Calculate traffic load at Access Point

There is no need to do any special kind of measurement for AP node. There are standard ways to do this kind of measurements through trace analysis.

Learn trace analysis of ns2 trace file.

Read the turorial called “NS by Example” by Jae Chung and Mark Claypool which is available at the following link.

http://nile.wpi.edu/NS/
http://nile.wpi.edu/NS/analysis.html

It will give you brief introduction to what is trace analysis.

You may lean much from similar online tutorials/web pages.