G.71l, also known as Pulse Code Modulation (PCM), is a very commonly used waveform codec. G.711 uses a sampling rate of 8,000 samples per second, with the tolerance on that rate 50 parts per million (ppm). Non-uniform quantization with 8 bits is used to represent each sample, resulting in a 64 kbps bit rate. There are two slightly different versions; U-law, which is used primarily in North America, and A-law, which is in use in most other countries outside North America. G.711 mu-law tends to give more resolution to higher range signals while G.711 A-law provides more quantization levels at lower signal levels. When using mu-law G.711 in networks where suppression of the all 0 character signal is required, the character signal corresponding to negative input values between decision values numbers 127 and 128 should be 00000010 and the value at the decoder output is -7519. The corresponding decoder output value number is 125.
Packet Loss Concealment (PLC) algorithms, also known as frame erasure concealment algorithms, can help hide transmission losses in a packetized network. Specifically, in an audio system where the input signal is encoded and packetized at a transmitter first before being sent over a network where it is received at an end point which will decodes the packet and play out the audio. Many of the standard CELP-based speech coders, such as G.723.1, G.728 and G.729, have PLC algorithms built into their standards. G.711 Appendix I outlines such a PLC algorithm for use with G.711.
An important technique for minimizing the network bandwidth necessary for voice transmission utilizes the fact that a significant amount of voice traffic may not contain any speech but only background noise. This can be detected and therefore fewer, smaller packets may be used to enable the receiver to play out the background noise. This technique is called Discontinuous Transmission (DTX), since audio is no longer transmitted in a continuoud stream. The method requires Voice Activity Detection (VAD) and the capability to mimic the background silence or Comfort Noise Generation (CNG).instead of transmitting the background noise exactly, G.711 Appendix II defines a standard method to do this for G.711 in a packet-based multimedia communication system. The use of the payload format is intended for packet-based systems with a large header overhead where the packet transmission rate plays a significant role in the overall system bit-rate.