| The flexibility, self-configurability and easy deployment of mobile ad hoc networks (MANET) are making these networks indispensable component in future mobile and wireless network architectures. However, most of the information is still stored on wired servers nowadays. And additionally, with the advent of future wireless systems consisting of an integration of different heterogeneous wireless technologies, the interconnection of MANETs to global IP networks like Internet is one of the areas which are becoming of paramount importance. In such scenarios, commonly known as hybrid ad hoc networks or wireless multihop access networks, mobile nodes are witnessed as an easily deployable extension to the existing infrastructure.There is being an on-going effort in the research community to efficiently interconnect MANET to Internet. However, many differences between such two heterogeneous networks make their interconnectivity is not trivial. Many key technology challenges for the internetworking, such as address autoconfiguration,gateway designing and service discovery, gateway forwarding strategy, QoS support, secure guarantee and accounting must be faced with. This paper focuses on some key issues about the hybrid ad hoc networks, including the performance evaluation of existing Internet gateway discovery algorithms and gateway forwarding strategies, internetworking scheme based on multipath, unidirectional links removing and broadcasting storm relieving during the global route computation. Several solutions have been proposed to deal with these issues and theory analysis as well as simulation results have confirmed their effectivities. These research conclusions will promote the fast deployment of MANET and increase the importance of MANET in the commercial areas.This dissertation makes the following specific contributions:(1) A simulation-based study of the current Internet gateway discovery protocols has been conducted. Besides the ordinary performance metrics, this study also has evaluated the global internetworking ability of AODV and OLSR, performance of gateway handover and scalability. To settle the instability of adaptive gateway discovery protocol, an enhanced mechanism which learns the Internet connectivity demand of MANET via Gateway Solicitation packets as well as Internet traffic has been presented. On the other hand, a prefix cache can be added into each ad hoc node to solve the path selection problem when global routing is organized hierarchically. These optimized mechanisms motioned above can improve the Internet connectivity performance greatly.(2) The properties of two proposed gateway forwarding strategies: traditional default routes and tunnels has been compared in an environment where there is no hierarchical addressing and reactive ad hoc routing protocol like AODV is occupied. Due to Inconsistent default gateway state, Internet host route state absence and cascading effect, the default route forwarding does not operate efficiently in multi-hop environment, and without modifications, it even operate incorrectly with multiple gateways. As a countermeasure, an improved default route forwarding strategy in which every RREP-I contains the Internet host reachability information is proposed. Simulation results show that this new default route forwarding strategy can effectively solve above problems of the traditional one. On the other hand, the principle and implementation of tunneling forwarding strategy has been studied, too. Performance comparison tunnel forwarding with default route forwarding, under different traffic type and varying node speeds, shows that the former is architecturally appealing with many properties like protocol transparency, ability to support multiple gateways and mutli-sessions. And these properties make it a suitable forwarding strategy in the scenarios where MANET and Internet two heterogeneous networks are integrated.(3) An on-demand multipath internetworking protocol called AOMDV-I that extends the AOMDV protocol to compute multiple paths between MANET nodes and gateways has been developed. Even alternate paths computed by AOMDV-I are leading to different gateways, their loop freedom and disjointness can be guaranteed and proved. If multiple paths are used simultaneously to route Internet traffic, the aggregate bandwidth of the paths may satisfy the bandwidth and QoS requirement of the application. On the other hand, if MANET node uses primary (or optimal) path and until this path either fails or becomes over-utilized, it tries alternate paths to continue the Internet connectivity, additional gateway discovery operations can be avoided. Performance comparison of AOMDV-I with its single path counterparts using NS2 simulations shows that AOMDV-I is able to effectively cope with global route failures in hybrid mobile ad hoc networks. In particular, it achieves a remarkable improvement in the end-to-end delay and decreases the packet loss and control overhead due to its reducing the frequency of gateway discovery operations.(4) An improved gateway discovery scheme has been proposed to enhance the Internet connectivity which is deteriorated due to unidirectional links and blindly rebroadcasting of gateway discovery packets for MANET. Through exchanging AODV HELLO packet which includes additional fields named symmetric neighbor list and asymmetric neighbor list, unidirectional links are removed from global route computation and broadcast storm also can be relieved simultaneously. And cooperated with the adaptive coverage of gateway discovery packets, the reliability of the broadcasting route computation upon which the Internet gateway discovering is based can be guaranteed and the profitless broadcasting is inhibited. Theoretical analysis and simulation results using NS2 show that this improved scheme provide better Internet connectivity for MANET than others due to its enhanced mechanism.(5) An effective Internet connectivity algorithm for MANET is proposed which directly uses HELLO messages to discovery available Internet gateways and to remove asymmetric links from global route computation simultaneously. Because no extra gateway discovery packets are introduced and the usage of half tunneling forwarding strategy, this new scheme can provide better performance than others and the simulation results using NS2 have confirmed its effectivity.
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