Configuring Rtp/Rtcp Settings; Configuring The Dynamic Jitter Buffer - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 15 Media

Configuring RTP/RTCP Settings

This section describes configuration relating to Real-Time Transport Protocol (RTP) and RTP
Control Protocol (RTCP).

Configuring the Dynamic Jitter Buffer

Voice frames are transmitted at a fixed rate. If the frames arrive at the other end at the same rate,
voice quality is perceived as good. However, some frames may arrive slightly faster or slower than
the other frames. This is called jitter (delay variation) and degrades the perceived voice quality. To
minimize this problem, the device uses a jitter buffer. The jitter buffer collects voice packets, stores
them and sends them to the voice processor in evenly spaced intervals.
The device uses a dynamic jitter buffer that can be configured with the following:
■ Minimum delay: Defines the starting jitter capacity of the buffer. For example, at 0 msec,
there is no buffering at the start. At the default level of 10 msec, the device always buffers
incoming packets by at least 10 msec worth of voice frames.
■ Optimization Factor: Defines how the jitter buffer tracks to changing network conditions.
When set at its maximum value of 12, the dynamic buffer aggressively tracks changes in delay
(based on packet loss statistics) to increase the size of the buffer and doesn't decay back
down. This results in the best packet error performance, but at the cost of extra delay. At the
minimum value of 0, the buffer tracks delays only to compensate for clock drift and quickly
decays back to the minimum level. This optimizes the delay performance but at the expense of
a higher 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.
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Mediant 4000 SBC | User's Manual