1. INTRODUCTION 1.1. NETWORKS

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1 1. INTRODUCTION 1.1. NETWORKS A set of two or more computer systems linked together forms a network. Networking is presently an additional progression of data communications, in various ways which began at the similar time as computer themselves [1]. A computer network essentially consists of an interconnection of devices (computers, printers, routers, etc.) that exchange information with each other. In order to do this, a device encodes data in a format that other devices can understand which is called protocol. To support a flexible configuration and ensure a good scalability, several encodings are usually stacked upon each other, so that data are first encoded (encapsulated) using a certain protocol, then the encoded data are encapsulated using another protocol, the resulting information is again encoded using a different protocol, and so on. After the last encapsulation step, information is actually sent to the destination, which performs the steps in reverse order: interprets the protocol used in received information and decapsulates the data, then again the decoded information is analyzed to interpret a different protocol and its payload is decapsulated, etc. After the last decapsulation step, the information that was originally forwarded by the sender is available for processing by the receiver [2]. According to the ISO OSI model, network protocols are organized as a stack consisting of 7 layers. There are various types of computer networks, including [3]: Local-area networks (LANs) Wide-area networks (WANs). Campus-area networks (CANs) Metropolitan-area networks (MANs) Home-area networks (HANs): In addition to these kinds, the subsequent characteristics are also used to classify different types of networks [3]: Topology: The geometric collection of a computer system. Protocol: The protocol describes a common set of rules and signals to facilitate computers on the network use to communicate. Architecture: Networks can be broadly classified as using moreover a peer-topeer or client/server architecture. 1

2 1.2. WIRELESS NETWORK Wireless network means to any type of computer network which will not be connected by cables of any kind. Wireless telecommunications networks are generally implemented and administered using a transmission system called radio waves. structure [4]. This implementation takes place at the physical level (layer) of the OSI model network Types of Wireless Networks Wireless PAN: Wireless personal area networks (WPANs) interconnect devices within a relatively small area that is generally within a person's reach. Intel "My WiFi" and Windows7 "virtual Wi-Fi" capabilities have made Wi-Fi PANs simpler and easier to set up and configure [5]. Wireless LAN: A wireless local area network (WLAN) links two or more devices over a short distance using a wireless distribution method, usually providing a connection through an access point for Internet access. Wireless Mesh Network: A wireless mesh network is a wireless network made up of radio nodes organized in a mesh topology. Each node forwards messages on behalf of the other nodes. Wireless MAN: Wireless metropolitan area networks are type of wireless network that connects several wireless LANs. WiMAX is a type of Wireless MAN and is described by the IEEE standard. 2

3 Wireless WAN: Wireless wide area networks are wireless networks that typically cover large areas, such as between neighboring towns and cities, or city and suburb. The wireless connections between access points are usually point to point microwave links using parabolic dishes. Mobile devices networks: With the development of smart phones, cellular telephone networks routinely carry data in addition to telephone conversations: Global System for Mobile Communications (GSM): (GSM is the most common standard and is used for a majority of cell phones [7].) Personal Communications Service (PCS) D-AMPS: Digital Advanced Mobile Phone Service, an upgraded version of AMPS, is being phased out due to advancement in technology Advantages of Wireless Networks The most important issue that wireless networks address is that of tether less connectivity which may carry out many advantages. Mobility Cost Installation and Rewiring Bandwidth Advantage of Wired Networks 1.3. WIRELESS MESH NETWORK (WMN) A Wireless Mesh Network (WMN) is a connection network during radio nodes structured in a mesh topology. Wireless mesh networks frequently consist of mesh clients, mesh routers and gateways. The mesh clients are regularly laptops, cell phones and additional wireless devices while the mesh routers promote traffic to the gateways which may not connect to the Internet. The coverage area of the radio nodes working as a single network is called a mesh cloud. A right to use this mesh cloud is reliant on the radio nodes running in harmony with all other to produce a radio network. A mesh network is consistent and proposes redundancy. 3

