[asterisk-ss7] Fax Detection with chan_ss7

Kevin P. Fleming kpfleming at digium.com
Fri Aug 5 12:51:49 CDT 2011

On 08/05/2011 12:06 PM, Robert Thomas wrote:

> I'm refering to the audio hook implemented to the detect the V21
> preamble fax on the T38 gateway implementation on asterisk 1.10.
> I think it depens whether they refer to fax detection to disable echo
> cancellation, or to renegotiate to T38.

Neither. In most cases, 'FAX detection' in Asterisk used to to redirect 
the channel to a different extension in the dialplan. This is done by 
detecting CNG, so that the destination of the call can be changed to one 
that is able to handle the FAX transaction. Asterisk does not handle 
echo canceller reconfiguration for FAX at all; that's done by DAHDI or 
the echo cancellers themselves.

> Is the fax audio hook detection responsibility of the channel used,
> chan_dahdi or chan_ss7?

CNG detection is the responsibility of the channel driver; V.21 preamble 
detection is handled by the FAX gateway code, and can be used on any 
channel type.

Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming at digium.com | SIP: kpfleming at digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org

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