VOIP should receive the same quality of voice transmission they receive with basic telephone services. Like other real-time applications, VoIP is extremely bandwidth- and delay-sensitive. For VoIP transmissions to be intelligible to the receiver, voice packets should not be dropped, excessively delayed, or suffer varying delay.VoIP can guarantee high-quality voice transmission only if the voice packets, for both the signaling and audio channel, are given priority over other kinds of network traffic. For VoIP to be deployed so that users receive an acceptable level of voice quality, VoIP traffic must be guaranteed certain compensating bandwidth, latency, and jitter requirements. QoS ensures that VoIP voice packets receive the preferential treatment they require. In general, QoS provides better (and more predictable) network service by providing the following features:
1) Supporting dedicated bandwidth
2) Improving loss characteristics
3) Avoiding and managing network congestion
4) Shaping network traffic
5)Setting traffic priorities across the network
For more information refer to these documents.
http://www.cisco.com/en/US/docs/ios/12_1/qos/configuration/guide/qcdvoice.html
http://cisco.com/en/US/tech/tk652/tk698/technologies_white_paper09186a00800d6b73.shtml
