Providing high QoS
When you install VoIP, you will be aware right away of the quality of the service isn't up to snuff. This tip discusses some basic considerations in providing high quality of service for your VoIP users.
For more information on quality of service, take a look at
Networking Quality of Service and Windows Operating Systems, by Yoram Bernet.
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When you're using VoIP, you have to have a very high quality of service, if for no other reason than your users will become highly irritated if the quality lags.
This leads inevitable to a discussion of the bandwidth required for high quality of service. To determine that, you have to understand how many users will be making calls over your IP network at a given time, and then determine how much bandwidth each of these calls needs.
A Microsoft white paper, available on InformIT, says that each user requires a guarantee for the network to provide 64 Kbps, with max end-to-end latency of 100 msec or less. Using this basic requirement, you can determine how many users you can have active on a VoiP setup at any given time, once you understand the basic bandwidth of your network.
But is it likely that you'll have that many users on at one time? You have to find that out either through measurement, or, when all else fails, solid estimation. It's isn't likely that the max number of users will be operating in a VoIP mode all at the same time, so you can go for something less and still get high QoS.
The white paper suggests that some small number of users (10 out of 1000) might be the average. But it you design a network for that number, and then more users place calls, you just blew the QoS requirement you had, because none of the users will then have appropriate quality, and all will be unhappy.
The result? You have to overprovision significantly to provide high QoS. What significant means, of course, is subject to your own situation, your finances, and more.
David Gabel is the Executive Technology Editor at TechTarget.
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This was first published in September 2001