IPerf

http://dast.nlanr.net/Projects/Iperf/

Free tool that adds load to a network to simulate live environments.
Runs on both UNIX & Win32

note when using this tool with the --udp option you should use -l and set the packet size based on the codec payload and the control overhead if testing to simulate IAX2
I believe iax2 packet size is 12 bytes + codec payload at gsm = 33 bytes, g723 = 27 bytes

http://dast.nlanr.net/Projects/Iperf/iperfdocs_1.7.0.html

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