Nortel RLC, Installation And Administration Manual

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RLC Installation and Administration Guide 23
June 2005 RLC description
Echo cancellation
The RLC performs echo cancellation in accordance with ITU G.168, and cancels
echo with a tail length of up to 32 milliseconds (32 ms).
Due to the network delay introduced by the IP network, echo is more noticeable
in VoIP networks than in traditional networks. All echo cancellation algorithms
require a small amount of time to adjust if the echo path changes. This results in
the initial syllable, or word, of the first sentence having echo. After that, no echo
is apparent.
This is a normal characteristic of the transition point between the VoIP network
and traditional analog facilities. If echo is not cancelled after the beginning of
the first sentence, this can be due to a more serious problem (such as impedance
mismatch). Also, note that some environments, such as conference calls, can
change the echo path. This also results in an adjustment period for Echo
Cancellation.
Note: The source of echo on the majority of installations is incorrectly
configured analog trunks. Please ensure that the trunks are configured correctly
in the areas of trunk card type (EXUT/XUT), TIMP, and BIMP settings, and
jumper settings on the actual trunk card itself. Also, measure the line loss and
compensate using the trunk class of service NTC or TRC as required.
Nortel recommends that the trunk card be an enhanced version (EXUT). An
EXUT supports more complex impedance situations. Please refer to NTP 553-
3001-106 for trunk cards and the NTP for your specific PBX for more
information. Also, if you are connecting recording equipment or Recorded
Announcement (RAN) devices to an analog trunk, ensure that the trunk
impedance matches the equipment impedance to prevent echo.
Trunk options for Reach Line Cards
The following is a list of the different trunk options supported by the Reach Line
Card:
„
VoIP trunk
(included through Ethernet interface)
„
PRI trunk
(clear-channel mode)