GATEWAY PLACEMENT AND FAULT TOLERANCE IN QOS AWARE WIRELESS MESH NETWORKS

Transcription

1 GATEWAY PLACEMENT AND FAULT TOLERANCE IN QOS AWARE WIRELESS MESH NETWORKS A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Yasir Drabu Dec 2010

2 Dissertation written by Yasir Drabu B.S., Bangalore University, 1998 M.S., Kent State University, 2003 Ph.D., Kent State University, 2010 Approved by Dr. Hassan Peyravi, Chair, Doctoral Dissertation Committee Dr. Javed E. Khan, Member, Doctoral Dissertation Committee Dr. Feodor F. Dragan, Member, Doctoral Dissertation Committee Dr. Kazim Khan, Member, Doctoral Dissertation Committee Accepted by Dr. Jonathan I. Maletic, Chair, Department of Computer Science Dr. Timothy S. Moerland, Dean, College of Arts and Sciences ii

3 TABLE OF CONTENTS LIST OF FIGURES vii LIST OF TABLES ix Acknowledgments xii Dedication xiii 1 Introduction Motivation Classification of Wireless Mesh Networks Wireless Mesh Networks (WMN) Architecture Components of WMNs WMNs Compared to Mobile Ad Hoc Networks (MANETs) WMNs Compared to Sensor Networks Advantages of WMNs Self organizing and self configuring Low deployment costs Increased reliability Scalability Interoperability Applications of WMNs iii

4 1.5.1 Broadband Wireless Access Industrial Applications Future Applications: Research Contributions Structure of this Dissertation Background and Related work Introduction WMN Standardization IEEE s WiFi Mesh IEEE Bluetooth IEEE Zigbee IEEE WiMAX WMN Deployments Academic Testbeds Commercial Implementations Problems and Challenges in WMNs Phycial Layer Issues Medium Access Layer Issues Transport Layer Issues Network Layer Issues Topological and Deployment Issues Gateway Placement in WMNs iv

5 3.1 Introduction Background and Related Work System Model and Assumptions Clustering with QoS Constraints Gateway Placement Algorithms Integer Linear Program Formulation Split-Merge-Shift Algorithm (SMS) Algorithm Illustration Algorithm Analysis Performance Summary Fault Tolerance in Cellular WMNs Background and Related Work An Augmented Tri-Sectorized System with Directional Antennas Sectorized cells directional antennas Honeycomb Networks Routing in Honeycomb Mesh QoS Routing Fault Tolerant Routing Fault Models Comparision of Routing Schemes Fault Tolerance in WMNs v

6 5.1 Background and Related Work System Model Network and QoS Model Fault Model Fault Recovery Topology Control Fault Recovery Algorithm Algorithm Evaluation Simulation Results Summary Falut-Tolerance Provsioning Through Cluster Overlapping in General WMNs Introduction Research Issues Preliminaries Planning with Disjoint Clustering Analysis Planning with Joint Clustering Simulation Results Summary Conclusion and Future Work BIBLIOGRAPHY vi

7 LIST OF FIGURES 1.1 Classification of Wireless Networks Infrastructure mesh network Survey of broadband access in rural and urban areas [47] Worldwide Internet usage peneration for 2010 (Courtesy: Internet World Stats) Roofnet testbed setup at Kent State University, Dept. of Computer Science Effect of distance on throughput, Roofnet testbed Effect of hops on throughput, Roofnet testbed Simulation setup to study effect of hop distance Load vs. throughput for varying hop distance Relay Load Flows Algorithm overview Shift operation overview Effect of S q on gateway count Effect of L q on gateway count Effect of S q on cluster size variation (a) Omni directional antennas, (b) Tri-sectorized antenna Transforming a Regular Cellular structure to a Trisectional Directional Cellular System vii

8 4.18 Tri-sectored node with three directional antennas and one omni directional antenna Brick network representation of a honeycomb network Detours, d and d, around faults Routing around a link failure A clustered wireless mesh with the overlay cluster graph Fault recovery algorithm working with three cases Effect of fault location on recovery probability (50 node network) Load Vs Throughput with two faults and CBR traffic Load Vs Delay Load Vs Jitter Maximum throughput performance across different traffic loads A linear multi-hop network A 100-node mesh network Initial clustering with h = 2 and I c = Inter-cluster distance Final disjoint clustering, 2 h 4, I c = Joint clustering with R = 2, I c = h = 1, I c = h = 1, I c = h = 2, I c = Overlapping clusters simulation setup Load vs. throughput with and without overlapping viii

