Contribution APC-1456

Yair Miranda yair at RADVISION.RAD.CO.IL
Tue Nov 10 12:33:26 EST 1998

> From:         Rich Bowen[SMTP:rkbowen at CISCO.COM]
> H.323 v2 section says, "Transmitters shall limit the content
> of their transmitted information to that which the receiver has
> indicated it is capable of receiving."
This was written back when "transmitted information" did not include H.245
messages. It referred only to the streams of video, audio, and data. That's
why it seems to contradict the following, which was added much later:

> H.245 v3
> section 7.12.6 says, "Endpoints shall use the alphanumeric indication
> to convey DTMF user input if the other endpoint has not indicated
> the ability to receive DTMF using UserInputCapability."
>    o Every endpoint is required to indicate capability of
>      receiving alphanumeric user input (not likely), or --
>    o Every endpoint is assumed to have the capability to
>      accept alphanumeric user input; i.e., each endpoint
>      *implicitly* indicates that it has this capability.  Or --
>    o I'm interpreting H.245 v3 too strictly.  What it really
>      means is that endpoints shall use alphanumeric if the
>      remote endpoint did not indicate DTMF capability but it did
>      indicate alphanumeric capability; otherwise don't send user
>      input at all.
> Is any of these interpretations correct?
The second one.

> Can someone please clarify the requirements here?
First, the intent was that all H.323 terminals have at least a basic ability
to signal "DTMF." In fact, this is a requirement. 6.2.8/H.323v2: "An H.245
indication, userInputIndication, is available for transport of user input
alphanumeric characters from a keypad or keyboard, equivalent to the DTMF
signals used in analogue telephony... H.323 terminals shall support the
transmission of user input characters 0-9, "*", and "#". Transmission of other
characters is optional."

All endpoints must be able to receive all of the H.245 messages, including
userInputIndication (6.2.8/H.245v3: "H.323 terminals shall be capable of
parsing all H.245 MultimediaSystemControlMessage messages..."); however, note
that there is no requirement to do anything with this information on the
receive side. Just like with the PSTN, it is up to special-purpose terminals
to decide whether and how to interpret these signals, but all terminals are
required to send them.

Paul Long
Smith Micro Software, Inc.

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