Comfort Noise Generation - AudioCodes Mediant 800B User Manual

error rate.
The default settings of 10 msec Minimum delay and 10 Optimization Factor should provide
a good compromise between delay and error rate. The jitter buffer 'holds' incoming packets
for 10 msec before making them available for decoding into voice. The coder polls frames
from the buffer at regular intervals in order to produce continuous speech. As long as
delays in the network do not change (jitter) by more than 10 msec from one packet to the
next, there is always a sample in the buffer for the coder to use. If there is more than 10
msec of delay at any time during the call, the packet arrives too late. The coder tries to
access a frame and is not able to find one. The coder must produce a voice sample even if
a frame is not available. It therefore compensates for the missing packet by adding a Bad-
Frame-Interpolation (BFI) packet. This loss is then flagged as the buffer being too small.
The dynamic algorithm then causes the size of the buffer to increase for the next voice
session. The size of the buffer may decrease again if the device notices that the buffer is
not filling up as much as expected. At no time does the buffer decrease to less than the
minimum size configured by the Minimum delay parameter.
In certain scenarios, the Optimization Factor is set to 13: One of the purposes of the
Jitter Buffer mechanism is to compensate for clock drift. If the two sides of the VoIP call are
not synchronized to the same clock source, one RTP source generates packets at a lower
rate, causing under-runs at the remote Jitter Buffer. In normal operation (optimization factor
0 to 12), the Jitter Buffer mechanism detects and compensates for the clock drift by
occasionally dropping a voice packet or by adding a BFI packet.
Fax and modem devices are sensitive to small packet losses or to added BFI packets.
Therefore, to achieve better performance during modem and fax calls, the Optimization
Factor should be set to 13. In this special mode the clock drift correction is performed less
frequently - only when the Jitter Buffer is completely empty or completely full. When such
condition occurs, the correction is performed by dropping several voice packets
simultaneously or by adding several BFI packets simultaneously, so that the Jitter Buffer
returns to its normal condition.
The following procedure describes how to configure the jitter buffer using the Web
interface.

To configure jitter buffer using the Web interface:
1. Open the RTP/RTCP Settings page (Setup menu > Signaling & Media menu >
Media folder > RTP/RTCP Settings). The relevant parameters are listed under the
General group, as shown below:
2. Set the 'Dynamic Jitter Buffer Minimum Delay' parameter (DJBufMinDelay) to the
minimum delay (in msec) for the Dynamic Jitter Buffer.
3. Set the 'Dynamic Jitter Buffer Optimization Factor' parameter (DJBufOptFactor) to the
Dynamic Jitter Buffer frame error/delay optimization factor.
4. Click Apply.

14.3.2 Comfort Noise Generation

The device can generate artificial background noise, called comfort noise, in the voice
channel during periods of silence (i.e. when no call party is speaking) for Gateway calls.
This is useful in that it reassures the call parties that the call is still connected. The device
detects silence using its Voice Activity Detection (VAD) mechanism. When the Calling
Tone (CNG) is enabled and silence is detected, the device transmits Silence Identifier
User's Manual
Figure 14-5: Configuring Jitter Buffer
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Mediant 800B Gateway & E-SBC
Document #: LTRT-10298