A VoIP system consists of an encoder-decoder pair and an IP transport network. The choice of voice codec is important because it has to fit the particularities of the transport network. Compressing voice signals while keeping the quality perceived by users at acceptable levels represents a daunting challenge. The methods that have been proposed for the compression of audio signals, which are referred to as voice codecs. Voice codecs are the algorithms that enable the system to carry analog voice over digital lines.