Digium, Inc.
Page 17
Chapter 1: Overview
Echo-Cancellation
Users connecting their TE120 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 TE120 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 VPMADT032 hardware echo cancellation module. The
VPMADT032 may be combined with both the TE121 and TE122 cards; it
may not be combined with the TE120P card. The TE121 and TE122 are
offered bundled with the VPMADT032 as, respectively: TE121B,
TE122B.
The VPMADT032 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
VPMADT032 will automatically operate and cancel all network echo
within its tail range (1024 taps). Users of TE120P cards, which do not
maintain the capability to support the VPMADT032, may also purchase
Digium's commercial HPEC software:
http://www.digium.com/en/products/software/hpec.php
VoIPon www.voipon.co.uk [email protected] Tel: +44 (0)1245 808195 Fax: +44 (0)1245 808299