4 Architecture Wireless mesh architecture is a first step towards cost effective and dynamic highbandwidth networks over a specific coverage area. Wireless mesh architectures infrastructure is, in result, a router network minus the cabling among the nodes. It is built of peer radio devices that don't have to be cabled to a wired port like conventional WLAN access points (AP) do. Mesh architecture maintain signal strength by breaking long distances into a series of shorter hops. Basically all the traffic in an infrastructure mesh network is moreover forwarded to or from a gateway, whereas in ad hoc networks or client mesh networks the traffic flows among arbitrary pairs of nodes. [10] Management This form of infrastructure can be decentralized (with no central server) or centrally supervised (with a central server), [2]. They are reasonably inexpensive, and very reliable and resilient, as each node needs only to transmit as distant as the next node Characteristics The characteristics of WMN s are explained as follows: Multi-hop wireless network. An objective to develop WMNs is to extend the coverage range of current wireless networks without sacrificing the channel capacity. Another objective is to provide non-line-of-sight (NLOS) connectivity among the users without direct line-ofsight (LOS) links. To meet these requirements, the mesh-style multi-hopping is indispensable [87], which achieves higher throughput without sacrificing effective radio range via shorter link distances, less interference between the nodes, and more efficient frequency re-use. Support for ad hoc networking, and capability of self-forming, self-healing, and selforganization: WMNs enhance network performance, because of flexible network architecture, easy deployment and configuration, fault tolerance, and mesh connectivity, i.e., multipoint-tomultipoint communications [130]. In WMNs, both backhaul accesses to the Internet and peer to peer (P2P) communications are supported [77]. In addition, the integration of WMNs with other wireless networks and providing services to end-users of these networks can be accomplished through WMNs. Dependence of power-consumption constraints on the type of mesh nodes: Mesh routers usually do not have strict constraints on power consumption. However, mesh clients may require power efficient protocols. 4

5 As an example, a mesh-capable sensor [115,116] requires its communication protocols to be power efficient. Thus, the MAC or routing protocols optimized for mesh routers may not be appropriate for mesh clients such as sensors, because power efficiency is the primary concern for wireless sensor networks [8,9]. Compatibility and interoperability with existing wireless networks: For example, WMNs built based on IEEE technologies [135, 71]. Such WMNs also need to be inter-operable with other wireless networks such as WiMAX, Zig- Bee [148], and cellular networks Applications Research and development of WMNs is motivated by several applications which clearly demonstrate the promising market while at the same time these applications cannot be supported directly by other wireless networks such as cellular networks, ad hoc networks, wireless sensor networks, standard IEEE , etc. Broadband home networking: Currently broadband home networking is realized through IEEE WLANs. An obvious problem is the location of the access points. Mesh networking, as shown in Figure 1.1, can resolve all these issues in home networking. The access points must be replaced by wireless mesh routers with mesh connectivity established among them. Therefore, the communication between these nodes becomes much more flexible and more robust to network faults and link failures. Protocols proposed for mobile ad hoc networks [34] and wireless sensor networks [8,9] are too cumbersome to achieve satisfactory performance in this application As a consequence, WMNs are well suited for broadband home networking as in the figure 1.1 Figure 1.1 WMNs for broadband home networking. 5

6 Community and neighborhood networking. In a community, the common architecture for network access is based on cable or DSL connected to the Internet, and the last-hop is wireless by connecting a wireless router to a cable or DSL modem. This type of network access has several drawbacks: Even if the information must be shared within a community or neighborhood, all traffic must flow through Internet. Large percentage of areas in between houses is not covered by wireless services. An expensive but high bandwidth gateway between multiple homes or neighborhoods may not be shared and wireless services must be set up individually. As a result, network service costs may increase. Only a single path may be available for one home to access the Internet or communicate with neighbors. WMNs mitigate the above disadvantages through flexible mesh connectivity s between homes, as shown in Figure 1.2. Figure 1.2 WMNs for community networking. Enterprise networking. This can be a small network within an office or a medium-size network for all offices in an entire building, or a large scale network among offices in multiple buildings. If the access points are replaced by mesh routers, as shown in Figure 1.3., Ethernet wires can be eliminated. 6