9 LIST OF TABLES 1.1 Network architectures Broadband Access Technology ix

10 Acronyms 3G 4G AODV BSS DOCSIS DSL DSR ESS HWMP IP Kbps LAN LQSR MAC MAN MANET Mbps MCL Third Generation Wireless Network Fourth Generation Wireless Network Ad Hoc On-Demand Distance Vector Basic Service Set, as defined in IEEE family of protocols Data Over Cable Service Interface Specification Digital Subscriber Line Dynamic Source Routing Extended Service Set, as defined in s draft IEEE standard Hybrid Wireless Mesh Protocol Internet Protocol Kilo Bits per Second Local Area Network Link Quality Source Routing Medium Access Control Metropolitan Area Network Mobile Ad-hoc Network Mega Bits Per Second Mesh Connectivity Layer x

11 MIMO MTM ORBIT PAN PDA PTM PTP QoS SDR STA SNR TCP WAN Wifi WMN WWW Multiple Input Multiple Output Multipoint to Multipoint Open-Access Research Testbed for Next-Generation Wireless Networks Personal Area Network Personal Digital Assistants Point to Multipoint Point to Point Quality of Service Software Defined Radio Mesh Station, as defined in s draft IEEE standard Signal to Noise Ratio Transport Control Protocol Wide Area Network Wireless Fidelity, refers to the family of protocols Wireless Mesh Networks World Wide Web xi

12 Acknowledgments I would like to thank my supervisor Dr. Hassan Peyravi for his guidance and support, during my research at the Internet Engineering Lab at Kent State University. He has inspired me to think critically and help me hone my analytical abilities. His hands on approach towards experimentation and simulation formed a corner stone of my research and have saved me countless hours in evaluating our algorithms and protocols. I would also like to thank my committee members, Dr. Javed Khan and Dr. Feador Dragan and Dr. Kazim Khan for their feedback. I would also like to thank my parents and my wife, Sabah Drabu, who have been a constant source of moral support and constant encouragement. I would also like to extend my special thanks to Sue Peti and Marcy Curtiss for their help throughout the years as a graduate student. I am grateful to the Internet2 foundation and the Ohio Board of Regents for supporting parts of this research. xii

13 To Sabah and Murad xiii

14 Abstract Wireless Mesh Networks (WMN s), in the form of WiFi (802.11x) or WiMax (802.16x), or their integrations have been proposed as an effective communication alternative for ubiquitous last mile wireless broadband access. They can be viewed as a hybrid between traditional cellular, point-to-point wireless systems, and ad-hoc networks. They offer more flexibility, mobility, coverage and expandability compared to their traditional counterparts at the expense of complex architecture and deployment structure. Though WMNs hold great promise in abetting network ubiquity, there still remain several challenges in the design and development of WMN s to support diverse services with different quality of service (QoS) requirements and large scale deployment. The focus of this dissertation is to address some of the core issues that directly affect the QoS in terms of delay, throughput, and fault tolerance. First we look at the deployment problem of the placement of wired gateways. This aspect of WMNs has a significant impact on the network s throughput performance, cost and capacity to satisfying the quality of service requirements. In the context of gateway placement, the QoS is influenced by the number of gateways, the number of nodes served by each gateway, the location of the gateways, and the relay load on each wireless router. While finding an optimal solution to simultaneously satisfy all the above constraints is known to be an N P-hard problem, near optimal solutions can be found within the feasibility region in polynomial time using various heuristic methods. In the initial part of this dissertation, we first present a near optimal heuristics algorithm for gateway

15 2 placement that facilitates QoS provisioning and fault tolerance in WMNs. We then investigate fault tolerance and recovery problems in WMNs. We present a fault recovery algorithm that can exploit the known geometry of a regular cellular mesh network. While keeping the QoS metrics intact, we consider a post-deployment fault recovery algorithm and pre-deployment fault tolerance planning.

16 CHAPTER 1 Introduction 1.1 Motivation One of the ultimate goals in the field of networking is to have ubiquitous broadband access for all, i.e. any time, any place access with satisfactory Quality of Service (QoS). Asymmetric and symmetric Digital Subscriber Line (xdsl) and Data Over Cable Service Interface Specification (DOCSIS) have fueled broadband access to many densely populated areas - residential as well as businesses. However their cost of deployment and the nature of wired medium limits their coverage and access to a broader spectrum of people. Wireless network systems, by virtue of their medium can provide mobility and wider coverage. Their usage is well established in narrow-band access systems, however their foray into broadband is relatively new. Wireless networks have changed the way we work and play. They have changed every facet of our lives to make it easier to communicate with our peers. Wireless networks tend to form a natural extension to the way we communicate making them more imbued in our daily lives. Researchers are pushing the envelope by improving the performance of traditional cellular networks. Great strides are are being made in bandwidth capacity in 3G and 4G networks. Today individuals and businesses use mobile devices like laptops, Personal Digital Assistants (PDAs), smart phones, etc, to send text, audio and video messages. Many use it to listen to streaming music and watch Video on Demand (VoD). As technology progresses we will see a greater demand for smarter and more efficient 1

