IP Telephony Codecs Defined and Explained

What is a Codec?

A Codec (coder-decoder) is a a piece of software that is used to compress or convert an analogue voice signal into a digitally encoded version that is ready for transmission. Once the data has been received, the receiving codec will convert the digitally encoded version of the voice signal, back into an analogue signal. Codecs, and the associated call quality can vary significantly following compression and transmission, which offers the organisations using them the flexibility to choose a codec that allows them to optimise the bandwidth and computational power that they have available.

It is of critical importance to ensure that the receiver of the compressed data is using, or has access to the same codec as the sender of the data to avoid the codecs mismatching. The outcome of a codec mismatch would be that both ends of the transmission wouldn’t be able to negotiate the same codec and the call will not complete. An explanation, and description of some of the most common IP telephony codecs can be found below:

ITU G.711

The ITU (International Telecommunications Union) G.711 codec comes in two varieties, namely, U-law and A-law. U-law is predominantly used in North America and Japan, whilst A-law is predominantly used in the rest of the world.

Using this codec will provide the best voice quality as no compression takes place during the transmission of the voice data – the codec only converts the analogue signal to a digital signal. The main downside to using this codec is that it takes up a lot of bandwidth due to the lack of compression. G.711 is supported by the vast majority of VoIP providers.

ITU G.729

The ITU G.729 codec offers good call quality at low bit rate of 8Kbps (Kilobits Per Second) which means that you are able to get much more out of your bandwidth when compared with other codecs such as G.711. It is worth noting however, that the benefits that you see in bandwidth usage, are negated by the amount of CPU processing required to compress the audio. As a result of this, you will see many VoIP phones that are only able to deal with one call at a time due to the processing demands of G.729. In order to use this codec, a licence must be obtained from a company that resells licences.


GSM is an acronym of Global System for Mobile Communications, which is the cellular system that has become standard outside of the USA.

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