When the going gets tough, the tough sniff packets.
Adding VoIP to any network can be a daunting challenge, but accomplishing the task can seem particularly impossible when problems begin to arise. To help troubleshoot any network problems, being proficient with the use of network-analysis tools can provide some restful nights. Ethereal is a network-analysis tool that allows for the "sniffing" or capturing of data packets on the network. Ethereal has some VoIP-specific features, too. By digging deep into VoIP signaling conversations with Ethereal and assessing SIP traffic problems using a conventional call-flow graph, you'll reveal the source of many problems you're likely to encounter. Because of the dominance of SIP, this exercise will concentrate on using only the features of the Ethereal sniffer that support analyzing this protocol rather than any of the protocols that came before it (MGCP, H.323, etc.). Feel free to learn those other protocols if you like, but know that almost every commercial VoIP provider uses SIP (when I say almost, I mean 95% or more).
You can use Ethereal to inspect network traffic from the Ethernet layer all the way up to the application layer. Packets are captured in a buffer and are displayed on the screen. Filters can be applied to restrict the capture to packets matching a certain source, destination, size, protocol, or service.
To obtain Ethereal, download it from http://ethereal.com/ and install it on your Windows, ...