Just required this detail for a customer so thought that you may all find it as useful as I (taken from MS detail).

Some information on call details:

When planning for media bandwidth usage per scenario, use the following tables, which describe the average amount of bandwidth used per media type.

Audio/Video Capacity Planning, Peer-to-Peer Sessions

Audio/Video Capacity Planning, Conferences

Audio Capacity Planning, PSTN

The network bandwidth numbers in these tables represent one-way traffic only and take silence suppression into account.

When you calculate the actual bandwidth usage for a certain scenario, it is important to understand the actual media flows, which are as follows:

In a two-party scenario:

• Users send audio streams only while they speak.
• Both participants receive audio streams.
• If video is used, both users send and receive video streams during the entire call.

In a Conferencing scenario (that is, a call with more than two participants):

• Users send audio streams only while they speak.
• All participants receive audio streams.
• If video is used, only two participants upload a video stream at a time (that is, the active speaker and the previous active speaker)
• If video is used, all participants receive video streams.

NeilC