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Basic troubleshooting tools for VoIP networks include packet sniffers like Ethereal, log analysis, a softphone like X-Lite, and a VoIP server like Asterisk that allows detailed debugging output in its logfiles
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Ethereal has built-in parsers that allow you to easily view SIP methods and responses, as well as SDP attributes and capabilities negotiation signals
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Ethereal's packet capture filters are configured using a string syntax. This syntax is described in Ethereal's documentation and in O'Reilly's Managing Security with Snort and IDS Tools
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Packet capture often reveals what is going wrong, but you may have to investigate diagnostic output on both ends of the signaling conversation in order to find out why
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X-Lite's diagnostic log records every SIP packet it sends or receives
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Asterisk uses an internal thread identifier that doesn't necessarily correspond to the SIP call ID, but can aid you in tracing an individual call through Asterisk's logs
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Troubleshooting quality-of-service issues is aided more by process of elimination than it is by packet analysis
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Network General Sniffer and WildPackets EtherPeek VX are packet capture tools that offer inspection of RTP traffic if needed
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IPerf can let you simulate traffic loads on a call path in order to determine its true VoIP performance capability