17 2 devices coupled with much faster wireless networks to address and aid the human appetite for information, communication and entertainment. In spite of all these advances, current wireless networks are marred with limited coverage, low throughput, unreliable connections, security concerns, and power limitation. Therefore, it is vital to build an economic, scalable and fault tolerant wireless system that can meet the exponentially growing demand. While improving existing wireless networks, new architectures like multi-point to multi-point wireless networks are being actively investigated and are providing solutions to some very challenging problems like the economics of last mile connectivity, community and neighborhood networks and other interesting applications. In this research, we focus on the deployment issues and challenges of wireless mesh networks.we specifically look into the pre-deployment planning in term of optimal placement of gateways and QoS provisioning, and post-deployment operation in terms of fault-tolerance and network connectivity. 1.2 Classification of Wireless Mesh Networks Wireless networks can be classified based on several criteria - like size, topology, power levels, applications or protocols used. In the context of this dissertation we classify WMNs based on the kind of connectivity between the various network elements. Broadly speaking, current wireless networks are either Point to Point (PTP) or Point to Multi- Point (PTM) networks. The complete taxonomy of this classification is show in the Figure 1.1. PTP networks are highly reliable but have low adaptability and they are not scalable.

18 3 Wireless Networks Single Hop (PTM) Point To Point (PTP) Multi-hop (MTM) Infrastructure-based (hub&spoke) Infrastructure-less (ad-hoc) Infrastructure-based (Hybrid) Infrastructure-less (MANET) Bluetooth Cellular Networks (GSM/CDMA) Wireless Sensor Networks Wireless Mesh Networks Car-to-car Networks (VANETs) Figure 1.1: Classification of Wireless Networks PTM are moderately scalable, but they have low reliability and adaptability. To overcome these limitations Multi-point to Multi-point (MTM) networks are evolving rapidly and they are offering features that provide high reliability, adaptability and scalability to accommodate a large number of users. The characteristics of each network are tabulated in Table 1.1. Table 1.1: Network architectures Topology Reliability Adaptability Scalability Routing Point-to-Point High Low None None Point-to-Multi-point Low Low Moderate Moderate Multi-point-to-Multi-point High High High High MTM wireless networks seem like a natural fit for wide area coverage. Using multiple hops provides increased coverage without the need for increasing the transmission power. As the number of nodes in the network increases the transmission power needed for each interface can be reduced. In this model if P t is the transmission power and d is

19 the transmission radius, P t d α, where 2 α 4. According to this model if the transmission distance (d) is decreased by a factor of 2, the transmission power (P ) will 4 decrease by a factor of 4 or 16. In this research we focus on a special class of PTP network called Wireless Mesh Networks. MTM wireless networks can also be realized using today s standard commodity based wireless networking equipment like the IEEE family of protocols [1]. These type of networks are called community mesh networks [12]. Projects like the Roofnet [17] address the setup and routing problems of wireless mesh networks based on b/g. Roofnet s proactive routing protocol probes the network for link quality and topology changes to provide the best possible route. Most of these commodity wireless technologies operate using a single radio and a shared channel. Most of the early protocols proposed were for Point-to-point wireless networks, that essentially extended the range of wired networks. However in the mid 1990s, with the commercialization of wireless radios, multiple hop wireless networks started to garner interest in solving many interesting problems like the last mile connectivity and ad-hoc networking. For example, community mesh networks like [33, 51] are working towards provide co-operative wireless broadband access to its users in their community. While the network formed using such technologies provide acceptable performance on smaller networks, they do not scale well for larger networks which have higher node density and multiple hops.

20 5 1.3 Wireless Mesh Networks (WMN) Architecture WMNs are multi-hop infrastructure based wireless networks that are interconnected by a set of relatively stationary wired gateways connected to the Internet. The routers that relay traffic and the client may or may not be mobile. Most of the traffic in a WMN flows from the client to the gateways. So, the traffic pattern may be asymmetric. A typical WMN is shown in Figure 1.2. Figure 1.2: Infrastructure mesh network Components of WMNs As mentioned, a wireless mesh network consist of three types of nodes: WMN Clients: These are the end-user devices like PDAs, laptops, smart phones, etc, that can access the network for using applications like , web surfing, VoIP, and alike. These devices are assumed to have limited power, mobile, having none

