Position-Based Packet Forwarding for Vehicular Ad-Hoc Networks
- Publisher: Universität Mannheim
Mobile Ad-Hoc Networks, or MANETs, are data communication networks between (potentially) mobile computer systems equipped with wireless communication devices and — in their purest form — in complete absence of communication infrastructure. Usage scenarios for these systems include communication during disaster recovery or battlefield communications. One of the great research challenges concerning MANETs is the Packet Forwarding Problem, i.e., the question to which neighbor node a data packet should be handed over to reach non-neighboring nodes. While this problem has been previously solved by the adaption of classic routing algorithms from wired networks, the availability of GPS enables to include information about the geographic position of nodes into the routing decision, by selecting forwarders that are geographically closest to the destination. While these algorithms have been shown to improve communication performance in networks with a high degree of node mobility, they require (a) a beaconing service that allows every node to build a table of its neighbors and (b) a so-called Location Service that allows to acquire the current position of non-neighboring nodes in the network. In this thesis, we propose Contention-Based Forwarding (or CBF), a greedy routing heuristic that is no longer in need of a beaconing service. Moreover, a forwarding node running CBF does not at all select the next forwarder explicitly but broadcasts the packet containing its own position and the position of the destination. The selection of the forwarding is now done in a contention period, where every possible forwarder, i.e., every receiver of the packet, considers its own suitability to forward by calculating the geographical progress for the packet if forwarded by itself. Then it waits for a time reciprocal to this suitability before simply retransmitting. If the retransmission of a packet is overheard, the own postponed retransmission process is canceled. In this thesis, we demonstrate that CBF outperforms beacon and position-based routing by delivering packets with constant overhead, almost ignorant of mobility. Also, we introduce two strategies to cope with the problem of packet duplication. A problem left open by greedy routing heuristics is routing in the presence of local optima, or voids. Voids are node placement situations, where — in spite of an existing route — no neighboring node is geographically closer to the destination than the current forwarder. In these situations, greedy forwarding fails and standard graph-based recovery well known from classical Position-Based Forwarding cannot be applied due to the lack of the beacon-based construction of neighbor tables. As a solution, we propagate Contention-Based Distance Vector Routing, a contention-based adaption of AODV that acquires topology information in the area of the void and does contention on the topological distance to the forwarder. Besides the forwarding algorithms, we extend position-based routing by two location services. The first, the Reactive Location Service or RLS is simple, purely on-demand and very robust to mobility, the second Hierarchical Location Service, is more complex but outperforms RLS in scalability. The second big column in this thesis is ad-hoc multi-hop communication in the context of Vehicular Ad-Hoc Networks , or VANET, i.e., networks where the communication system is carried by vehicles. These systems very elegantly fit into the propositions and requirements for our more general routing approaches since they have (a) easy access to position information an (b) "suffer" from high mobility. For VANETs, we separate the routing problem into highway and city scenarios and study various routing algorithms in both. In the end, we advocate the usage of position-based routing in both scenarios; moreover, the contention-based approaches are most promising. While a lot of ad-hoc research has been deemed to be theoretical, we have also built a multi-car communication system. For this system, we provided the network and system architecture and provided the communication software. In this thesis, we will describe these efforts as a proof-of-concept and provide measurement results.