H323 Audio and Video Codecs

Codecs define the format of audio and video information and represent the way audio and video are compressed and transmitted over the network.

H.323 provides a variety of options for audio and video coding. Two codecs, G.711 for audio and H.261 for video, are required by the H.323 specification.

H.323 terminals must be able to send and receive A-law and μ-law coding algorithms (also known as G.711), as determined by the International Telecommunications Union, Telecommunication Standardization Sector (ITU-T).

Additional audio and video codecs provide a variety of standard bit rate, delay, and quality options that are suitable for a range of network selections.

Using H.323, products can negotiate nonstandard audio and video codecs.

Here follows a short description of the required audio and video codecs (G.711 and H.261), as well as the two popular codecs (G.723 and H.263), which offer the low-bit rate connections necessary for audio and video transmission over the Internet.

• G.711 This codec transmits audio at 48, 56, and 64 kilobits per second (Kbps). This high-bit-rate codec is appropriate for audio over higher speed connections.

• G.723 This codec specifies the format and algorithm used to send and receive voice communications over the network. G.723 is a high-speed codec that transmits audio at 5.3 and 6.3 Kbps, which reduces bandwidth usage.

• H.261 This codec transmits video images at 64 Kbps (VHS quality). This high bit-rate codec is appropriate for video over higher speed connections.

• H.263 This codec specifies the format and algorithm used to send and receive video images over the network. This codec supports common interchange format (CIF), quarter common interchange format (QCIF), and sub-quarter common interchange format (SQCIF) picture formats and is superior for Internet transmission over low-bit-rate connections, such as a 28.8 Kbps modem.