Please see the SearchNetworking.com Webcast on network performance. You want to plan as if your network is always at its peak when you add real-time or voice applications. That said, technology has improved and you always hear the argument to plan on averages, but this will prove insufficient over time. We are also adding additional services to these systems which will increase the network burden.
Compression will always be around. But a good rule of thumb is to look at the number of calls you process at the same time now, and multiply that by the throughput needed for your compression rate (typically about 64k per call in the simultaneous range, but that will vary by manufacturer). Also, if your network does not understand QoS or DiffServe, it gets a bit more complicated.
VoIP calls use erlangs to determine bandwidth. Erlangs are a function of simultaneous calls, number of users, etc. I hope this helps. If you can send me some more specifics on manufacturer and number of users, I will be happy to help further.
This was first published in November 2005