Explanation:
In the equation N (dproc + dtrans + dprop), N is the number of checkpoints/stops that the packet makes as it reaches the destination. The stops are made at each router and the final destination node. Now, since N = number of routers + final node, then number of routers = N – final node. As we know, there is only 1 final node in a path, thus, number of routers = N – 1. Suppose, There is a path A->R1->R2->B for a packet where A is the source node, B is the final node and R1 and R2 are routers. The total delay would be given by N (dproc + dtrans + dprop) where N = 3, since the packet would stop at R1, R2 and B. The number of routers here are 2, and (N – 1) is also 2.