive (and decode)" capabilities.

Furthermore, most of section 5.2 talks about capabilities and not about
their exchange.

The fourth paragraph in H.245, "5.2 Capability Exchange", states: "The
absence of _a_ transmit capability indicates that ....". This phrasing (the
"a") suggests that the absence of just one of the capabilities indicates
that no choices are offered.

Section 7.2.3 "Terminal Capability Set Acknowledge", states: "This is used
to confirm receipt of a TerminalCapabilitySet from the peer CESE". In other
words upon reception of a TCS, the Acknowledge should always be sent. So
when should a Reject be sent? The logical succession of TCS messages between
two CESEs is not well documented and clarifying text is required to make
this clear.


Frank Derks                    |Tel  +31 35 6893238 |
Advanced Development           |Fax  +31 35 6891030 |
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