Linksys SPA932, SPA962, SPA942, SPA922 manual Factors Affecting Voice Quality

Setting System Features

Ensuring Voice Quality

Codec	Est. Bandwidth	2 Calls	4 Calls	6 Calls	8 Calls
	Budget
G.726-16	63 kbps	126 kbps	252 kbps	378 kbps	504 kbps
G.729	55 kbps	110 kbps	220 kbps	330 kbps	440 kbps

NOTE: The use of silence suppression can reduce the average bandwidth budget by 30% or more.

For more information about bandwidth calculation, refer to the following websites:

http://www.erlang.com/calculator/lipb/

http://www.packetizer.com/voip/diagnostics/bandcalc.html

Factors Affecting Voice Quality

The following factors contribute to voice quality:

•Audio compression algorithm—Speech signals are sampled, quantized, and compressed before they are packetized and transmitted to the other end. For IP Telephony, speech signals are usually sampled at 8000 samples per second with 12–16 bits per sample. The compression algorithm plays a large role in determining the voice quality of the reconstructed speech signal at the other end. SPA9x2 phones support the most popular audio compression algorithms for IP Telephony: G.711 a-law and u-law, G.726, G.729a, and G.723.1.

•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—SPA9x2 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 (comfort noise generator, or CNG).

•Packet loss—Audio packets are transported by UDP, which does not guarantee the delivery of the packets. Packets may be lost or contain errors that can lead to audio sample drop-outs and distortions and lower the perceived voice quality. SPA9x2 phones apply an error concealment algorithm to alleviate the effect of packet loss.

•Network jitter—The IP network can induce varying delay of received packets. The RTP receiver in SPA9x2 phones keeps 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.

Linksys SPA9x2 Phone Administration Guide 39