An efficient broadcast relay scheme for MANETs
Introduction
Mobile ad hoc network (MANET) is a kind of self-organizing mobile wireless networks that do not rely on a fixed infrastructure to communicate. Because of the ever-changing topology, MANETs use broadcasting as one of fundamental approaches, especially when packets are transmitted to multiple hosts in an ad hoc network, broadcast is more efficient than unicast or multicast. Furthermore, broadcast may be the only reliable delivery method when mobile hosts move arbitrarily with high speed. There are many routing protocols [4], [5], which rely on broadcast to discover routing information and implement routing recovery. Blind flooding is the traditional broadcast approach. It is straightforward, but not optimal. Blind flooding generates a large number of redundant packets that waste valuable resources such as bandwidth and energy. The large number of redundant broadcast packets caused by flooding has been known as the Broadcast Storm Problem [6]. Current research on optimal broadcasting mechanisms in ad hoc wireless networks has been focusing on minimization of rebroadcast number and efficiency to deliver packets to most hosts in the network. According to the types of information required, these protocols can be grouped into two categories: topology-based and geometry-based.
In topology-based broadcast protocols [2], [3], [10], [11], hosts determine whether the received packet is forwarded according to the connectivity information in their neighborhood. Since an optimal broadcast forwarding in ad hoc network is proven to be NP-complete [1], topology-based protocols attempt to approximate the minimal connected cover set. Each host is assumed to know the local connectivity information up to 2 hops. Based on the topology information, topology-based protocols can generate a small forwarding host set, so that the redundant rebroadcasts can be significantly reduced while the maximum broadcast reachability is maintained. However, the exchange of local topology information under such topology-based protocol may cause a large amount of overhead, since each host must maintain a long list of the neighboring hosts, especially when the host density in the network is high. In addition, hosts in the network need to take long time to collect all connectivity information with other neighboring hosts within 2 hops [1], [2], [3]. During such long convergence time, the existing hosts may leave and new hosts may join the 2-hop neighborhood range, which make the created forwarding host set incorrect. Hence the topology-based broadcast protocols cannot maintain good performance in a highly dynamic environment.
By contrast, geometry-based broadcast protocols choose the forwarding hosts according to the geometry location information of direct neighboring hosts. Each host obtains its location information either through low power low cost Global Positing System (GPS) receivers, or by measuring signal strengths and calculating relative coordinates [16]. The location information is exchanged among direct neighboring hosts via periodical beacons or broadcasting packets. The exchange of location information only consumes a small amount of bandwidth. Compared with topology-based protocols, geometry-based protocols usually have less convergence time to obtain the location information, and they give each host less computational load to generate the cover set. Therefore, geometry-based protocols are more efficient for an ad hoc network with a high level of host mobility. On the other hand, because of insufficient network topology information, the performance of geometry-based protocols is usually poorer than that of topology-based protocols. Some protocols such as the distance-based approach and the location-based approach [6], [7], maintain a low broadcast reachability in order to reduce redundant broadcasts. The others such as the angle-based approach [8], generate massive rebroadcasts for the purpose of achieving a high reachability.
This paper presents a novel relative position-based broadcast relay (RPBR) scheme to improve the performance of geometry-based protocols, in terms of increasing broadcast coverage area. The broadcast scheme with high broadcast coverage area can achieve a high delivery rate with low number of rebroadcasts. First, the analyses of the coverage area for both single-hop broadcast relay case and multi-hop broadcast relay case are presented, in which each forwarding host is assumed to have the same number of neighboring hosts taking part in the rebroadcasts. The upper bound of broadcast coverage is obtained, which is used for the design of the RPBR scheme. Differing from the common geometry-based protocols, the RPBR scheme is based on both relative distance and forward angle information of forwarding neighboring hosts. Using the RPBR scheme, each host that wishes to forward the received packet calculates the coordinates of the symmetrical area around its proceeding host. The hosts within the symmetrical area have higher priority to rebroadcast the packet than other hosts. With little extra overhead and computational load, RPBR can help many geometry-based broadcast protocols [6], [7], [8], [9] to achieve high delivery rate and low number of rebroadcasts.
