G.711 is an ITU-T Guidelines for digitizing analog audio signals by pulse code modulation (PCM). This codec are the traditional fixed-line telephony and IP telephony.
When G.711 is in time steps of 125 microseconds, which corresponds to a sampling rate of 8000 Hz, respectively, a sample of the audio signal generated. The sample will be doing lossy compressed bit to 8. The generated data stream has a data transmission rate of 8000 Hz x 8 bit = 64 kbit / s The highest frequency of the analog signal may not exceed 4000 Hz according to the Nyquist -Shannon sampling theorem. G.711 only the frequency range of 300 to 3400 Hz encoded in the digitization. For subsequent non-linear coding of the digital signal, two different methods for quantization are used: In Europe, A-law, in North America and Japan, the μ - law method. Because the overhead data transmission rates of 80 kbit / s to 128 kbit / s are required for the transmission of IP G.711 voice channels.
G.711 is one of many codes for IP telephony ( VoIP) and is used in ISDN. The voice quality of a VoIP call with G.711 therefore corresponds to that of ISDN. In the classification of the Mean Opinion Score ( MOS) G.711 reaches a value of 4.4. The MOS determines the subjective perception of voice quality of a user. Thus achieved G.711 higher subjective speech quality than the other codec, G.726 and G.729 like. These have for the advantage of a lower data transmission rate is required by data compression.