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

[Nick Matthews]

For agent calls that doesn't really sound right.  I believe the specs
for the modem detection are pretty strict - something like 5%
frequency deviation over 500ms or longer for 2100hz.  From what I
understand it's pretty hard to do that with human voices.  It sounds
like a bug where a customer didn't get a switchover, we made it less
strict, and now we're going back the other way.  Seems to be a
sinusoidal VOIP trend.

-nick

On Wed, Jun 8, 2011 at 1:40 PM, Wes Sisk <wsisk at cisco.com> wrote:
> 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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