Cisco 521SG Administration Guide - Page 34
samples to absorb the network jitter, instead of playing out all the samples
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Getting Started Ensuring Voice Quality 1 • The encoder and decoder pair in a compression algorithm is known as a codec. The compression ratio of a codec is expressed in terms of the bit rate of the compressed speech. The lower the bit rate, the smaller the bandwidth required to transmit the audio packets. Although voice quality is usually lower with a lower bit rate, it is usually higher as the complexity of the codec gets higher at the same bit rate. • Silence suppression-Cisco SPA IP phones apply silence suppression so that silence packets are not sent to the other end to conserve more transmission bandwidth. IP bandwidth is used only when someone is speaking. Voice activity detection (VAD) with silence suppression is a means of increasing the number of calls supported by the network by reducing the required bidirectional bandwidth for a single call. A noise level measurement is sent periodically during silence suppressed intervals so that the other end can generate artificial comfort noise by using a comfort noise generator (CNG). • Packet loss-Audio packets are transported by UDP. Packets might be lost or contain errors that can lead to audio sample drop-outs and distortions and lower the perceived voice quality. The Cisco SPA IP phones apply an error concealment algorithm to alleviate the effect of packet loss. • Network jitter-The IP network can induce varying delays of received packets. The RTP receiver in Cisco SPA IP phones keep a reserve of samples to absorb the network jitter, instead of playing out all the samples as soon as they arrive. This reserve is known as a jitter buffer. The bigger the jitter buffer, the more jitter it can absorb, but this also introduces bigger delay. - Jitter buffer size should be kept to a relatively small size whenever possible. If jitter buffer size is too small, many late packets might be considered lost and thus lower the voice quality. Cisco SPA IP phones dynamically adjust the size of the jitter buffer according to the network conditions that exist during a call. - The minimum jitter buffer size is 30 ms or 10 ms plus the current RTP frame size, whichever is larger, for all jitter level settings. However, the starting jitter buffer size value is larger for higher jitter levels. This setting controls the rate at which the jitter buffer size is adjusted to reach the minimum. - Jitter Buffer Adjustment-Controls how the jitter buffer should be adjusted. Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide 33