Voice Packetization

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.

Table 1. Packetization for different Codecs
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

G.722—64 Kbps

10, 20, 30

Configuring Transrating for a Codec

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. dial-peer voice number voip
  4. codec codec-name bytes voice-payload-size [fixed-bytes]
  5. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device> configure terminal

Enters global configuration mode.

Step 3

dial-peer voice number voip

Example:

Device(config)# dial-peer voice 1 voip

Enters dial peer configuration mode for the specified VoIP dial peer.

Step 4

codec codec-name bytes voice-payload-size [fixed-bytes]

Example:

Device(config-dial-peer)# codec g729r8 bytes 30 fixed-byte

Configures a different packetizations for a voice codec.

Step 5

end

Example:

Device(config-dial-peer)# end

Exits to privileged EXEC mode.