Cisco SPA1001 Administration Guide - Page 18

Ensuring Voice Quality Chapter 1 Introducing Linksys Analog Telephone Adapters 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. The Linksys ATA supports the most popular audio compression algorithms for IP Telephony: G.711 a-law and μ-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 The Linksys ATA applies silence suppression so that silence packets are not sent to the other end to conserve more transmission bandwidth. Instead, a noise level measurement can be sent periodically during silence suppressed intervals so that the other end can generate artificial comfort noise that mimics the noise at the other end (using a CNG or comfort noise generator). 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 lowers the perceived voice quality. The Linksys ATA applies an error concealment algorithm to alleviate the effect of packet loss. Network Jitter The IP network can induce varying delay of the received packets. The RTP receiver in the Linksys ATA 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. Therefore, the jitter buffer size should be kept to a relatively small size whenever possible. If the jitter buffer size is too small, late packets may be considered lost and this lowers the voice quality. The Linksys ATA can dynamically adjust the size of the jitter buffer according to the network conditions that exist during a call. Echo Impedance mismatch between the telephone and the IP Telephony gateway phone port can lead to near-end echo. The Linksys ATA has a near-end echo canceller with at least 8 ms tail length to compensate for impedance match. The Linksys ATA also implements an echo suppressor with comfort noise generator (CNG) so that any residual echo is not noticeable. Linksys ATA Administrator Guide 1-4 Document Version 3.1