21 6 or limited routing capabilities, and may or may not be always connected to the network. Mobile Ad-hoc Networks (MANETs) can be assumed to be special case of WMNs that are formed purely by WMN clients. WMN Routers: These network elements are primarily responsible for routing traffic in the network. Traffic does not originate or terminate at a router. The routers are characterized by limited mobility and relatively high reliability. Compared with a conventional wireless routers, a wireless mesh router can achieve the same coverage with much lower transmission power consumption through multi-hop communications. Additionally, the Medium Access Control (MAC) protocol in a mesh router supports multiple-channels and multiple interfaces to enable scalability in a multi-hop mesh environment. With the capability of self-organization and selfconfiguration, WMNs can be deployed incrementally, one node at a time. WMN Gateways: This is a router which has direct access to the wired infrastructure (i.e. Internet). Most of the client nodes on a WMN, communicate with the wired infrastructure [5] for information deliverted over the World Wide Web (WWW), s, audio and video. Since they have multiple interfaces (wired and wireless), the gateways are typically more expensive, both to install and operate. Typically they are fewer in number and their placement has a significant impact the performance of the network. The distributed nature of WMN infrastructure in terms of control and coordination and multi-hop and multi-point connectivity enables development of self-configuration and self-healing capabilities. They do not have a centralize controller like today s cellular

22 7 networks, therefore no central point of failure or bottleneck exists. Further since they can communicate wirelessly they are more economical to deploy. In terms of requirements and design principals, WMNs are ideal to meet the growing demands of the wireless community. However, fundamental technical challenges need to be addressed before they become commercially viable. There is a lot of commercial and academic interest in this area due to the impact this technology can have on the economics of access networks. WMNs also have the potential to bring much larger throughput to the last mile in sparsely populated areas and rural areas which have limited economic resources WMNs Compared to Mobile Ad Hoc Networks (MANETs) Wireless Mesh Networks share a lot of properties with MANETs. The most important characteristic is the that nodes are connected without any wired infrastructure and routing is done across multiple hops in both networks. However, the similarity ends there. Mobile Ad hoc Network (MANET) assume all nodes have similar functionality in terms of packet forwarding and other node attributes like power, mobility and reliability. On the other hand, WMNs have different network elements like client, wireless router and gateways with wide range of practical applications. Thus ad hoc networks can be considered as a special case of WMNs where only the client nodes are connected. MANET may or may not depend on wired infrastructure, where as mesh networks have wired gateways that they rely on. Further, the traffic in a MANET is between users where as in WMNs the traffic is between the client/user and the gateway. Last, but not

23 8 least, MANNET nodes typically have higher mobility in comparison to WMN routers WMNs Compared to Sensor Networks Sensor networks and WMNs are both multi-hop networks with intermediate nodes acting as routers. Both direct traffic from the client (users or sensors) towards a gateway or data acquisition device. However, there are significant difference mainly due to the intent and applications. Typically, sensor networks have low bandwidth in order of tens of Kilo Bits Per Second (Kbps) whereas WMNs typically are designed for higher bandwidth in excess of 1 Mbps. Sensor network nodes are constrained by power, hence power efficiency is a big design consideration. However, in WMNs power efficiently, while recommended, is not a design constraint. Finally, due to the fact that sensor networks collect data within their fixed sensory radius, they are normally stationary. In the case of WMNs the client nodes can be highly mobile. 1.4 Advantages of WMNs Self organizing and self configuring While dependant on the implementation and the protocols used, the ultimate goal of WMNs is to be self healing and self configuring. This reduces the setup time and maintenance cost. This also allows the network service providers the ability to change and adapt the network to meet the demands of the end users. Wireless mesh nodes are easy to install and un-install, making the network extremely adaptable and expandable as more or less coverage is needed.

24 Low deployment costs Since WMNs use wireless only routers, deploying them over larger areas of coverage is cheaper compared to single hop cellular networks. This is primarily due to fewer wired routers/access points that are more expensive to install and maintain. This coupled with easier maintenance leads to a lower operation cost Increased reliability WMNs have multiple paths between any given source and destination node. This allows for alternate routing in case the currently used route fails. Alternate routes can also serve as a means to balance the network load. If part of a network becomes congested, a communication pair can chose an alternate path thus minimizing bottlenecks. Load balancing via alternate routing, if implemented in WMNs, can significantly increase network reliability Scalability Unlike traditional wireless networks, as the number of nodes increases in a WMN, the greater its transmission capacity become. This is mainly achieved by better load balancing traffic along alternate routes. In some configurations, WMNs allow local networks to run faster, because local packets don t have to travel back to a central server. Obviously this is limited by the configuration and protocols that manage the contention domain of the medium.

25 Interoperability WMNs can work with existing WiFi and WiMaX standards, thus making them very attractive for incremental deployment and reuse of existing infrastructure. The IEEE set of standards actually has protocols defined that enables it to be configured as a WMN. Futher s, as discussed in Section defines protocols that will enable mesh networking on existing WiFi protocols. While it is necessary to improve protocols across most layers of the network stack, - most of the improvements can augment the current standards to maintain interoperability. 1.5 Applications of WMNs Broadband Wireless Access Broadband access has become critical for today s information economy. It enables applications like video on demand, video telephony, online-gaming and telecommunications. Each new application has a significant impact on quality of life. Telecommuting in particular can lead to increased productivity due to the time saved traveling back and forth from the place of work. It also reduces traffic on the roads during busy hours, thus having a positive impact on the environment. Studies have shown that people with broadband access versus those that use dial-up/narrow-band are more likely to publish content online and participate in social and community based activities [48]. Present day back-haul bandwidth is very large, in the order of 10 6 Gbps, primarily due to the capacity of optical fiber cables. However providing access to this large bandwidth to the end user needs expensive termination into each and every home and office. The current last mile technologies are highlighted in Table 1.2.

