Analysis of Optimal Strategies to Minimize Message Delay in Mobile Opportunistic Sensor Networks

Wireless sensor networks (WSNs) are autonomous and self-healing networks of small battery powered sensors. Besides sensing their physical environments, these sensors are capable of communicate wirelessly, store, and compute data locally. The small size and its capability attract academics as well as industry for real-time monitoring of an area for potential events like wild fire, intruders, and hazardous gas. Since multi-hop communications from sources to sink node were unavoidable in WSNs, it is hard to achieve a longer life time.To reduce multi-hop communication, the idea of using mobile nodes as relay nodes which collect data and deliver it to sink, was introduced to WSNs. In addition to life time improvement, the mobile relay nodes can also keep wireless bandwidth capacity to a constant level while the node density is high. The mobile relay nodes moves independently and random from the perspective of WSNs. Mobile Opportunistic sensor Network (MOsN) specifically denotes overlays of the mobile opportunistic network on top of a static wireless sensor network where the time taken for relay nodes to deliver the data from static sensors to sink is completely opportunistic and unbounded. However, many applications related to security, emergency, and bio-hazard cannot tolerate this unbounded message delay. So we begin by analyzing the average time taken for relay nodes to deliver the message to sink in MOsN by modeling the delivery of message as a randomly moving particle with certain biasness towards the sink. The result shows this delay is a function of the message bias level and distance d from the sink to origin of message. The sink as placed in the center of tilted grid with the radius of D and the message bias level α which varies [0, 1], the delay can vary from Ο(d) to Ο(D log d) to Ο(D² log d). Based on this result, we propose an heuristic algorithm which forward the data to other relay nodes if it has the bias level larger than a threshold 1/(2x+1), where x is distance of relay node carrying message to the sink.Finally, we deduced that the lower average message delay can be achieved by a static sensor wait for a relay node with its bias level higher than some threshold for its message delivery. Before we delve into optimal message handover policy, the movement of relay nodes modeled as a directed random walk with biases in their mobility. This bias random mobility model well represents the network of multiple independent but not identically moving relay. The relationship between the message bias level and mobility bias level is derived from a help of the simulation results. The optimal message handover policy based on the observed mobility bias level is proposed for static sensors at different locations when the inter-arrival time of relay nodes to a given static sensor is close to a constant. We also propose an optimal relay node selection algorithm in the case of inter-arriving time of relay nodes are linearly increasing.