Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

Evaluation of the AODV and DSR Routing Protocols 27The weight of a path P i in G i is denoted by w i (P i ). For additive path metrics,it is the sum of the link weights along the edges of the path. The weight of amobile path includes the individual weights of each path in the sequence, and atransition cost c trans that represents the overhead incurred by the protocol torespond to a topology change. Thus, the weight of a mobile path P in G, denotedby w(P), is defined asw(P) =T∑i=1T∑−1w i (P i )+ c trans (P i ,P i+1 ). (2)Given these definitions and assuming a given cost model, the shortest mobilepath (SMP) problem is defined within the framework as follows.Problem 1. Given a mobile graph G = G 1 ...G T and a specified source-destinationpair (s, t), find a mobile path P = P 1 ...P T from s to t, such that theweightT∑T∑−1w(P) = w i (P i )+ c trans (P i ,P i+1 )i=1of the mobile path is minimum.In [6], a simple 2-valued transition cost function was considered initiallybecause the SMP problem is tractable in this case.i=1i=1MERIT Assessment Measures. Two assessment measures are proposedwithin the MERIT framework: the MERIT ratio and the MERIT spectrum.For a given mobile graph G and s-t pair, let P real be the actual mobilepath generated by the MANET routing protocol R. The weight w(P real ) of thismobile path is computed directly from the routing state trace for the s-t path ineach G i . The paths generated in turn directly fix the transition costs. Similarly,let P ideal be the shortest mobile path for G. Both the path P ideal and its weightw(P ideal ) are computed by the Shortest Mobile Path algorithm [6] run onthis instance G of the mobile graph for ) source-destination pair s-t.The MERIT ratio = Eis the expected value of the cost ratio of(w(Preal )w(P ideal )the actual mobile path to the shortest mobile path. The ratio represents howfar the routes in protocol R deviate in cost from the theoretical optimum. Wecompute the mean of the ratio of a large enough sample size of randomly drawns-t sessions in order to obtain a 95% confidence interval. The distribution of thesample is implicit from the simulation parameters.The value of the MERIT ratio becomes meaningful when we understand howit changes as a function of a some parameter. The MERIT ratio expressed asthe function of some independent parameter defines the MERIT spectrum ofthe protocol R. Some examples of such parameters include the node velocity,the average actual path length, the average node density, transmit power levelsand the related energy consumption, and even the transition cost.