Re: T.30 Fax and Echo Cancelation

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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.

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