7 Figure 1.3. WMNs for enterprise networking. Metropolitan area networks. WMNs in metropolitan area have several advantages. The physical-layer transmission rate of a node in WMNs is much higher than that in any cellular networks. For example, an IEEE g node can transmit at a rate of 54% Mbps. Moreover, the communication between nodes in WMNs does not rely on a wired backbone. Compared to wired networks, e.g., cable or optical networks, wireless mesh MAN is an economic alternative to broadband networking, especially in underdeveloped regions. Wireless mesh MAN covers a potentially much larger area than home, enterprise, building, or community networks, as shown Figure 1.4. Thus, the requirement on the network scalability by wireless mesh MAN is much higher than that by other applications. Figure 1.4. WMNs for metropolitan area networks. 7

8 Transportation systems. Instead of limiting IEEE or access to stations and stops, mesh networking technology can extend access into buses, ferries, and trains. To enable mesh networking for a transportation system, two key techniques are needed: the high-speed mobile backhaul from a vehicle (car, bus, or train) to the Internet and mobile mesh networks within the vehicle, as shown in Figure 1.5. Figure 1.5 WMNs for transportation systems. Building automation. In a building, various electrical devices including power, light, elevator, air conditioner, etc., need to be controlled and monitored. Currently this task is accomplished through standard wired networks, which is very expensive due to the complexity in deployment and maintenance of a wired network. If BACnet (building automation and control networks) access points are replaced by mesh routers, as shown in Figure 1.6, the deployment cost will be significantly reduced.. Figure 1.6: WMNs for building automation. 8

9 Health and medical systems. In a hospital or medical center, monitoring and diagnosis data need to be processed and transmitted from one room to another for various purposes. Data transmission is usually broadband, since high resolution medical images and various periodical monitoring information can easily produce a constant and large volume of data. Traditional wired networks can only provide limited network access to certain fixed medical devices. Security surveillance systems. As security is turning out to be a very high concern, security surveillance systems become a necessity for enterprise buildings, shopping malls, grocery stores, etc. In order to deploy such systems at locations as needed, WMNs are a much more viable solution than wired networks to connect all devices Mesh Connectivity Mesh Connectivity Layer (MCL) is a technology that allocates a computer user to connect to a wireless mesh network that uses Wi-Fi or WiMax. The wireless mesh network can be connected to the Internet all the way through a single computer by means of a leased T-1 line or broadband satellite connection, allocating all network users high-speed Internet access. The MCL system is being developed by Microsoft, in union with numerous major universities, for computers that make use of Windows operating systems Mesh Routers Wireless mesh routers united the industry's most complicated mesh networking aptitude with purpose-built hardware to bring out high reliability and most favorable performance. Fixed and mobile mesh routers are obtainable and can be mixed on a single network to generate a mesh network with dynamic coverage areas Multi-hop Networking Multi-hop or ad hoc, wireless networks utilize two or more wireless hops to communicate information from a source to a destination. There are two different applications of multi-hop communication, with common features, but different applications. Mobile ad hoc networks (MANETS) consist of a collection of mobile nodes that communicate without requiring a fixed wireless infrastructure. 9

10 1.3.8 Applications Mesh networks might involve moreover fixed or mobile devices. The solutions are as varied as communication needs, for example in difficult environments for instance emergency situations, tunnels, oil rigs, battlefield surveillance, high speed mobile video applications on board public transport or real time racing car telemetry. A significant possible application for wireless mesh networks is VoIP. By means of a Quality of Service scheme, the wireless mesh can sustain local telephone calls to be routed through the mesh. Some Current Applications Even though they need wired or cell phone or other physical connections in their area, U.S. military forces are currently using wireless mesh networking to connect their computers, mainly ruggedized laptops, in field operations. Electric meters now being organized on residences transmit their readings from one to another and ultimately to the central office for billing exclusive of the need for human meter readers or the need to connect the meters with cables. [3] The laptops in the One Laptop per Child program use wireless mesh networking to enable students to exchange files and get on the Internet. The 66-satellite Iridium collection operates as a mesh network, with wireless links between adjacent satellites. Calls between two satellite phones are routed through the mesh, from one satellite to another across the constellation, without having to go through an earth station. This enables a smaller travel distance for the signal, reducing latency, and also permits for the constellation to operate with far fewer earth stations that would be essential for 66 traditional communications satellites. The Commotion Wireless Project intends building a 'device-as-infrastructure' distribution encrypted communication platform Operation The principle is related to the way packets travel in the region of the wired Internet data will skip from one device to another until it reaches its destination. Dynamic routing algorithms executed in each device allow this to happen. To implement such dynamic routing protocols, each device needs to communicate routing information to other devices in the network. Each device then establishes what to do with the data it gets either pass it on to the next device or keep it, depending on the protocol. The routing algorithm used should attempt to always make sure that the data obtains the most appropriate (fastest) route to its destination. 10

