The first step in a PERT or CPM network is to divide the entire project into significant activities in accordance with the work breakdown structure. There are two approaches for drawing a project network, and one of them is the Active On Arrow (AOA), whose details are demonstrated by the following figure.

Please, clearly describe what dummy activities are and why it's essential to keep them in the project.

Activity-on-Arrow (AOA) A B B B 00 B A Dummy activity B D B B D Dummy activity