[Asterisk-Users] GSM ISDN gateway to quadbri NT port under bristuffed Asterisk - where to set gain (I have unreliable inband dtmf recognition )?

[Steve Underwood]

Leandro Morgado wrote:

>Steve Underwood wrote:
>
>  
>
>>Robert Rozman wrote:
>>
>>    
>>
>>>Hi,
>>>
>>>I'm getting unreliable dtmf recognition (it works fine for 4-5
>>>digits, errors (duplicates) on more), when transferred inband from
>>>gsm gateway to NT port of quadbri under bristuffed Asterisk.....
>>>
>>>Since Asterisk is claimed to have good dtmf recognizer, I suspect
>>>there are some settings to workarouned... I've tried dtmf relax, but
>>>didn't help, so I suspect gain settings....
>>>
>>>Is there any other possible cause of unreliable dtmf inband
>>>recognition ? Where can I set gain on voice channel (I guess majority
>>>of settings under bristuff in zaptel.conf are dummy) ?
>>>
>>>Any other advice on this problem or similar experience ?
>>>
>>>Thanks in advance,
>>>      
>>>
>>I kind of amazed if works at all when getting DTMF out of a GSM phone.
>>You really shouldn't expect it to.
>>    
>>
>
>We have sucessfully "read" incoming DTMF from:
>
>a) Nokia32 Analog GSM connected to TDM400 (had to use relaxdtmf with
>chan_zap)
>b) Ateus BRI ISDN GSM connected to AVM Fritz (had to patch chan_capi
>0.3.5 to support relaxdtmf)
>
>
>Question (I'm from a software eng. background, not telco):
>What would be the reason for not receiving DTMF from a GSM
>phone/gateway? Do you have the time to explain why? (I'm really
>interested in learning :)
>  
>
The low bit rates codecs used for GSM cannot carry DTMF without 
seriously corrupting it. To allow DTMF to be sent to things like IVRs 
the GSM protocol alows the handset to send a message to the basestation 
to tell it to send a DTMF digit to the wireline network. The timing of 
these digits is completely unrelated to the user pressing keys on the 
phone, so any input method based on DTMF timing does not work. There is 
usually no need for DTMF to be sent reliably from a basestation to a 
handset, so the GSM protocol makes no provision for it.

DTMF might or might not get through. The signals are quite distorted, 
but they are still present. Its pretty hit and miss. GSM 06.10 (the 
original GSM codec, which things like Asterisk also support) tends to 
work quite a bit of the time. The newer codecs (EFR, half-rate and AMR) 
tend not to.

Regards,
Steve