Packet Collision is a theoretical concept. In the real world a packet is not a "physical" entity that can collide. Therefore a "collision" is the interference of two radio waves (of the packets being sent). 


In NetSim the collision model is as follows -


At the transmitter, when a packet is being transmitted, if the sum of received power from all other nodes is greater than the receiver sensitivity then that packet is marked as collided


At the receiver, when a packet is being received, if the sum of the received power from all other nodes, except the node from where this packet is being transmitted, is greater than the receiver sensitivity then that packet is marked as collided.