Codec is an algorithm usually installed as a program or software on a server or embedded in the hardware and is commonly used to encode and decode audio, video, fax and text data. Codec is a combination of 'compressor-decompressor' or 'coder-decoder'.

A VoIP Codec is utilized to convert voice signals into digital data during a VoIP call. Codecs typically accomplish three tasks:

Encoding - Decoding

A normal PSTN call transports voice as analog signals over phonelines. In VoIP, these analog signals are converted into digital signals by a process called encoding using a codec. When the digitized voice reaches its destination, it is decoded back to the original analog state in order for the receiver to hear and understand it.

Compression - Decompression

A codec also compresses digitized voice to make it less bulky and allow more data to be sent at a time, thus improving performance. A compression algorithm is used to compress the digitized voice, which is then sent over the network to be decompressed at the other end before being decoded.

There are two types of compression: lossless and lossy. Lossless compression is a technique that preserves the original quality of the digitized voice during compression. If an original file size is 2.0MB, lossless compression can reduce it to about half that size without losing much of its original quality.

Lossy compression, as the term implies, reduces quality by some amount in order to achieve compression by stripping a file of some of its redundant data. Lossy compression are typically used on graphics, audio, and video.

Encryption - Decryption

Encryption is another use for codecs to secure data sent over the wire. Encryption protects the data in such a way that no one could understand it, thus remaining confidential while in transit. Upon reaching the destination, it is decrypted back to its original state using the same codec.

Below are common codecs for VoIP:

G.711
Bandwidth/kbps: 64
Description: Delivers precise speech transmission. Very low processor requirements. Needs at least 128 kbps for two-way.

G.722
Bandwidth/kbps: 48/56/64
Description: Adapts to varying compressions and bandwidth is conserved.

G.723.1
Bandwidth/kbps: 5.3/6.3
Description: High compression with high quality audio.

G.726
Bandwidth/kbps: 16/24/32/40
Description: An improved version of G.721 and G.723 (different from G.723.1)

G.729
Bandwidth/kbps: 8
Description: Excellent bandwidth utilization. Error tolerant. License required.

GSM
Bandwidth/kbps: 13
Description: High compression ratio. Free and available in many hardware and software platforms. Same encoding used in GSM cellphones.

iLBC
Bandwidth/kbps: 15
Description: Robust to packet loss.

Speex
Bandwidth/kbps: 2.15 / 44
Description: Minimizes bandwidth usage by using variable bit rate.

SOURCES: Wikipedia and About.com