SIP User's Manual 142 Document #: LTRT-83310
Mediant 600 & Mediant 1000
12.1.2 Silence Suppression (Compression)
Silence suppression (compression) is a method for conserving bandwidth on VoIP calls by
not sending packets when silence is detected. The device uses its VAD feature to detect
periods of silence in the voice channel during an established call. When silence is
detected, it stops sending packets in the channel.
The procedure below describes how to enable silence suppression using the Web
interface.
To enable silence suppression using the Web interface:
1. Open the Voice Settings page (Configuration tab > VoIP menu > Media submenu >
Voice Settings).
2. Set the 'Silence Suppression' (EnableSilenceCompression) field to Enable.
3. Click Submit to apply your changes.
12.1.3 Echo Cancellation
The device supports adaptive linear (line) echo cancellation according to G.168-2002.
Echo cancellation is a mechanism that removes echo from the voice channel. Echoes are
reflections of the transmitted signal.
In this line echo, echoes are generated when two-wire telephone circuits (carrying both
transmitted and received signals on the same wire pair) are converted to a four-wire circuit.
Echoes are reflections of the transmitted signal, which result from impedance mismatch in
the hybrid (bi-directional 2-wire to 4-wire converting device).
An estimated echo signal is built by feeding the decoder output signal to an RLS-like
adaptive filter, which adapts itself to the characteristics of the echo path. The ‘estimated
echo signal’ (the output of this filter) is then subtracted from the input signal (which is the
sum of the desired input signal and the undesired echo) to provide a clean signal. To
suppress the remaining residual echo, a Non Linear Processor (NLP) is used, as well as a
double-talk (two people speak at the same time) detector that prevents false adaptation
during near-end speech.
The procedure below describes how to configure echo cancellation using the Web
interface:
To configure echo cancellation using the Web interface:
4. Open the Voice Settings page (Configuration tab > VoIP menu > Media submenu >
Voice Settings).
5. Set the 'Echo Canceller' field (EnableEchoCanceller) to Enable.
6. Open the General Media Settings page (Configuration tab > VoIP menu > Media
submenu > General Media Settings).
7. From the 'Max Echo Canceller Length' drop-down list (MaxEchoCancellerLength),
select the maximum echo path delay (tail length) for the echo canceller.
Note: The following additional echo cancellation parameters are configurable only
through the ini file:
• ECHybridLoss - defines the four-wire to two-wire worst-case Hybrid loss
• ECNLPMode - defines the echo cancellation Non-Linear Processing
(NLP) mode
• EchoCancellerAggressiveNLP - enables Aggressive NLP at the first 0.5
second of the call
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