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During data call or fax call echo cancellers are switched off in entire path from phone to exchange and exchange to phone. Echo canceller are used to cancel echo during voice call. Modulation in data and fax call also done in baseband carrier. Why echo cancellers are switched off during fax or data call. Why echos has dont effect on fax and data call signals.

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    \$\begingroup\$ Probably QAMdemonium :-) \$\endgroup\$
    – Russell McMahon
    Sep 17 '17 at 2:15
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Echo cancelers are turned off in the telephone network during fax and data calls when the 2100 Hz answer tone with phase reversals is sent. If they are not disabled, they cause a time varying channel for the cancelers in the modem, and thus cause reduced performance.

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Fax's use their own algorithm for minimum echo or Return Loss optimization (1) , better modems (with DSP) @ 56k can cancel 2 echoes. THis is part of the training cycle for frequency equalization, phase flattening and echo cancellation before communication begins.

( the weird tones before white noise)

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  • \$\begingroup\$ Thanks Tony. But why echocancellers are switched off in exchange and in phones. What will happened if echocancellers are not switched off at exchanges? \$\endgroup\$ Feb 25 '18 at 6:00
  • \$\begingroup\$ itu.int/rec/… \$\endgroup\$ Feb 25 '18 at 7:14

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