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VoIP Hacks by Theodore Wallingford

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Hack #74. Audit a Network's QoS Capabilities

Networks without Quality of Service (QoS) measures aren't always suitable to carry voice traffic. So how do you know whether a network path supports QoS?

Using pathping and traceroute during peak traffic periods, you'll be able to establish whether a particular IP route is a good place for time-sensitive traffic like VoIP media streams. You'll know the jitter and latency qualities of the network, you'll have identified problem routers and potential traffic bottlenecks, you'll know whether each router supports Resource Reservation Protocol (RSVP, a QoS standard that allows network bandwidth to be reserved for each call), and you'll know how well the network supports 802.1p precedence tagging. (I explain what 802.1p is in this hack; keep going!)

Though Linux is better equipped to provide VoIP services and to serve as a base for troubleshooting, Windows does have a nifty command-line tool that you can use to determine if IP routing supports basic class-of-service measures. pathping, which ships with Windows 2000 and Windows XP, lets you see how well your Internet provider—or your corporate network—supports 802.1p and RSVP. This makes pathping a particularly useful Windows-only VoIP networking tool.

On non-Windows boxes, though, you still have traceroute, of course. While not implicitly a QoS measurement tool, traceroute can gather useful performance data from the VoIP network.

Using pathping

pathping is similar to traceroute. It first determines ...

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