11 Multi-radio mesh Multi-radio mesh refers to a unique pair of dedicated radios on each end of the link. This means there is a unique frequency used for each wireless hop and thus a dedicated CSMA collision domain. This is the right mesh link wherever you can achieve maximum performance with no bandwidth degradation in the mesh and devoid of adding latency. Thus voice and video applications work presently as they would on a wired Ethernet network. In true networks, there is no concept of a mesh. There are only Access Points (AP) and Stations. A multi-radio wireless mesh node determination dedicates one of the radios to act as a station, and connect to a neighbor node AP radio. The merging of WMNs with several conventional wireless networks like WSNs, Wi-Fi, WiMAX, Wireless Region Area Networks (WRANs) Cellular networks Network Topology In communication networks, a topology is a generally diagrammatic explanation of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology. The physical topology of a network is the actual geometric layout of workstations. There are numerous general physical topologies, as illustrated below and as shown in the figure 1.7. Figure 1.7. Network Topology 11

12 In the bus network topology, all workstation is connected to a main cable called the bus. Consequently, in result, each workstation is in a straight line connected to every other workstation in the network. In the star network topology, there is a central computer or server to which all the workstations are straightly connected. Every workstation is directly connected to other workstation through the central computer. In the ring network topology, the workstations are linked in a closed loop configuration. Adjacent pairs of workstations are connected in a straight line. Other pairs of workstations are not directly connected, the data passing through one or more intermediate nodes. If a Token Ring protocol is used in a star or ring topology, the signal travels in only one direction, passed by a so-called token from node to node. The mesh network topology occupies either of the two schemes, called full mesh and partial mesh. In the full mesh topology, each workstation is related directly to each of the others. In the partial mesh topology, some workstations are connected to all the others, and a few are connected only to those other nodes with which they exchange the most data. The tree network topology uses two or more star networks connected mutually. The central computers of the star networks are linked to a main bus. Thus, a tree network is a bus network of star networks. Logical (or signal) topology refers to the nature of the paths the signals go behind from node to node. In several instances, the logical topology is similar as the physical topology. But this is not always the case. For example, some networks are physically placed out in a star configuration, but they operate sensibly as bus or ring networks TOPOLOGY CONTROL AND TOPOLOGY CONSTRUCTION FOR WIRELESS MESH NETWORKS Topology Control Topology control is a method used in circulated computing to modify the underlying network (modeled as a graph) in order to decrease the cost of distributed algorithms if run over the new resulting graphs. It is a fundamental procedure in distributed algorithm. For instance, a (minimum) spanning tree is used as a backbone to decrease the cost of broadcast from O (m) to O (n), where m and n are the number of edges and vertices in the graph correspondingly. The term "topology control" is used mostly by the wireless ad hoc and sensor networks research community. The major aim of topology control in this domain is to save energy, decrease interference among nodes and extend lifetime of the network. 12

