Research On Preemptive On-Demand Routing In Ad Hoc Networks

Existing on-demand ad hoc routing protocols do route selection only according to the route hops and route fresh state and something else. The state of links that construct the route is not considered at all. This neglect increases the risk of using a potential unreliable route, which results in frequent route broken. Meanwhile, existing on-demand ad hoc routing protocols take route maintenance actions only after a path breaks, incurring a significant cost in detecting the disconnection and establishing a new route. During the route reconstruction, packets can be delayed or even be dropped, which will cause significant performance degradation of ad hoc routing protocol and affect the QoS of the network. In this thesis, we proposed a preemptive extension to on-demand ad hoc routing protocols. The extension consists of three modules: link state analysis, preemptive route discovery and preemptive route maintenance. The essential part of the preemptive extension is the link state analysis module, which is able to predict the broken time of a link. The prediction is based on the signal history of a link and activated by a prediction algorithm. The route broken time is equal to the minimum broken time of the links that construct the route. During the route discovery, the route selection depends on not only the route hops and the other conventional parameters but also the predicted route broken time. When a path is likely to be broken soon (which is indicated by the predicted link broken time), a route warning is sent to the source indicating the likelihood of the disconnection. The source can then select a new route to use or initiate a new route discovery. So, a new route can be got before the old route actually broken, potentially avoiding the delay-jitter and packets lost because of the disconnection.We have done an extension to AODV protocol, using our algorithm. A lot of simulation experiments have been done under NS2. The result of the simulations demonstrates that the proposed preemptive route discovery and route maintenance mechanism added to AODV significantly reduces the number of path breaks, therefore enhances the packet delivery ratio and decreases the packet delay, with a small increase in protocol overhead. We also show and analyze some experimental results obtained by running TCP on top of the preemptive routing schemes .