TELECOM Digest OnLine - Sorted: Re: How a Telephone Works

Re: How a Telephone Works

John Levine (
14 Sep 2005 19:41:16 -0000

> I think your explanation is right on, however in my world, and this
> may be too technical for this discussion and if I remember ... If the
> switch does sense data flowing, it turns off the echo cans for the
> duration of that call on any long haul circuit. Having been retired
> for about 7 years now, I wonder if the scheme is still the same ...

I believe that the pilot tone the modem sends is defined to tell the
echo cancellers to go away. Of course, now that phone calls are
typically digitized at the originating CO and turned back to analog at
the callee's CO, whether it's across the street or half way around the
world, how much echo cancelling do we need?


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