13 Topology Construction and Maintenance Recently, topology controls have been divided into two sub problems: topology construction, in charge of the initial reduction, and topology maintenance, in charge of the maintenance of the condensed topology so that characteristics like connectivity and coverage are sealed. This is the initial stage of a topology control protocol. Once the initial topology is positioned, especially when the place of the nodes is random, the administrator has no control over the design of the network; for instance, some areas may be extremely opaque, showing a high number of redundant nodes, which will raise the number of message collisions and will give several copies of the same information from similarly located nodes. However, the administrator has control over some limitations of the network: transmission power of the nodes, state of the nodes (active or sleeping), role of the nodes (Cluster head, gateway, regular), etc. By modifying these limitations, the topology of the network can change. At the same time a topology is condensed and the network starts serving its intention, the selected nodes begins expending energy: The "optimal" reduced topology prevents it being the first second of full activity. After some time being active, some nodes will begin to run out of energy. In particular, wireless sensor networks with multihoping, it is a truth that nodes that are closer to the sink use higher amount of energy than those farther away due to packet forwarding. The network must restore the performance periodically in order to preserve connectivity, coverage, density and any other metric that the application requires Need for a New Routing Metric Given a source and destination node, a routing protocol provides one or more network paths over which packets can be routed to the destination. A routing metric that accurately captures the quality of network links and thus aids in meeting such criteria is central to computation of good quality paths. The design of routing metrics for wireless multi-hop networks is challenging due to the following three unique characteristics of wireless links: Time varying channels and resulting variable packet loss: A routing metric should accurately capture this time varying packet loss. Packet transmission rate. 13

14 Cost Reduction Rural areas (especially in developing regions) include populations with very low paying capacity. For this reason, a chief factor in network deployment is the cost of the infrastructure and the network equipment. In this context, efficient algorithms are to be examined for the minimum cost topology construction problem in rural wireless mesh networks. The cost of wiring to rural areas is prohibitively expensive. Moreover, traditional wireless technologies for instance cellular data networks (e.g., EV-DO) and upcoming technologies like IEEE WiMAX have prohibitively expensive equipment costs. Accordingly there has been considerable interest in recent times [2], [8], [16], [17] in the design of rural mesh networks by means of IEEE (WiFi) equipment. The cost of an radio has a magnitude less than that of cellular/wimax base stations. Hence, this approach is a viable option for building low cost networks. Rural mesh networks contain two key characteristics, 1) A fixed topology (a node in this network is a village), and 2) Long distance links between the nodes (about 7-8 kms). A typical IEEE supported rural mesh network consists of a group of villages connected with each other through point-to-point wireless links. Some exceptional nodes in this mesh, called the gateway nodes, are connected to the wired internet. Other mesh nodes connect to the gateway nodes (and thus, to the rest of the internet) through one or more hops in the mesh. An essential requirement to establish longdistance links is that line-of-sight is sustained between the radio antennas at the end-points. To ensure that line-of-sight across such long distances (over obstacles such as trees, buildings and the terrain), would need the antennas to be mounted on tall towers. The required height of the towers depends together on the length of the link, and the height of the obstructions next to the link. The cost of the tower depends on its height as well as the type of material used. For comparatively shorter heights (10-20 meters) antenna masts are adequate. For greater heights, sturdier and much added expensive antenna towers are required. The cost of building towers and masts for various heights is given below: To cover a distance of 7-8 kms needs the tower height of at least one end-point should be approximately meters [3]. The cost of building such a tower ($ $5000) is greater than the cost of the communication equipment at a node. 14

15 Given this considerable difference between the cost of towers and other equipment, the principal problem in building rural mesh networks is to construct a topology with the lowest total cost of antenna towers. A basic requirement of any topology is that it should connect all villages to the gateway node. The cost of the sub graph is the sum of the cost of antenna towers required to establish all the links in the sub graph. In this work, the first algorithms for this topology construction problem with provable performance bounds Energy Optimization Energy consumption of communication systems is becoming a fundamental issue and, among all the sectors, wireless access networks are largely responsible for the increase in consumption. In addition to the access segment, wireless technologies are also gaining popularity for the backhaul infrastructure of cellular systems mainly due to their cost and easy deployment. In this context, Wireless Mesh Networks (WMN) are commonly considered the most suitable architecture because of their versatility that allows flexible configurations. In this work the flexibility of WMN is combined with the need for energy consumption reduction by presenting an optimization framework for network management that takes into account the tradeoff between the network energy needs and the daily variations of the demand. A resolution approach and a thorough discussion on the details related to WMN energy management are also presented. Green Networking consists of a rethinking of the way networks are built and operated so that not only costs and performance are taken into account but also their energy consumption and carbon footprint. It is quickly becoming one of the major principles in the world of networking; given the exponential growth of Internet traffic that is pushing huge investments around the world for increasing communication infrastructures in the coming years. In fact, the Information and Communication Technology (ICT) sector is said to be responsible for 2% to 2.5 % of the GHG annual emission [1,2,3] as it generates around 0.53Gt (billion tonnes) of carbon dioxide equivalent (CO 2 e). This amount is expected to increase to 1.43GtCO 2 e in 2020 (data from [4]). Among Internet related networking equipment, it is the access the one with the major impact in energy expenditures. It has been estimated that access networks consume around 70 % of overall telecommunications network energy expenditures and this percentage is expected to grow in the next decade [5,6]. 15