26 11 Technology Advantage Disadvantage Fiber to customer Very high bandwidth Very expensive - infrastructure costs and expensive Customer Premise equipment Cable/DOCSIS Uses existing infrastructure Shared bandwidth limits quality of service, reliability and scalability xdsl Uses existing copper Distance for Central office, un-bundling the local loop, not all copper is up to specification Satellite Access anywhere High investment cost, shared bandwidth, higher latency Wireless Low cost and fast to deploy Needs line of sight Table 1.2: Broadband Access Technology It is clear that wired access like Cable and DSL are economically feasible only in urban and sub-urban areas with a reasonably high population density. Rural area have limited coverage using wireless technologies like satellite and cellular networks. Satellite access is expensive and has much higher latency due to the distance between the end client and the satellite. In the case of cellular networks the towers are expensive to install and operate. As can be seen in Figure 1.3, rural areas in the US, have a much lower broadband access penetration - primarily due to the lack of service providers and the higher cost of the service itself. Further the problem of access is exacerbated, when considering access disparity at a global level as shown in Figure 1.4. Inadequate Internet access in rural areas and developing countries creates a greater digital divide between well connected towns and cities and rural areas, thus limits social communication and puts rural businesses at an disadvantage [36]. To foster the wider adoption of Internet access, WMNs offer a novel and cost effective

27 12 Figure 13: Main Reason for No High-Speed Internet Use at Home, Rural/Urban, 2009 Can Use Somewhere Else 3.6% Rural: 10.9 Million Households Not Available 11.1% No Computer or Computer Inadequate 16.3% Lack of Skill 2.3% Don't Need/Not Interested 37.7% Can Use Somewhere Else 4.7% Urban: 32.1 Million Households Not Available 1.1% No Computer or Computer Inadequate 19.0% Lack of Skill 3.2% Don't Need/Not Interested 38.1% Other 6.4% Too Expensive 22.3% Other 6.2% Too Expensive 27.6% Figure 1.3: Survey of broadband access in rural and urban areas [47] When other types of non-use are examined, however, the rankings can and do change. For example, respondents who do not use the Internet anywhere ranked the value proposition significantly higher than affordability. Figure 14: Main Reason Given for No Internet Use at Any Location, 2009 No Computer or Computer Inadequate 22.3% Lack of Skill 4.3% Too Expensive 18.6% Other 5.5% Not Available 0.7% Can Use Somewhere Else 1.4% Don t Need/ Not Interested 47.2% This contrasts with the category of households that do not access the Internet at home, which rated cost as the clear-cut top concern. Figure 1.4: Worldwide Internet usage peneration for 2010 (Courtesy: Internet World Stats) 13

28 alternative in areas where cable TV or DSL lines aren t available. They can be deployed 13 quickly and the service provider can see a quick return on investment. Other than network service providers, community mesh networks like [61] are also ways to provide broadband access to a local community. City wide municipal area networks that provide free broadband access to its residents are also become more feasible due to the lower deployment and operational costs of mesh networks. Many such networks already exist, and more are on the way Industrial Applications Healthcare: Many hospitals are spread out through clusters of densely constructed buildings that were not built with computer networks in mind. Wireless mesh nodes can sneak around corners and send signals short distances through thick glass to ensure access in every operating room, lab and office. The ability to connect to the network is crucial as more doctors and care-givers maintain and update patient information test results, medical history, even insurance information on portable electronic devices carried from room to room. Hospitality: High-speed Internet connectivity at hotels and resorts has become the rule, not the exception. Wireless mesh networks are quick and easy to set up indoors and outdoors without having to remodel existing structures or disrupt business. Temporary Venues: Construction sites can capitalize on the easy set-up and removal of wireless mesh networks. Architects and engineers can stay wired to the office, and Ethernet-powered surveillance cameras can decrease theft and vandalism. Mesh nodes can be moved around and supplemented as the construction project progresses.

