 1016). Unfortunately, this method introduces an end-to-end delay up to 100 ms (due to block processing and searching the codebook) that is not accept- able for voice conversational applications. In 1992, AT&T Bell Laboratories standardized a Speech-Coding Principles Waveform Coders AbS Coders Fig. 1.4 Quality comparison of speech-coding schemes. Source: Reconstructed from Kondoz (2005); used with permission. Bitrates (kbps) version of CELP (G.728 Recommendation), which uses a backward prediction and satisfies the ITU G.721 requirements. G.728-compatible coders are referred to as low-delay CELP coders, with the approximate end-to-end delay of 2.5 ms. AbS techniques can generally achieve better compression ratios than waveform coders; however, they are known for pro- viding a more artificial speech quality. In multimedia applications, the Mean Opinion Score (MOS) is the most common subjec- tive quality measure, which provides a numerical indication of the perceived quality of received media after compression or transmission. The MOS is expressed as a single number in the range 1 to 5, where 1 is lowest perceived quality and 5 refers to highest perceived quality. Examples of other subjective quality measures are Diagnostic Rhyme Test (DRT) and Paired Comparison Test (PCT). Mean Squared Error (MSE) and signal-to-noise ratio (SNR), on the other side, are examples of widely used objective measures. Figure 1.4 com- pares the quality of the waveform-based and AbS coders in terms of MOS. Standards for Voice Communications This section describes the relevant details of current standardized voice codecs in the context of the network for which they were designed. Most speech coding systems in use today based on telephone bandwidth narrowband speech, nominally limited to about 300 to 400 Hz and sampled at a rate of 8 kHz. Public Switched Telephone Network (PSTN) is the most familiar network for conversational, circuit-switched, Time Division Multiplexed (TDM) voice communications. The primary voice codec used for PSTN is a 6