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VoIP Codecs

By: Amy Wolters

VoIP phones use codecs to convert analog signals into digital signals. Essentially, codecs encode, and decode speech. There are several different codecs VoIP phones can use, in which they vary in sound quality and the amount of bandwidth they use.

The kind of codec a VoIP phone can use varies among phones. In some cases, people will have some of their phones using different codecs than others. Where you might see a couple of different codecs being used is if a company has some phones that support “wideband” audio (also known as “HD voice”) and others phones that do not. A codec used to achieve wideband audio is G.722, which is not a codec all VoIP phones support. To find out what codecs your phones support, check the technical specifications.

The following is a diagram that that lists the different codecs that may help you choose the best one for your phones.

Codec Algorithm Frame Size/Lookahead Usual Rate Comments
G.711 PCM 0.125 ms/0 64 Kb/s Universal use
G.722 0.125 ms/1.5 ms 48, 56 or 64 Kb/s Wideband coder
G.726 ADPCM 0.125 ms/0 32 Kb/s High quality, low complexity
G.728 LD-CELP 0.625 ms/0 16 Kb/s High quality in tandem; Recommended for cable
G.729(A) CS-ACELP 10 ms/5 ms 8 Kb/s Widespread Use
G.729e Hybrid CELP 10 ms/5 ms 11.8 Kb/s High quality/complexity; Recommended for cable
G.723.1(6.3) MPC-MLQ 30 ms/7.5 ms 6.3 Kb/s Video conferencing origin
G.723.1(5.3) ACELP 30 ms/7.5 ms 5.3 Kb/s Video conferencing origin
IS-127 RCELP 20 ms/5 ms Var. 4.2 Kb/s avg.
AMR ACELP 20 ms Var. 4.75-12.2 Kb Compatible w. No. Amer. & Japanese digital cellular, WCDMA (not CDMA2000); Nokia IPR

Source: B. Goode. Voice Over Internet Protocol (VoIP). Proceedings of the IEEE, 90:1495--1517, September 2002.