Apple – Support – Downloads – AirPort Utility 5.3.1 for Leopard. Wow, incredibly hard to find, but the Airport Utility let’s you look at individual client performance. Very cool, but impossible to find but it shows you the nominal bit rate of the link I think that is active and changes over time.

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here is the sequence:

Start /Utilities/Airport Utility

Click on the left hand side on the Airport or Time Capsule you want

Click on Manual Setup

Click on the Advanced Button at the upper right

Click on the Logging &SNMP tab

Click on the Logging and Statistics button

Click on Wireless Clients

And I thought Microsoft was obscure!