[VoiceOps] ATA CPE and RFC2833

Peter Childs PChilds at internode.com.au
Thu Feb 16 03:16:12 EST 2012

In a isolated environment I plugged one of our lab switching nodes (ip->tdm) into a loopback device on the other side to 'loop' back audio.

Using a soft-client that only sends rfc2833 (rtp.p_type==101) I can 'see' the DTMF digits in both directions (and hear them fed back via the loopback), however in Wireshark (1.4.2 on MacOSX) they generate no visible or audible artefacts in the RTP stream playback (they are visible in the capture, and the 'flow diagram').

Then using my suspect ATA I see rtp.p_type==101 digits again (and confirmed by 'debug voip rtp named-event') however in my wireshark RTP stream I can 'see' audio and 'hear' it as well as seeing the nte events.

I also just created a 'display filter' in wireshark 'rtp.p_type != 101 || mgcp' to show only the MGCP and RTP (without named events) and then saved a copy of only displayed packets.   Loading this capture (the filtered capture) and decoding the voice playback shows audio tones in packet.

 Peter Childs - Voice Engineer
 Ph: 08 8228 2380 Mb: 0406 388 405 Fax: 08 8235 6801

On 16/02/2012, at 6:29 PM, Alex Balashov wrote:

> On 02/16/2012 01:40 AM, Peter Childs wrote:
>> I'm working through one of those lovely DTMF issues and have notice
>> that a particular CPE that is experiencing a DTMF fault on our network
>> sends both rfc2833 rtp packets _and_ the tone in audio stream (not
>> named rtp packets, but how the device has 'heard' the tones from the
>> originating handset).
> Out of curiosity, how did you arrive at this conclusion?
> If you play back such a stream in, say, Wireshark, it will generally inject some sort of acoustic tone when it encounters the out-of-band named events.
> -- 
> Alex Balashov - Principal
> Evariste Systems LLC
> 260 Peachtree Street NW
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