Section snippets
The upper bound of broadcast coverage area
In order to keep the average area covered by each broadcasting host to be maximum, the rebroadcast neighboring hosts around each forwarding host must be carefully selected. This section focuses on the upper bound of broadcast coverage area under the condition that each forwarding host is assumed to have the same number of neighboring hosts which are involved in the rebroadcasts. An ad hoc network is modeled using Unit Disk Graph [12] with the same coverage range of radius r=1. Two hosts X and Y
The RPBR scheme
To approach the upper bound of broadcast coverage, a relative position-based broadcast relay (RPBR) scheme is proposed in this paper. It is assumed that mobile hosts are randomly located in a two-dimensional homogeneous ad hoc network. Position devices such as low power low cost GPS receivers are equipped to obtain the location information of mobile hosts, which is exchanged by periodical ‘Hello’ packets among neighboring hosts. The skeleton of the RPBR scheme can be described by the following
Performance evaluation
To evaluate the performance of the RPBR scheme, simulation experiments have been conducted in a homogeneous network. All hosts have the same communication range with radius r=1 length unit (250 m). Hosts are randomly distributed in an area of S=30×30 square units, and the number of hosts (N) in the network ranges from 1000 to 5000. Accordingly, the average host degree1 (the number of neighboring hosts around each host) varies from 3.5 to
Conclusion
Firstly, this paper presents the conditions to achieve the upper bound of broadcast coverage in ad hoc networks, which is useful for the design of optimum broadcast relay scheme to improve the performance of geometry-based protocols. Secondly, this paper presents a novel RPBR scheme, which is able to forward packets effectively, especially when network host density is heavy. Simulation shows that the broadcast performance improved by the RPBR scheme increases when the network host density
Acknowledgements
Authors gratefully acknowledge the anonymous reviewers for their helpful comments to improve this paper.
Supeng Leng is a PhD student in the School of Electrical and Electronic Engineering at Nanyang Technological University, Singapore. He received his BE degree from University of Electronic Science and Technology of China in 1996. He has experience as an engineer in the field of computer communications. His current research focuses on wireless ad hoc networking, including mobility management, location service and MAC protocols.
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Cited by (0)
Supeng Leng is a PhD student in the School of Electrical and Electronic Engineering at Nanyang Technological University, Singapore. He received his BE degree from University of Electronic Science and Technology of China in 1996. He has experience as an engineer in the field of computer communications. His current research focuses on wireless ad hoc networking, including mobility management, location service and MAC protocols.
Dr Liren Zhang is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his BE degree from Shandong University in 1982, ME degree from the University of South Australia in 1988, and PhD from the University of Adelaide, Australia in 1990, all in electrical engineering. From 1990 to 1995 he was a Senior Lecturer in the Department of Electrical and Computer Systems Engineering, Monash University, Australia. Dr Zhang has vast experience as an engineer, academic and researcher in the field of multimedia communications, switching and signaling, teletraffic engineering, network modeling and performance analysis for ATM networks, high speed data networks, mobile networks, satellite networks and optical networks. He has published more than 100 research papers in international journals and conferences. He has been the associate editor for the Journal of Computer Communications since 2000.
L.W. Yu received his BE and ME degrees in Electrical Engineering from the University of Auckland, New Zealand. His industrial experience included work in the areas of optoelectronics and telecommunications with Hewlett-Packard. Since 1986, he has been with Nanyang Technological University (NTU) as an academic staff. His areas of research are optical fibre, wireless and mobile communications.
C.H. Tan received his BE (Electrical), MSc (Industrial Engineering) and ME (Electrical) from the former University of Singapore. After his first degree, he worked in the area of telephone switching systems in the former Singapore Telecom, and also researched into the areas on the performance of Manchester coded frequency shift keying scheme and telephone network optimization for his postgraduate degrees. In 1982, he joined Nanyang Technological University (NTU) as an academic staff and his broad areas of interest are data communication, computer networking, queueing systems and operations research. His research areas are ATM networks and switching, ATM source characterization, traffic management and control, and IP telephony.