A codec (Coder/Decoder) converts analog signals to a digital bitstream, and another identical codec at the far end of the communication converts the digital bitstream back into an analog signal.

In the VoIP world, codecs are used to encode voice for transmission across IP networks.

Codecs for VoIP use are also referred to as vocoders, for "voice encoders".

Codecs generally provide a compression capability to save network bandwidth. Some codecs also support silence suppression, where silence is not encoded or transmitted.

Codec Algorithm Bit Rate (Kbps) Comments
ITU G.711 PCM (Pulse Code Modulation) 64 G.711 with mu-law used in North America and Japan, while G.711 with A-law used in the rest of the world.
ITU G.722 SBADPCM (Sub-Band Adaptive Differential Pulse Code Modulation) 48, 56 and 64  
ITU G.723 Multi-rate Coder 5.3 and 6.4  
ITU G.726 ADPCM (Adaptive Differential Pulse Code Modulation) 16, 24, 32, and 40  
ITU G.727 Variable-Rate ADPCM 16-40  
ITU G.728 LD-CELP (Low-Delay Code Excited Linear Prediction) 16  
ITU G.729 CS-ACELP (Conjugate Structure Algebraic-Code Excited Linear Prediction) 8  
ILBC Internet Low Bitrate Codec 13.33 and 15.20  
Speex CELP (Code Excited Linear Prediction) 2.15-44.2 Part of the GNU Project and available under the Xiph.org variant of the BSD license
GSM – Full Rate RPE-LTP (Regular Pulse Excitation Long-Term Prediction) 13  
GSM – Enhanced Full Rate ACELP (Algebraic Code Excited Linear Prediction) 12.2  
GSM – Half Rate CELP-VSELP (Code Excited Linear Prediction – Vector Sum Excited Linear Prediction) 11.4  
DoD FS-1016 CELP (Code Excited Linear Prediction) 4.8