29 14 Other temporary venues like street fairs, outdoor concerts and political rallies can set-up and tear down wireless mesh networks in minutes. Warehouses: There is simply no effective way to keep track of stock and shipping logistics without the types of Ethernet-enabled handheld scanners used in modern warehouses. Wireless mesh networks can ensure connectivity throughout a huge warehouse structure with little effort Future Applications: The U.S. military, which helped develop wireless mesh technology, foresees a day when thousands of microchip-size mesh nodes can be dropped onto a battlefield to set up instant scouting and surveillance networks. Information can be routed to both ground troops and headquarter personnel 1.6 Research Contributions In this research we have focused on the deployment, fault tolerance and quality of service issues in WMNs. We first investigated and developed a new heuristic which is able to places wired gateways subject to multiple constraints on delay and relay load. We then studied and developed routing and fault recovery algorithms in WMNs with a known brick topology. We also developed a probabilistic fault recover algorithm in more generalized WMNs while maintaining the QoS. The contribution of this dissertation is three-fold in both pre-deployment and postdeployment of wireless mesh networks under some QoS constraints. First, we investigate the problem of optimal placement of gateways in wireless mesh network with some QoS constraints including hop-count delays, relay capacity, and gateway (cluster) capacity.

30 15 We introduce a pre-deployment clustering algorithm for wireless mesh network that incorporates the QoS constraints into the clustering technique, and then incorporates links and node faults in post-deployment while keeping the QoS metrics intact. We further analyses the clustering approach in terms of its complexity and its limitation. This result has been published in [24] and [22, 25]. Second, we introduce a new pre-deployment near optimal gateway placement by finding the maximal clique among all disjoint clusters, while keeping the hop count constant to control the delay. In order to support fault tolerance for distant nodes from a cluster head (gateway), we allowed inter-cluster nodes to join neighboring clusters. We also considered overlapping clustering in which a node dynamically seeks membership in neighbor clusters. This results has been submitted in [26]. Third, we studied the problem of fault-tolerance has been investigated in the context or wireless cellular mesh network. This result has been published in [23] 1.7 Structure of this Dissertation In chapter 2, we discuss wireless mesh networks in depth. We see what makes them unique. We look at some of standardization efforts, review the enabling technologies and the various open research issue in deploying WMNs. We also look at some of the academic and commertial implementations. In chapter 3 we investigate one of the fundamental problems of placement of wired gateways in WMN deployment. We propose a multi-constraint QoS aware linear equation and present a heuristic to solve the equations. our algorithm to those proposed in literation. We also compare the performance of We show through simulation that our approach produces better placement of gateways in terms of the number of gateways

31 16 and overall network performance. Using wired gateways, typical deployments of WMNs use an overlay spanning tree that is associated with a wired gateway to deliver the mesh traffic. While this approach makes Quality of Service (QoS) and deployment more attainable, it introduces sparseness in the routing paths and that limits the alternate paths available to recover from failure. In chapter 4 we look a a special case of wireless Mesh networks, that make use of directional antennas to form a network topology with a known geometric properties. We propose and design a fault-tolerant WMN based on a hexagonal topology with multiradio and directional antennas. For this model, we introduce an addressing and routing scheme that simplifies the network operations. Further, we extend the routing approach to cope with one or multiple network link failure, as link failure is common in wireless networks. To address this, we exploit the regularity and multi-path characteristics of an augmented tri-sectioned hexagonal system to route around link or node failures. In chapter 5 we investigate fault tolerance in a WMN without any assumption about the topological properties. As mentioned, the overlay of spanning trees introduces sparseness in a WNM. We propose a reactive fault recovery algorithm that works on these overlay based WMNs. The algorithm attempts to recover from failed links or nodes by making minimally localized path and clustering changes to the network topology, while maintaining the initial QoS constraints. Through simulation, we show that using our approach makes WMNs more resilient to faults and gives better throughput/delay performance when compared to similar fault scenarios in a non-overlay WMN. In chapter 6 we a pre-deployment fault tolerance approach using clustering with overlapping. Based on our simulations we observe that nodes that are further way from

32 17 the gateway have are more likely to suffer a link failure. This is due to the increase number of hops and a larger contention domain. We present algorithms that allow for the boundary nodes to be members of adjacent clusters. This allows them to form alternate paths in case of congestion or link failure to their primary cluster gateway. Finally in Chapter 7 we summarize our finding and discuss ideas on how to extend this research.

