[cisco-voip] Incorrectly switch to modem passthrough mid-call

[Wes Sisk]

Any more info on the topology and configurations?

Generally speaking gateways must monitor inband audio and perform 
frequency analysis to identify tones.  Configuration may allow or 
prohibit that.  Generally speaking tone detection is not the most 
reliable.  In CoolEdit or similar you can perform frequency analysis on 
audio to identify composite frequencies.  Just because you can't hear it 
doesn't mean it's not there.

A bit of background information taken from the TAC Hot Issues RSS feed:
Voice/Fax fails when mgcp fax t38 inhibit command is configured , Fixed 
CSCsj24114
V.21 Fax Tone is not detected if the signal level is less than -22 db , 
Fixed CSCso69794
Trunk circuits can get stuck in "fax" mode , Fixed CSCsl58919
Fax Passthrough Improperly Configured by CM auto-configuration , Fixed 
CSCsv98322
MGCP fax T38 inhibit & default-package displayed after reload , Open 
CSCto24769
Voice/Fax fails when mgcp fax t38 inhibit command is configured , Fixed 
CSCsj24114
DTMF tone detector too picky , Fixed CSCsv54864

On casual observation this one sounds really interesting:
dsp falsely detects modem tones leading to one way audio. , Fixed CSCsc29824


And my personal favorite:
CSCsd92242    Gateway erroneously detects DTMF during speech

Regards,
Wes

On 6/8/2011 12:18 PM, James.Brown at barclayswealth.com wrote:
> All,
>
> I've noticed recently that around 1 in 10k calls arriving into our Contact Centre appear to change codec half way through the call. After many hours of Wireshark analysis, I realised the gateway was incorrectly detecting certain sounds from the client as SG3 fax and consequently upspeeding. To the human ear, these are obviously not faxes, so I raised a TAC case, thinking I might have come across a DSPWare issue.
>
> The TAC response is that this is normal. I wanted to gauge opinion on this one, because I have honestly not come across it before. Has anyone else experienced voice calls incorrectly converting to fax mid-call? Presumably this also has implications for the recently added T38 support for SG3, because our agents aren't trained to speak that yet ;-)
>
> Regards
>
> James.
>
>
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