[asterisk-users] T.30 Fax and Echo Cancelation

Kevin P. Fleming kpfleming at digium.com
Fri Apr 13 10:33:53 CDT 2012

On 04/13/2012 02:25 AM, Anita Hall wrote:

> I know that all echo cancelers are supposed to shut down when they hear
> the CNG or CED fax tone. Is this always a good thing ? I know that T.30
> is supposed to be half duplex, so echo should not be there but in my
> session recordings, I have seen both sides trying to send at the same
> time (is that a sign of some other bad thing?)

Your statement is not quite correct. G.168-compliant echo cancellers 
don't respond to the CNG tone at all, and their response to the CED tone 
varies. Some will completely disable themselves, others (modern and 
better ones) will continue to operate, but without their NLPs 
(non-linear processors) engaged.

All FAX modems are half-duplex, but that doesn't mean that echo won't 
occur. Echo occurs regardless of whether the modems are half- or 
full-duplex. If you see both FAX endpoints sending modem data at the 
same time, then yes, you have some other type of problem.

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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