说明：  Three wireless nodes are created and they are configured with specific parameters of a mobile wireless node. After creating the nam file and trace file, we set up topography object. set node_ ($i) [$ns node] is used to create the nodes. Location of the nodes is fixed by specifying X, Y coordinates. Z coordinate is always zero. Here we set the initial size for the every node by using initial_node_pos. AODV routing protocol is used here. $val(stop) specifies the end time of the simulation.