Digium, Inc.
Page 17
Chapter 1: Overview
Echo-Cancellation
Users connecting their TE430 Series cards to the PSTN or other devices
are likely to be placing calls that will result, at some point, in an
unbalanced 4-wire/2-wire hybrid. The result of this hybrid is the
reflection of a near-end echo to the calling party. Elimination of this echo
is the responsibility of echo cancellation.
The TE430 Series cards, unless otherwise equipped, utilize Asterisk to
perform software-based echo cancellation. Asterisk maintains a number
of open source echo cancelers. These open source echo cancelers provide
a moderate level of echo cancellation, but are not capable of dealing with
higher levels of, or more advanced, echoes.
Digium recommends that those users concerned about echo cancellation
purchase the VPM128 hardware echo cancellation module. The VPM128
may be combined with the TE435 (as TE435B) or TE436 (as TE436B)
cards.
The VPM128 is designed to handle up to 128ms of echo cancellation
across all channels and provides a G.168 compliant echo cancellation
solution.
If equipped and not explicitly disabled in chan_dahdi.conf, the VPM128
will automatically operate and cancel all network echo within its tail
range (1024 taps). Users may also purchase Digium's commercial HPEC
software:
http://www.digium.com/en/products/software/hpec.php
