G.721 is an ITU-T standard codec that uses Adaptive Differential Pulse Code Modulation (ADPCM); a form of Pulse Code Modulation (PCM), to produce a digital signal with a lower bit rate than standard PCM. This ITU standard for speech codecs uses ADPCM on a  32 kbit/s channel. NOTE: G.721 was first introduced in 1984. In 1990 this standard was folded into G.726 along with G.723. (See also ADPCM).

See G.7xx for more information on how these standards are used in telephony networks.

Enterprise VoIP Planet
The IT Manager's Guide to Voice over IP

ITU Recommendation G.721
32 kbit/s adaptive differential pulse code modulation (ADPCM)

Six Steps to Getting Your Network Ready for Voice over IP
Using low bandwidth CODECs helps reduce bandwidth requirements however due to the overhead of IP, UDP and RTP protocols, such reduction is limited.

The International Telecommunication Union
The International Telecommunication Union is an international organization within which governments and the private sector coordinate global telecom networks and services. This home page offers numerous links to highlights and current topics, standardization, radiocommunication, telecommunication, and general secretariat sectors.

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