[reSIProcate] rtp ringing

[Adam Roach]

For what you describe, a full RTP stack would be overkill. You can 
probably get by just sending your packets with an appropriately encoded 
RTP header every 20 ms. You'll also need to listen for incoming RTP 
packets and discard them. (Ideally, you'd calculate statistics based on 
these packets and generate proper RTCP; however, in practice, things 
will pretty much work just fine if you don't).

Also: while what you're proposing will work _mostly_ fine for codecs 
that don't rely on prior frames for encoding (like G.711), you are very 
likely to run into difficulty coming up with a single G.729 or G.723 
packet to represent "ringing." Even with G.711, you'll run into problems 
because the standard North-American 350+440 ringback tone doesn't repeat 
neatly on 20 ms boundaries.

You might try sending an RFC4733 representation of ringback (e.g., if 
you're targeting the US market: modulation = 0, volume = -6 dBm0, 
frequency = 350, frequency = 440, duration = 2 seconds; sent every 6 
seconds or so). Support for this approach may be more limited, so make 
sure you test it against all your target devices.

/a

On 11/20/07 10:37 AM, David Thompson wrote:
>
> Quick question if anyone has an idea for me:
>
> I have built an sbc using the reciprocate stack. (THANK YOU to all who 
> have worked on this stack)
>
> I want to provide my own “ringing” early media rtp for comfort 
> reasons. I know reciprocate does not offer an RTP stack but I was 
> wondering if someone could point me in the right direction on where I 
> could look.
>
> Basically I would already have the ringing file encoded in g729, g723, 
> g711 and would simply want to issue a 180 early media from the sip 
> stack then start transmitting the rtp in the proper codec. Once the 
> call completed I would then let the media pass from end point to 
> endpoint directly.
>
> Any thoughts would be appreciated……
>
> David
>
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