Effect of Channel Asymmetry on Reputation Based Cooperation Mechanisms in Mobile Ad-Hoc Networks

Enforced cooperation among MANET nodes is an active research issue. One proposed class of protocols to enforce cooperation is based on reputation systems. In Reputation systems, nodes maintain reputation of other nodes. A node after sending a packet to its neighbour to forward further, increases reputation of neighbour if it overhears the same packet from the neighbour. If it does not overhear the packet, reputation of neighbour is reduced. This method of updating reputation requires that wireless channel must be symmetric. Even when two nodes are in range of each other, channel may become asymmetric due to fast fading effects. We have examined the probability of channel asymmetry as a function of ratio r (of inter node distance and transmission range), for given values of relative average velocity of nodes V and Ricean parameter K. Ricean parameter K represents type of the environment i.e. obstructed or unobstructed. An enhancement of existing reputation system ROBUST is also proposed. In the enhanced protocol, observing node probabilistically updates reputation of neighbour when it does not overhear. This probabilistic update takes into account probability of channel asymmetry. Proposed scheme results in reduction of false positives, and even false negatives remain in control. As a result, packet delivery ratio is increased.