Re: [rtcweb] Working group last call for draft-ietf-rtcweb-audio

[Roman Shpount <[e-mail]>]

On Fri, Dec 18, 2015 at 2:34 PM, Ted Hardie <[e-mail]> wrote:

> On Fri, Dec 18, 2015 at 10:32 AM, Roman Shpount <[e-mail]> wrote:
>
>> Hi All,
>>
>> Since the decision not to support A-D DTMF digits was never made on the
>> IETF list (I have looked through the archive and cannot find it), I do not
>> think they should be removed.
>>
>>
> ​Can you say more about what use there would be in supporting them?
>

First of all, DTMF support is there for legacy support. I generally dislike
when features are removed from an established capability set without a good
reason. Can anyone name me a single commercial PSTN VoIP gateway which does
not support A-D DTMF tones? I cannot think of one. I believe it would
generally make sense to implement whole set defined in
https://tools.ietf.org/html/rfc4733#section-3.2 instead of just 3/4. From
what I know there is no extra implementation to support A-D DTMF tone. It
looks like these tones are removed for aesthetic reasons only.

As far as more specific examples are concerned:

DTMF tones A-D are used to provide a way to access hidden flows in IVR
systems. For instance, when inbound call center is integrated with an
external auto-dialer, auto-dialer places a call to call center IVR and uses
a hidden flow accessible via one of those tones to connect directly to a
waiting agent.

DTMF tones A-D are also commonly used by ham radio operators to integrate
with PSTN, such as http://www.echolink.org/Help/dtmf_functions.htm

DTMF tone A-D are used to provide prioritized call handling service in ITU
Q.955.3 specification which is still used by some PBX and phone systems.

Finally, some calling card like systems allow remote control of functions
such as flash by mapping them to extended DTMF tones.

Regards,
_____________
Roman Shpount