Reply by Huo Jiaquan ●November 24, 20032003-11-24
The papers i read gave me the impression that the
hybrids at the central offices are equipped with ECs.
So they are not? Or those ECs do not provide
sufficient suppression?
--- "Shaw, David G (David)" <> wrote:
> I will discuss V.32, since it is the first case of
> this
> type and all others are at least similar.
> The calling modem sees answer tone (2100 Hz) and
> sends a tone at 1800 Hz.
> The answer modem then switches from answer tone to a
> pair of tones
> at 600 and 3000 Hz. After a short delay, the answer
> modem
> reverses the phase of the 2 tones and starts a
> timer. When
> the calling modems sees the phase change, it waits a
> predetermined
> 64 symbols at 2400 symbols per second and reverses
> the phase
> of the 1800, while starting a timer. When the
> answer modem sees
> the phase change at 1800 it now has an estimate of
> the round trip
> delay by subtracting the 64 symbols from the total
> on the timer.
> The answer modem now waits that same 64 symbols and
> reverses the
> phase of the two tones again. The calling modem
> sees the phase
> changes again and also has an estimate of the round
> trip delay, again by subtracting the 64 symbols from
> its timer.
> The assumption here is that the echo producing
> mechanism is the
> hybrid in the central office closest to the remote
> modem, so that
> the measured delay end to end is just a little
> longer than the round trip
> delay I would expect to see before the remote echo
> comes back.
> All of this does not take into account that
> possibility
> of another/other echo(es) somewhere else in the
> network.
>
> Regards,
> Dave Shaw
>
> -----Original Message-----
> From: Huo Jiaquan [mailto:]
> Sent: Friday, November 21, 2003 2:10 AM
> To:
> Subject: RE: [echocancel] Re: LEC and NEC > Sorry, i have been working with acoustic echoes, not
> very familiar with modem designs. Could u explain
> 'determined through an exchange between the
> end-points
> during startup' in a bit more details? Or sugguest
> some good ref.?
>
> thx
>
> --- "Shaw, David G (David)" <>
> wrote:
> > For modem, the bulk delay is determined through an
> > exchange between the end-points during the startup
> > of the modem.
> > For voice calls, you will have to come up with a
> > clever algorithm to determine the delay.
> >
> > Dave Shaw
> >
> > -----Original Message-----
> > From: Huo Jiaquan [mailto:]
> > Sent: Monday, November 17, 2003 8:50 PM
> > To:
> > Subject: Re: [echocancel] Re: LEC and NEC
> >
> >
> > How do u determine the bulk delay for the far-end
> > echo?
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