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Wikipedia VoIP description and explanation

VoIP data rates with different speech encoding standards

CCITT Standard G711 G729A G723.1
Packet size 120 10 24
Sample duration ms 15 10 30
Date rate kbit/s 64 8 5.3 or 6.4
Packets per second 66 100 33
Data rate with IP encapsulation kbit/s 87 40 16.5

G711 voice codec: Encodes analogue speech into pulse code modulation (PCM) narrowband speech signals using A-law or μ-law. Decodes digital values into quantised analogue output values using A-law or μ-law.   Reference: ITU-T Recommendation G.711, "Pulse Code Modulation (PCM) of Voice Frequencies," General Aspects of Digital Transmission Systems; Terminal Equipments, International Telecommunication Union (ITU), 1993.

The ITU-T G729 standard is an encoding standard using a device called a "vocoder" to provide toll-quality speech at 8 kbit/s. The algorithm has been designed with consideration for tolerance to bit errors and keeping the digital signal processing requirements down so as to minimise delay times. It uses 10ms frames to keep delays short.  It is intended as an improvement over 32kbit/s ADPCM in terms of bitrate while maintaining toll-quality speech.  The G729A, G729B & G729AB are varients. G729A is slightly less complex. G729AB and G729B incorporate a silence compression scheme in accordance with ITU Recommendation V.70.

Comtech CDM570 modems are able to process VoIP packets and reduce the IP header significantly.  The result is G729a at 10.8 kbit/s, but there is a maximum of 64 simultaneous calls per modem (approx 700 kbit/s carrier and 64 voice calls).

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