A decision to consider implementing voice over IP generates several questions that are not easily answered. Two specific questions our company has addressed in a couple of applications are:
How much bandwidth can be saved by using voice over IP, compared to traditional 64 kbps circuit switching?
For this there is a free Excel spreadsheet called "VoIP-Calc," available at: http://www.infotel-systems.com/downloads_registration.htm. A tutorial on how to use this spreadsheet is also available here.
What is the expected mouth-to-ear delay for a given pair of users?
This calculator is available from http://www.infotel-systems.com/Software_files.htm, and is an .exe file that is useable for a one month free trial, then can be purchased for $39 US. This tool includes the bandwidth calculator as well as mouth-to-ear delay. It also includes a transit delay and jitter estimator that can be used in the calculation.
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