Research Of Topology Control In Wireless Ad Hoc Networks

Ad hoc wireless networks, or simply ad hoc networks, consist of a collection of geographically distributed nodes that communicate with one other over a wireless medium. Ad hoc networks differ from cellular networks in that there is no fixed infrastructure and the communication capabilities of the network are limited by the battery power of the network nodes. The topology of ad hoc networks,which is determined by positions and transmission ranges of nodes,has a significant effect on the network performance.Topology control deals with how to quickly deploy and maintain the network topology, increase the network capacity,reduce node energy consumption and enhance the network survivability. Furthermore, the integration of operational topology control techniques in the protocol stack is one of the main open research areas in this field.Topology control is the art of coordinating nodes'decisions regarding their transmitting ranges, in order to generate a network with the desired properties (e.g. connectivity) while reducing node energy consumption and/or increasing network capacity. In this thesis, the topology control problem and survey state-of-the-art solutions are proposed to tackle them. Topology control techniques in network simulation tool are implemented, and experimental results illustrate the effectiveness of topology control algorithms. However, several aspects related to topology control are not carefully investigated yet. In this research,the problem of topology control from four aspects are discussed about topology control for ad hoc networks using directional antennas, low-interference topology control, fault-tolerant topology control and implementation of topology control. The main contributions of this thesis are listed as follows:At first, a possible solution to integrate topology control mechanisms into the medium access control protocol is proposed. Through RTS/CTS message exchange, the MAC layer can trigger execution of the topology control protocol in case it detects new neighbors by overhearing the network traffic and analyzing the message headers. The integrated solution establishes a solid research foundation for topology control techniques. Then, OPNET network simulator is added processing module for topology control to the wireless MAC protocol and verif y its feasibility by experiments. Secondly, the topology control for ad hoc networks with directional antennas is investigated, in order to maintain network connectivity while reducing node energy consumption and increasing network capacity.A distributed neighbor-based topology control approach for ad hoc networks with steered beam antennas, referred to as the DK-Neigh, is proposed to maintain the outdegree of every node equal to or slightly below a specific value K. Quadrat statistical methods are employed to derive analytical expressions to determine the critical transmission range and neighbor number K. The approach can build the resulting communication graph with high probability connected. Extensive simulations are carried out, which show that the DK-Neigh is to achieve a high probability (more than 96 percent) of connectivity. If width of beam is smaller than 60 degree, average energy cost of nodes is reduced at least 15 percent than the topology control protocol K-Neigh using omni-directional antennas. Simulations show the DK-Neigh is more energy-efficient while maintaining network connectivity.A localized topology control algorithm for Ad hoc networks with steered beam directional antennas, referred to as the BATC, is proposed in this thesis. The topology is controlled not only by adjusting the transmission powers of nodes but also by changing the directional antenna's direction. The centralized algorithm preserves the connectivity of the resulting topology. At the same time, the resulting network topology reduces energy consumption, decreases traffic interference and improves network throughput while transmission power reduced. Simulation results show that the proposed algorithm significantly improves the network performance.A tight lower bound for the critical neighbor number is deduced that is necessary to obtain an almost surely connected network with randomized beamforming on a bounded area of given size. In this thesis, a topology control protocol, referred to as the RBNTC, is proposed which is fully distributed, asynchronous, and localized. Not only does the constructed topology demonstrate significant reductions in the power required keeping the network connected, but also simulation results show that the proposed algorithm significantly improves the network performance.Thirdly, a new protocol interference model is presented as a measure to describe the interference of the entire network in this thesis. Furthermore, a distributed interference-avoidance topology control approximation algorithm, referred to as the ISPT, is proposed. The algorithm minimizes the interference in the network according to our metrics while preserving the connectivity of the resulting topology. The simulation results show that ISPT decreases interference and improves network capacity in terms of throughput.Finally, An arguement is proposed that survivability of topologies is not equivalence to connectivity of the multiply connected graph by illustrating some practical examples, then the concepts of network survivability into the topology of Ad hoc networks to obtain topology robustness and transmission capacity is introduced. Furthermore analytical expressions to determine the critical neighbor number for K-vertex connected topology are derived. A distributed neighbor-based topology control protocol, referred to as the k2TC, is proposed to maintain the degree of every node equal to or slightly below a specific value k when nodes fail, in a timely manner. The topology constructed under k2TC is high probability connected and bidirectional, which can be implemented at a reasonable cost. Simulation studies show that the resulting topology has good network survivability in terms of invulnerability and availability.