Subject: Re: Simple Gateway Control Protocol -- control signals should be in RTP

From: "Scott Petrack"<[email-masked]>
Date: Mon, 25 May 1998 09:55:07 +0200

From: Scott Petrack@VOCALTEC on 05/25/98 09:55 AM

Two more problems, this time with "SignalRequests" and "RequestedSignals".
SGCP currently mentions the following signals:

These might be received by a Gateway and (maybe) reported to the CA:
?    fax tones,
?    modem tones,
?    continuity tone,
?    continuity detection (as a result of a continuity test),
?    on-hook transition,
?    off-hook transition,
?    flash hook,
?    DTMF digits (or pulse digits).

These might need to be sent to an endpoint from a Gateway, on the
instruction of a CA:
?    Ringing,
?    Distinctive ringing, which can occur in 8 variants numbered 0 to 7,
?    Ring back tone,
?    Dial tones,
?    Intercept tone,
?    Network Congestion tone,
?    Busy tone,
?    Confirm tone,
?    Answer tone,
?    Call waiting tone,
?    Off hook warning tone,
?    Preemption tone,
?    Continuity tones,

The problem concerns "control signals" which are often sent to/from an
endpoint terminal via the audio path. Note that the endpoint in question
could be either an IP endpoint or a PSTN endpoint. I have a problem that
many of these signals need to be co-ordinated with an actual media stream.
DTMF is a particularly obvious example, and we spent a lot of time in the
VoIP Forum explaining the need to have an RTP payload type for DTMF, which
is currently an internet draft. Sending these audio/control signals in RTP
has three main advantages: it allows the signals to be precisely
synchronized with any media stream, it allows one to use standard RTP
redundancy techniques to ensure the reliable delivery of these signals, and
it allows a single IP message to be used in every country and
administration (this is because the actual audio waveforms, durations, etc.
can be subject to local regulation)

I propose that all these "telephony audio control signals" be added to the
current DTMF draft.

Yesterday I said that SGCP == SNMP+SIP/H.323
I see now that that 'audio control signals' require RTP as well.

Scott Petrack
VocalTec Communications Ltd.
[email-masked]