I've had cause to do some investigation recently into best practices for planning, deploying, and managing VoIP. While I won't go into all of the details tonight, I did run across an interesting inconsistency.
Most VoIP hardware/software vendors and consulting agencies that I've dealt with recommend a fairly telephony heavy planning measurement period prior to any VoIP deployment. This phase will involve, among other things, pulling reports from your old school telephone on phone usage so that you complete your erlang calculations to estimate the additional bandwidth that the VoIP traffic will require. With this method, bandwidth is provisioned prior to the VoIP deployment and based upon non-optimized network configurations. While this may seem like a method for oversubscribing bandwidth, what usually happens over time is that the amount of traffic on the network increases, which caused MOS to degrade, and then the network is optimized over time with advanced services such as QoS to mitigate the circumstance.
That said, the process that I'm seeing most engineers use is somewhat different. In most cases, rather than invest significantly in planning before rolling out VoIP, I'm seeing a significant planning investment during the VoIP deployment. Many engineers have found that they can be very effective by deploying VoIP on a limited basis, to a pilot user group or WAN site, with little to no planning so long as there is available bandwidth and they're willing to react in real-time to performance issues as they arise. During this initial deployment they experiment with different CODECs, QoS parameters, queueing strategies, and bandwidths until they find the model that is "right" for their environment. At this point, they've come up with what is effectively a best practice for their particular environment and can go forward with planning and deploying VoIP for the rest of the enterprise.
As I was researching this I came across several free, web based erlang calculators. I thought this one was pretty cool:
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