Content Dissemination in Mobile Ad Hoc Networks

Abstract

In this thesis, we are concerned with content dissemination in mobile ad hoc networks. The scope of content dissemination is limited by network capacity, and sometimes the price to be paid for securing faster delivery. In the first part of the thesis, we address the issue of finding the maximum throughput that a mobile ad-hoc network can support. We have assumed that there is no price involved, and all nodes work as a team. The problem of determining the capacity region has long been known to be NP-hard even for stationary nodes. Mobility introduces an additional dimension of complexity because nodes now also have to decide when they should initiate route discovery. Since route discovery involves communication and computation overhead, it should not be invoked very often. On the other hand, mobility implies that routes are bound to become stale, resulting in sub-optimal performance if routes are not updated. We attempt to gain some understanding of these effects by considering a simple one-dimensional network model. The simplicity of our model allows us to use stochastic dynamic programming (SDP) to find the maximum possible network throughput with ideal routing and medium access control (MAC) scheduling. Using the optimal value as a benchmark, we also propose and evaluate the performance of a simple threshold-based heuristic. Unlike the optimal policy which requires considerable state information, the proposed heuristic is simple to implement and is not overly sensitive to the threshold value. We find empirical conditions for our heuristic to be near-optimal. Also, network scenarios when our heuristic does not perform very well are analyzed. We provide extensive numerical analysis and simulation results for different parameter settings of our model. Interestingly, we observe that in low density network the average throughput can first decrease with mobility, and then increase. This motivates us to study a mobile ad-hoc network when it is sparse and in a generalized environment, such as when movement of nodes is in a two-dimension plane. Due to sparseness, there are frequent disruptions in the connections and there may not be any end-to-end connection for delivery. The mobility of nodes may be used for carrying the forwarded message to the destination. This network is also known as a delay tolerant network. In the rest part of the thesis, we consider the relay nodes to be members of a group that charges a price for assisting in message transportation. First, we solve the problem of how to select first relay node when only one relay node can be chosen from a given number of groups. Next, we solve two problems, namely price-constrained delay minimization, and delay-constrained price optimization.