[asterisk-dev] Proposal for T.38 transparent gateway design in Asterisk

[Klaus Darilion]

Am 28.04.2010 23:17, schrieb Lee Howard:
> Kevin P. Fleming wrote:
>> Klaus Darilion wrote:
>>
>>
>>> Further, FAXOPT should be allow configure the faxdetection flexible,
>>> e.g. CNG and/or CED tone, incoming or outgoing)
>>>
>>
>> CED tone is not a reliable discriminator for FAX. CNG tone is, for
>> detection of calling FAX machines, and is already supported by
>> chan_dahdi (and could easily be supported by the other TDM channel
>> drivers as well).
>
> CNG isn't reliable, either.  A faxmodem dialing with ATDT<number>@ will
> not produce CNG, and many fax callers will do this in order to not
> disturb the receiver if they happen to call a human instead of a machine.
>
> Fax detection is not entirely reliable either way ... neither detecting
> CED nor detecting CNG.

 From a pragmatic point of view: yes it is not reliable but as soon as I 
hear CED or CNG I would want to initiate T.38. Maybe then T.38 might be 
initiated falsely in case of modem calls, but who cares about modem 
support over VoIP anyway?

So, even if not all fax will send CNG we should listen for CNG because 
if we hear it, we know it is a fax. And CED can be a modem too, but as 
we do not care about modem we can listen for CED too.

regards
klaus