Portmanteau of 'Coder/Decoder'; In VoIP the codec is an audio format that encodes the digital representation of the audio on that call, whether that is the ringing tone, busy tone, or the actual conversation. There are many varieties of codecs which describe the method of encoding and in most cases compressing the audio stream. Many codecs operate at multiple sampling rates (8kHz, 16kHz, 22kHz, etc), therefore the sampling rate of the audio stream is generally specified in the VoIP set up messages (ie, a SIP INVITE). The most commonly used codec is G.711 (ulaw/alaw), however other popular codecs include G.722 and G.729. Various codecs have advantages and disadvantages over others and the choice of which codecs are offered differ depending on the use case.

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