Section 11.7. Simulating Media Loads


11.7. Simulating Media Loads

The bandwidth requirements of a VoIP call can be calculated by adding overhead factors such as RTP and IP headers and Ethernet framing to the size of the actual audio frame. That gives you the packet size , which you can multiply by packet rate in order to figure out the bandwidth required for the call (this was expanded upon significantly in Chapter 6). But that's all academic.

A great tool for adding load to a network is IPerf, which you can download from http://dast.nlanr.net/Projects/Iperf. This package can create streams of UDP traffic that use up the amount of bandwidth you specifyperfect for simulating VoIP load. IPerf has a server and client that each set up a socket for sending/receiving a large stream. You can control the port number, protocol, and size of the stream.


In a real-world scenario, you may need to fill, or nearly fill, a call path with traffic in order to test its behavior under heavy loads. By tweaking the amount of load you place on the call path , you can figure out the failure thresholds for it. Use IPerf to simulate a fixed load, and continually increase it, recording the call quality and completion rate each time you do.

Once you hit the performance ceiling, you should be able to state that a particular link will carry no more than x G.711 calls or that a certain call path through the network will carry no more than y G.729A calls. You would almost certainly have to do this kind of simulation during off-hours.

But how can you account for the bandwidth consumption of non-VoIP applications on the network? One way is to measure it during regular business hours, and then add it into your IPerf simulations.

Use your managed switches or MRTG (multi-router traffic grapher ) to record traffic levels during regular hours. Then, later, use an IPerf client/server pair to simulate that traffic during off-hours. Use a second IPerf client/server pair to simulate VoIP traffic, and continually raise the traffic level, until your test call breaks down or fails.



Switching to VoIP
Switching to VoIP
ISBN: 0596008686
EAN: 2147483647
Year: 2005
Pages: 172

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