After the voice wavelength is digitized, the DSP collects the digitized data for an amount of time until there is enough data to fill the payload of a single packet.
With G.711, either 20 ms or 30 ms worth of voice is transmitted in a single packet. 20 ms worth of voice corresponds to 160 samples per packet. With 20 ms worth of voice per packet, 50 packets are created per second: 1 sec / 20 ms = 50.
The packetization rate has a direct effect on the total amount of bandwidth needed. More packets require more headers, and each header adds 40 bytes to the packet. The Table 2 table shows the effect of packetization rates on bandwidth utilization.
Codecs such as G.729 also compress the digitized output. G.729 creates a codeword for every 10 ms of voice. This “codeword” is a predefined representation of a 10-ms sample of human voice. Two codewords are contained in each packet at 50 packets per second or three codewords at 33.3 packets per second. Because the codewords need fewer bits, the overall bandwidth required is reduced.
|Supported Codecs||Packetization (ms)|
|G.711 a-law 64 Kbps||10, 20, 30|
|G.711 law 64 Kbps||10, 20, 30|
|G.723 5.3/6/3 Kbps||30, 60|
G.729, G.729A, G.729B, G.729AB 8 Kbps
|10, 20, 30, 40, 50, 60|
|10, 20, 30|