33 CHAPTER 2 Background and Related work 2.1 Introduction As we have discussed in Chapter 1, the demand for ubiquitous and seamless broadband wireless Internet access has been the major driving force behind the development of multi-hop wireless mesh networks (WMNs). In this chapter we discuss the standardization efforts, academic testbeads and commercial deployments, and the problems and challenges associated with the performance of WMNs. While packet radio networks [6] have been extensively researched in different contexts for several decades, wireless packet networks pose several challenges to deploy, mainly due to the unreliable medium, limitation of radio technology, and power management. The hidden and exposed nodes problems, initially identified in [64], have been addressed by a 3-way hand shake in the IEEE family of protocols. Most early architectures were based on point-to-multipoint communications. In these networks, a wireless base station (access point) provides connectivity to a wired backbone network and served as an extension to the wired infrastructure. However, in the recent years, due to the advances in radio technology and lower communication cost, wireless mesh networks have been investigated as an alternative for point-to-multipoint and multipoint-to-multipoint systems. 18

34 WMN Standardization IEEE s WiFi Mesh The family of standards (802.11a/b/g) where intended to extend the range and connection flexibility of wired networks. Consequently, they were designed only for single hop operations. Networks based on this standard did not have any aspect that would enable the working of multi-hop networks. To overcome this limitation many companies like developed their own WMN solutions due to the flexibility and cost effectiveness WMNs offer compared to typical WiFi networks. Though most of these products enable the end client to connect using standard MAC, they are not interoperable. To fulfill the need and extend the coverage of , the work group was formed in The goal of the s standard include: Increasing range/coverage of existing networks Increases throughput and performance Increased flexibility in deployment and use Provide seamless security Maintain backward compatibility with the existing standards The s standard defines an Extended Service Set (ESS), and a Wireless Distribution System (WDS) that provides a protocol for auto-configuring paths between APs. over self-configuring multi-hop topologies in a WDS to support both broadcast/multicast and unicast traffic s has three key components:

35 20 1. Mesh Portal(MP): MP acts as a gateway/bridge to external networks. 2. Mesh STA (station): STA relays frames hop-by-hop in a router-like fashion. 3. Mesh AP (Access Point): AP provides relaying functions as well as the connectivity services for clients. The s protocols defines how the APs can discover the Mesh station (STA). It further defines an extensible path selection framework that allows routing between various STAs using Hybrid Wireless Mesh Protocol (HWMP) which is a combination of Ad hoc On Demand Distance Vector (AODV) [50] and tree-based proactive routing algorithms s also includes mechanisms to provide deterministic network access, a framework for congestion control, power saving and Quality of Service. As of July 2010, the standard is in a 6th version of the draft and the task group is working towards finalizing the standard by early 2011 [2] IEEE Bluetooth Bluetooth was developed as a replacement for wires and initially was more of a wire replacement technology and not a real networking standard. It was then standardized by the IEEE task group to form a Personal Area Network (PAN). The standard define the MAC and PHY protocols that target a bit rate of up to 1Mbps. This standard is not suitable for mesh networking due to low bandwidth and limited hardware support. However it does have a provision for defining multi-hop scatternets that allow for creating small mesh network that allow multiple devices within a very short range connect to each

36 21 other IEEE Zigbee Zigbee was initially proposed by Motorola as a way to support a class of sensory networks that have multi-month to multi-year lifetime using small batteries. This standard is used to create low data rate (20-250Kbps) sensory or Personal Area Networks. This standard supports mesh topology by defining a coordinator that is responsible for setting up the multi-hop network. This standard is very suitable for setting up sensor mesh networks IEEE WiMAX a is a wireless communications specification for metropolitan area networks (MANs). It was approved in January 2003 and released in April 2003 as part of a set of standards known as or WiMax. The standards complement the older (WiFi) family of specifications. The a standard was developed for wireless MANs operating on licensed and unlicensed radio-frequency (RF) bands between 2 GHz and 11 GHz, at data speeds of up to 75 megabits per second (Mbps), with low latency and efficient use of spectrum space. The security of is enhanced by encryption features. Forward error correction (FEC) and space/time coding optimize accuracy under marginal signal conditions. The maximum range can be extended to approximately 30 miles (48 kilometers) with some sacrifice in throughput. The a specification is ideally suited for advanced communications methods such as voice over IP (VoIP) and prioritized data traffic.

37 WMN Deployments Several academic institutes have created created mesh network testb beds. Additionally, comapanies have created software and equipment for commercial deployments. We list some of the most notable testbeds and companies below Academic Testbeds Roofnet (MIT) Roofnet is an experimental b/g mesh network in development at MIT which provides broadband Internet access to users in Cambridge, MA. There are currently around 40 active nodes in the network. Roofnet defines a routing and MAC protocols that enable based nodes to work in a multi-hop environment. The protocols include link-level measurements of , finding high-throughput routes in the face of lossy links, adaptive bit-rate selection. The software is available for public usage and new protocols are being developed that take advantage of radio s properties. We have also used this for our experimental test bed that is covered in Section Emulab (University of Utah) Emulab is a network test bed, setup initial at the University of Utah. The name Emulab refers both to a facility and to a software system. It provides three experimental environments - simulated, emulated, and infrastructure. Emulab unifies all of these environments under a common user interface, and integrates them into a common framework. This framework provides abstractions, services, and namespaces common to all, such as allocation and naming of nodes and links [63]. Currently the test bed provides access to hundreds of PCs (168 as of this writing), several with wireless NICs ( a/b/g) and a