16 An important part of the energy consumption is given by the wireless part of the access and it has been estimated that the base stations represent 80% of the total wireless consumption [7]. Being able to minimize base station consumption represents an important green networking objective. An increasingly popular type of wireless access is the so-called Wireless Mesh Networks (WMNs) [8] that provide wireless connectivity through much cheaper and more flexible backhaul infrastructure compared to wired solutions. The nodes of these dynamically self-organized and self-configured networks create a changing topology and keep a mesh connectivity to offer Internet access to the users. Obviously, the use of wireless technologies also for backhauling can potentially make the issue of energy performance even more severe if appropriate energy saving strategies are not adopted. As a matter of fact, the resources of Wireless Access Networks are, for long periods of time, underemployed, since only a few percentage of the installed capacity of the Base Stations (BS) is effectively used and this results in high energy waste [9,10]. In WMNs also, network devices are active both in busy hours and in idle periods. This means that the energy consumption does not decreases when the traffic is low and that it would be possible to save large amounts of energy just by switching off unnecessary network elements. The focus of our work is to combine the versatility of Wireless Mesh Networks with the need for optimizing energy consumption by getting advantage of the low demand periods and the dynamic reconfigurations that are possible in WMNs. It is proposed to minimize the energy in a time varying context by selecting dynamically a subset of mesh BSs to switch on, considering coverage issues of the service area, traffic routing, as well as capacity limitations both on the access segment and the wireless backhaul links. To reach the objective, an optimization framework is provided based on mathematical programming that considers traffic demands for a set of time intervals and manages the energy consumption of the network with the goal of making it proportional to the load. Energy management in wireless access networks has been considered very recently in a few previous works [11, 9, 12, 2, 13, 14, 15, and 1]. In this work, a novel approach is presented for the dynamic energy management of WMNs that provides several novel contributions: Not only the access segment is considered but also the wireless backhaul of wireless access networks. The issue of wireless coverage is combined together for the access segment, and the routing, for the backhaul network, and to optimize them jointly. 16

17 Traffic variations over a set of time intervals are included to show how it is possible to have energy consumption following these variations. A rigorous mathematical modeling of the energy minimization problem based on Mixed Integer Linear Programming (MILP) is provided to and solve it to the optimum Interference Wireless mesh networks are characterized by static mesh routers connected by wireless links to each other and to a few gateway nodes. The WMN routers effectively form a multihop wireless access backbone. Recently, the deployment and use of WMNs have increased significantly and several cities have planned and/or deployed WMNs ([17, 21, 20, 15, 18, and 19]). Thus, improving WMN performance will have a direct impact on the growing population of users. Since the most significant application of such networks is to provide broadband Internet access to static or mobile hosts in areas where wired infrastructure is difficult or economically not feasible to deploy, it is important to optimize the network throughput of WMNs. A well-known fundamental technique to improve throughput is to exploit parallelism. In wireless networks, parallelism is achieved through spatial reuse, i.e., enabling simultaneous transmissions of packets at multiple sender-receiver pairs. Since WMNs operate over a shared broadcast medium, such parallelism is fundamentally limited by signal interference, i.e., the nature and amount of interference caused by simultaneously operating transmitters to other receivers determines the amount of parallelism that can be exploited and consequently the network throughput achievable. Thus, it is important to study the nature and extent of interference in WMNs and its impact on the network throughput Causes of Interference in Wireless Mesh Networks The primary causes of interference in wireless mesh networks and their characteristics are first reviewed. A variety of factors may cause interference in wireless networks. The most common ones are listed below. Intentional interferers that transmit in the same band and in the same area usually include other nodes which make the interfering signal to have very similar structure with the desired signal. 17

