SIP Codecs - SIP

What is a codec and what is it used for ?

Coder-decore in short called as a codec is used for two simple operations –

  • For easy transmission, a codec converts an analog voice signal to an equivalent digital form.
  • It gets the original analog form from this compressed digital signal so that it can be replayed.

There are free and licensing codecs available in the market. Each of them differ in the sound quality and bandwidth.

Hardware devices (like phones and gateways) negotiate the codec they will use because they support many different codecs.

What is G.711 ?

G.711 was introduced by ITU in 1972 as a codec for digital telephony use. It has two variants : A-Law and uLaw. A-Law is used in Europe and international telephone links whereas uLaw is used in Japan and USA.

  • G.711 compresses each 16-bit sample (2 bytes) to 8 bits (1 byte) using logarithmic compression resulting in a 1:2 compression ratio.
  • One call consumes 128 Kbits/s as the birate is 64 Kbits/s in one direction.
  • G.711 provides the best voice quality as it uses the same codec which is used by the PSTN network, but uses more bandwidth compared to other codecs.
  • Local Area Networks have lot of available bandwidth and hence G.711 works best in LANS.

What is G.729 ?

A codec with a low bandwidth requirement which provides good audio quality is G.729.

  • The audio is encoded by the codec in the frames of 10 ms long, containing 80 audio samples.
  • Each frame is encoded with the codec algorithm into 10 bytes, such that the resulting bitrate is 8 kbit/s in one direction.
  • The hardware that implements the licensed codec need to be bought by the end-users buying the licensed codec.
  • A frequently used variant of G.729 is G.729a. It is wire-compatible with the original codec but has lower CPU requirements.

What is G.723.1?

The results of the competition that was announced to design a codec which allows the calls over 28.8 and 33 kbit/s modem links is G.723.1.

  • There are to variants of G.723.1 and both operate on the audio frames but both differ in their algorithms.
  • The bitrate for the first vibrant is 6.4 kbit/s, while for the second variant, it is 5.3 kbit/s.
  • The encoded frames for the two variants are 24 and 20 bytes long, respectively.

What is GSM 06.10?

The codec that is designed for the GSM mobile networks is GSM 06.10, which is also known as GSM Full Rate.

  • GSM coded variant is freely used and is often found as a open source VoIP application.
  • This codec operates on audio frames 20 ms long (i.e. 160 samples) and it compresses each frame to 33 bytes, so the resulting bitrate is 13 kbit/.

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