38 wide-area network nodes geographically distributed across approximately 30 sites. 23 ORBIT (Rutgers Winlab) The ORBIT (Open-Access Research Testbed for Next-Generation Wireless Networks) radio grid emulator is an indoor wireless network test bed. ORBIT is a two-tier laboratory emulator/field trial network test bed designed to achieve reproducibility of experimentation, while also supporting evaluation of protocols and applications in real-world settings. The project was started in September 2003 under the NSF Network Research Test beds (NRT) program that was a collaborative effort of Rutgers, Columbia, Princeton, Lucent Bell Labs, Thomson and IBM Research. It currently has a wireless system of over 400 nodes supporting x set of protocols. Technology For All(TFA) (Rice University) The TFA project aims to develop fundamental information technology advances that address the unique needs of underserved communities and developing regions [62]. Since 2004, the project has operated a research test bed in an under-resourced community in Houston s East End. The network serves over 4,000 community residents via fully programmable network nodes. Community residents can access the network via legacy devices or custom instrumented mobile phones. The test bed jointly serves as a community resource and a platform for test-driving the above research advances Commercial Implementations The flexibility of deployment and the business opportunity, many companies have developed WMN solutions by extending standard protocols. Most of these are proprietary

39 24 extensions and are not interoperable between two different implementations. Tropos Networks Tropos is based in Sunnyvale, California and focuses on providing broadband mesh networks for cities and providing distributed area networks for building and utilities. They have over 27 patents that are productized as wireless mesh router hardware and a Tropos Mesh Operating System. The operating system creates a self-organizing and self-healing wireless mesh topology and intelligently selects the optimum path through the network. It leverages the router s on-board intelligence to monitor and maximize performance, minimizing network congestion and adapting in real-time basis to interference and other variables common in an outdoor environment. Currently, Tropos has several network deployments including Avista Utilities, Oklahoma City wireless network and Google s network in Mountain View, California. BelAir Networks BelAir is based in Ontario, Canada and has developed a patented WMN architecture. They provide b coverage for large zones - like large hotels, municipalities, etc. BelAir use wireless mesh routers (like BelAir 200) that have three radios and eight fixed directional antennas. They have implemented dynamic transmission power control and use the directional antennas to communicate with other mesh routers. They also use cross layer optimization to improve routing, by getting congestion and latency feedback from the PHY layer. They have many small and large mesh network deployments in educational institutes, municipal governments, hotels and other wireless internet access providers.

40 25 Firetide Firetide is a provider of hardware and software for wireless infrastructure mesh networks, based in Los Gatos, California. They have a software called HotView Pro that centrally manages the mesh routes that are based on their hardware platform and Mobility Controller that enables mesh networks to work on mobile units like mass transport. They also have developed a proprietary AutoMesh routing protocol that not only provides routing but also load balancing and congestion control. They have installations in airports like Singapore International Airport, mass transit system like Seoul Metropolitan Subway and educational institutions like California State University, Long Beach. Mesh Dynamics Mesh Dynamics is based in Santa Clara, California and has developed software and hardware to deploy wireless mesh networks. Their networks are compatible with a/b/g. They have developed proprietary software that uses multiple radios (1 to 4 radios) to create a dynamic tree tropology to implement WMNs. They also have developed protocols for dynamic channel selection, enabling them to maintain high level of throughput over multiple hops. Their technology is radio agnostic and they provide a centrally managed software to monitor the network. Mesh Dynamics have deployments in the area of video surveillance, broadband access, public safety and campus networks. They have broadband access deployments in Fresno, California and Red River, New Mexico where the subscriber density is very low and wired networks are not economically feasible.

41 26 Microsoft Microsoft has actively been researching WMN with an application focus on community access networks. They have developed software called Mesh Connectivity Layer (MCL) that enables Windows based computers to use multiple radio cards to increase throughput and act as mesh routers. They have developed a routing protocol called Link Quality Source Routing (LQSR) which is based on Dynamic Source Routing (DSR). It has been designed to be transparent to higher and lower layers and therefore can be used with existing software and hardware. Currently, there are no commercial implementations or products based on MCL. Apart for these companies - Intel, Philips, Motorola, Cisco and many other companies are working towards the standardization of mesh networks in the form of IEEE s family of protocols. 2.4 Problems and Challenges in WMNs WMNs have been made possible due to significant advances in current technology and the maturing of protocols. While siginificant advances have been made to make WMNs possible, many problems that still remain, to fully realized their potential. There challenges are at different layers of a WMN, like capacity management, fairness, addressing and routing, mobility management, energy management, service levels, integration with the Internet, etc. Many of these aspects have been briefly discussed in [34] Phycial Layer Issues The radio models used in modeling a WMN fundamentally define how the protocols and algorithms would work to develop an efficient wireless network. Each of these models