November 5, 2004

One downside to VoIP

We've been trying to track down a performance problem on our web server lately. When we test the server using a crossover cable, our server can serve up about 400rps. (Our old server could only serve up 300rps, so that's a pretty sizable increase.) We can serve up more than that, but our testing machine gets CPU limited at that point.

However, the moment that the server touches the network, we're lucky to get 50rps. So, what is causing this massive drop in performance? Cisco QOS.

We recently went to a VoIP phone system. It's a lot cheaper to maintain than a POTS or PBX system, and it makes it easier for our public safety guys to record all of the lines that they are legally required to, as well as to retrieve those records.

Cisco's QOS has seven levels for the QOS, with the higher the level, the more important the packet. Voice packets are higher priority than data packets, so anything VoIP-related takes higher precedence than data requests.

For us, we made the decision to share the same network for VoIP and data. Due to the QOS stuff, the more phone traffic we have, the slower our system goes.

Our calls outside of the offices use up about 32Kbps, but calls inside the offices use 160Kbps. Each phone also uses a chunk of bandwidth every 30 seconds that there is no call. If you are planning on migrating to VoIP and want to share the same fiber, do some load testing on your network and ensure that any necessary business services will not be affected by the addition of several high-load devices.

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