298 ◾ IP Communications and Services for NGN
“toll-quality speech” is used to refer to the quality of speech received from a fixed-
line telephone handset. Usually, it describes the fact that the output of the telephone
handset can be recognized as belonging to the speaker without confusion. It indicates
the extent to which the voice of the speaker remains unchanged at the receiving end
of the telephone line. Each codec is therefore rated in terms of how close it represents
the speaker’s voice or how much it has altered the naturally sounding voice of the
person. e most accepted method of measuring the quality of speech from speech
coders is obtained from a subjective response of the listener. To estimate the MOS, a
wide range of listeners ...