고속 이동성을 갖는 애드 혹 무선망에서 실시간 응용 서비스 제공을 위한 선택적 포워딩 기법에 관한 연구

Abstract

Ad hoc wireless networks were recently considered to be the most suitable wireless communication system in order to support various applications in mobility environments. Routing in mobility environments is challenging since the network comprises mobile nodes with different mobility characteristics (such as movement direction, velocity and residual battery capacity) and constrained range in transmission. In ad hoc wireless networks, a flooding-based packet forwarding is usually preferred in order to set up the source-destination route and deliver the packets to the destination with high reliability. Most ad hoc routing protocols using flooding-based packet forwarding in route discovery procedure generally expect the shortest route with minimum hop count from the source to destination. These routing protocols usually chooses the neighbor nodes farthest away from the sender, i.e., located at transmission range boundary, as intermediate relaying nodes along a route. However, due to unrestricted mobility of mobile nodes, this leads to frequent route discovery procedures since the probability that adjacent relaying nodes are connected is rapidly weakened as times goes by. This results in decreasing reliability over the established route and increasing packet delivery latency. Consequently, the performance of ad hoc routing protocol is considerably affected by link (or route) duration between adjacent intermediate relaying nodes. Meanwhile, vehicle-based applications were recently introduced to improve traffic safety and efficiency. These applications are classified into two types: safety-oriented and non-safety-oriented applications. In this classification, safety-oriented applications are usually provided by means of vehicle-to-vehicle (V2V) communications in order to support reliable and fast alert message propagation to all surrounding vehicles when an emergency occurs on the road. In vehicular ad hoc networks (VANETs), broadcast-based packet forwarding is usually preferred in order to propagate urgent traffic-related information to all reachable nodes within a certain dangerous region. This approach, however, may cause a serious contention in transmission between adjacent nodes, collisions at the medium access control (MAC) layer and redundant rebroadcast packets, which can lead to high packet loss rate and high packet delivery latency. This dissertation proposes two different selective forwarding schemes that aims to i) alleviate network overhead problems, and ii) guarantee reliable and fast packet delivery to the destination, especially in supporting real-time applications over mobile ad hoc networks (MANETs) and safety-oriented applications over vehicular ad hoc networks (VANETs). In MANETs, the Relay Region based selective forwarding is proposed in order to establish an optimal source-destination route being able to support specific application requirements. The proposed Relay Region is defined as a limited distance progress in transmission to next relaying node in route discovery procedure, and introduced to maintain the established route for an arbitrary length of time (i.e., route duration), while meeting packet delivery reliability and delay constraints. In VANETs, selective forwarding based alert message propagation scheme is proposed in order to deliver real-time alerts to all reachable vehicles in potentially dangerous situations, while optimizing network resource utilization. The proposed scheme ensures that all vehicles maintain forwarder information before detecting an emergency event on the road. When an emergency event is detected, the vehicle prepares an alert message that contains information for multiple forwarders including the selected forwarder's identifier, message holding time, and hop count. The forwarder with the smallest message holding time has the highest priority for retransmitting the received alert message. This strategy can effectively solve the network overhead problems caused by flooding-based packet forwarding. The experiments demonstrated that the proposed schemes can significantly improve the performance of packet delivery reliability and latency, alleviating network overhead problem, especially in ad hoc wireless networks with high mobility. The proposed schemes can be utilized to various applications including mobile wireless systems for real-time multimedia, and vehicle safety systems to enhance driver safety.