Quick Bandwidth Calculation
Total packet size Total bandwidth requirement
Payload size Nominal bandwidth requirement
Total packet size = All headers + payload
Payload size Nominal bandwidth requirement
Total packet size = All headers + payload
|
Parameter |
Value |
|
Layer 2 header |
6 to 18 bytes |
|
IP + UDP + RTP headers |
40 bytes |
|
Payload size (20-ms sample interval) |
20 bytes for G.729, 160 bytes for G.711 |
|
Nominal bandwidth |
8 kbps for G.729, 64 kbps for G.711 |
Example: G.729 with Frame Relay:
Total packet size * nominal bandwidth requirement
Example: G.729 with Frame Relay:
Total packet size * nominal bandwidth requirement
Total bandwidth requirement payload size (6 + 40 + 20 bytes) * 8 kbps --= 26.4 kbps
20 bytes
A quick way to calculate total bandwidth requirement for a voice packet is to remember that the ratio of the total bandwidth requirement to the nominal bandwidth for the payload is the same as the ratio of total packet size to payload size.
The payload size is dependent on the sample interval and the codec used and is usually 20 bytes for G.729 and 160 bytes for G.711, assuming a 20-ms sample interval.
The headers are always 40 bytes for the IP, UDP, and RTP headers, plus the Layer 2 header size. This size might be 6 bytes for Frame Relay or PPP or 18 bytes for Ethernet, for example.
To calculate the total bandwidth, find the total packet size, including all the headers plus payload and divide by the payload size. Multiply the result by the nominal bandwidth for the codec, and the result will be the total bandwidth requirement.
Note At http://tools.cisco.com/Support/VBC/do/CodecCalc1.do, you can try to calculate the bandwidth using the Voice Codec Bandwidth Calculator. Access to the URL requires a valid Cisco.com account.
2-62 Optimizing Converged Cisco Networks (ONT) v1.0 © 2006 Cisco Systems, Inc.
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