18 The IEEE b medium access protocol handles this problem by allowing nodes to transmit packets only when there is no other transmitting node. If traffic is sensed in the medium, nodes wait for a predetermined amount of time before they attempt to listen and transmit their packets. As a result, this source of interference causes a direct reduction to the network throughput. Non-intentional interferers that transmit in the same band and in the same area are bluetooth nodes, microwave ovens, cordless phones and similar equipment. These sources typically emit signals whose structure is very different from the desired signal. For example, unlike b nodes that occupy a relatively wide bandwidth of nearly 30MHz, the spectral mask of a bluetooth signal is limited to 1MHz. In addition, bluetooth employs the frequency hop spread spectrum (FHSS) technique that causes nodes to hop over 79 frequencies of 1MHz bandwidth [16]. Such non-intentional interferers may cause two effects: (1) They may occupy the medium not allowing the desired nodes to transmit; or (2) They may transmit their signal while a desired transmission is in progress leading to damaged packets that need to be re-transmitted. In both cases, the network throughput will be impaired. Multipath fading that leads to inter-symbol interference, occurs when the desired signal arrives at the intended node through several different paths (at least two). Multipath is caused by objects (and/or humans) that happen to exist in the vicinity of two communicating nodes (Figure 1.8). The physics that cause multipath signals is typically quite complex and described statistically by appropriate models such as in [13]. Nevertheless, they can be qualitatively described by three fundamental phenomena: reflection/transmission, diffraction and scattering. Reflection and transmission take place when the desired propagating wave impinges on an object whose dimensions are large compared to its wavelength (e.g. building walls, large desks, etc.). Diffraction occurs when the desired signal impinges on sharp edges such as wall edges. The wave bends around these edges and therefore can reach locations that are not optically visible from its source. 18

19 Scattering happens when the desired wave meets objects with dimensions significantly smaller than its wavelength (e.g. foliage). Scattering causes the wave to disperse in many different directions. Figure 1.8: Multipath caused by reflected, scattered and diffracted signals In typical indoor and outdoor environments, all these mechanisms occur several times as the desired wave propagates from its source to its destination. As a result, several copies of the desired signal arrive at the intended node. This effect is called delay spreading which is described by an average time delay, which represents the time window that delays copies of the signal reach the receiver. Typical values range from 0.1 s in weak multipath environments to over 1 s in dense urban environments [8]. The delayed signal copies are typically weaker than the line-of-sight (LOS) signal (if it exists) but exhibit various phases depending on the followed wireless path and the objects they encountered. This may lead to inter symbol interference in the receiver that degrades the signal-to-noise ratio (S/N), thus leading to reduced throughput for the overall network PROBLEM SPECIFICATION Topology construction in Rural Wireless Mesh Networks provided wide field for research. But there are several problems in topology construction for wireless mesh networks. Many Topology Construction models exist already, but all those models lack certain abilities. So, the performance of the topology construction is not very significant. Some of the drawbacks of the existing models are construction cost, energy conservation and selforganization approach. Construction cost mainly depends on the cost required for building the antenna towers at nodes. 19

20 The factor that determines the cost of tower is its height that based on the length of its link and physical barrier across those links. Energy conservation will be based on the quantity of energy required for transmission and reception of messages among the Wireless Mesh network (WMN). The network lifetime will be greatly improved if the transmission power is decreased in the nodes. Self-Organization is very important for interference reduction and to increase the aggregate capacity of the network OBJECTIVES OF THE RESEARCH The main objective of the research work is the topology construction for rural wireless mesh network. There are various factors to be considered in this topology construction. The most important factors include (1) Minimizing the cost (2) Minimizing the energy loss (3) Self Organization 1.7. PROPOSED METHODOLOGY The methodology aims at constructing a topology for rural wireless mesh networks. The methodologies used in the proposed approach are 1. An Efficient and Minimum Cost Topology Construction for Rural Wireless Mesh Networks 2. A Novel Topology Construction for Rural Wireless Mesh Networks with Minimum Cost and Energy. 3. A Topology Construction for Rural Wireless Mesh Networks with Minimum Cost, Energy Reduction and Self Organization 20