Dynamic association in wireless mesh networks

23 4 Factors Considered in the Association in Wireless Mesh Networks 25 4.1 Association Mechanisms in WMNs Proposed Previously.. All unique characteristics andadvantages of WMNs owe to t

DYNAMIC ASSOCIATION IN WIRELESS

MESH NETWORKS

Wang Hui (B.Eng, XJTU )

A THESIS SUBMITTEDFOR THE DEGREE OF MASTER OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2010

First of all, I would like to thank my supervisors, Prof Lawrence W C.WONG, Dr SOH Wee Seng and Dr Mehul MOTANI for their invaluable guidance,generous advice and unwavering patience throughout my research This thesis willnot be possible without their continuous support

I would also like to express my appreciation to my parents and the wholefamily It is them that give me endless love and encouragement throughout mylife

Also to all the staff and students in Ambient Intelligence Lab and cations Lab and all my friends Their precious friendship and encouragement haveput me through many tough times

Communi-Finally, I am grateful for the partial support from Interactive and DigitalMedia Institute, which was approved by my supervisors

ACKNOWLEDGEMENTS

2.2.2 Typical Features 11

2.2.3 Application Scenarios 12

2.3 Issues and Challenges 14

2.4 Standard Activities 17

2.4.1 IEEE 802.11s 17

2.4.2 Other Standards for Wireless Mesh Networks 18

2.5 Real Deployments 20

3 Association Mechanisms in Wireless Networks 21 3.1 Association Procedure in IEEE 802.11 Specifications 21

3.2 Association Mechanisms Already Proposed 22

3.3 Association Game in Wireless Networks 23

4 Factors Considered in the Association in Wireless Mesh Networks 25 4.1 Association Mechanisms in WMNs Proposed Previously 25

4.2 Link Quality 26

4.3 Load Balancing 27

4.4 Cross-layer Association 28

4.5 Reflecting Dynamic Network Conditions 29

4.6 Association Time Overhead 30

4.7 Accelerate Network Convergence Speed - Association Oscillation Avoidance 31

4.8 Conclusion 31

5 Dynamic Association in 802.11 Based Wireless Mesh Networks 33 5.1 System Model 33

5.2 Framework of Dynamic Association Mechanism 35

5.3 Elaboration of Dynamic Association Mechanism 36

5.3.1 Association Cost of the Access Link 36

5.3.2 Association Cost of the Multi-hop Wireless Backbone 39

5.3.3 Procedure of Dynamic Association Mechanism 40

5.3.4 Dynamic Re-association, Association Oscillation Avoidance and Network Convergence 42

5.4 Basic Analysis of Dynamic Association Mechanism 43

5.4.1 New Aspects of Dynamic Association 43

5.4.2 Basic Analysis 44

5.5 Conclusion 47

6 Evaluation of the Dynamic Association Mechanism 49 6.1 Introduction of Network Simulation Tools 49

6.2 Performance of the Association Mechanism 52

6.3 Dynamic Re-association versus Static Association 58

6.4 Oscillation Avoidance 60

6.5 Network Convergence 66

6.6 Conclusion 67

SUMMARY

Nowadays, network communications with end devices are increasingly wireless.IEEE 802.11 based wireless local area networks (WLANs) and cellular networkbased mobile phones are widely used in our everyday life With increasing de-mand for multimedia transmission and unrestrained roam, all the existing wirelessnetworks should be updated Hence, many standards for wireless networking arenow taking the next step to support mesh architectures in which data is commonlyforwarded on paths consisting of multiple wireless hops Wireless mesh networks(WMNs) are one of these new technologies which support higher link capacity andwider wireless network coverage

As a promising next generation wireless networking technology, WMNs havebeen attracting considerable research and industrial focus, and they are undergoingrapid progress and inspiring numerous applications All unique characteristics andadvantages of WMNs owe to the multi-hop wireless mesh backbone, which is formedthrough the self-organization of mesh routers Mesh end stations (STAs) shouldassociate with mesh access points (MAPs) to obtain network access and be part

of the network STAs can roam freely over the wireless network coverage area

by handoff among MAPs nearby Furthermore, WMNs can be integrated withother types of networks, such as Internet, Wi-Fi, WiMax, cellular networks and so

on, with gateway functionalities in some of the MAPs This unique architecturebrings in many advantages to WMNs Thanks to the multi-hop wireless backbone,WMNs enable rapid deployment with lower cost backhaul, self-healing, resilienceand good scalability Also, WMNs support widespread wireless network coveragedue to multi-hop forwarding, higher bandwidth due to shorter hops, better batterylife in end devices (e.g., STAs) due to lower power transmission

In order to enable these unique characteristics and advantages of WMNs, manyissues and challenges should be solved Many conventional management mecha-nisms which have significant effect on network performance should also be recon-sidered Such a mechanism is the association of STAs with MAPs of the wirelessbackbone Previous standards and literatures have already devised many associa-tion mechanisms in wireless networks IEEE 802.11 specifications define a simpleReceived Signal Strength Indication (RSSI) based association mechanism, in whichSTAs simply select the access point (AP) with the highest RSSI value to associatewith Other literatures either highlight link quality or load balancing solely or makesome impractical assumptions Several works on association mechanisms in WMNspropose a cross-layer framework considering jointly the association cost of accesslinks and cost of the multi-hop wireless backbone, which suits well in WMNs.Based on the cross-layer association framework, we propose a dynamic asso-ciation mechanism in the context of IEEE 802.11 based WMNs Our dynamicassociation mechanism takes wireless link quality, load balancing and associationoscillation avoidance into consideration The metric introduced in this associationmechanism measures the real traffic load through channel based load detectionand suits both coordinated and uncoordinated networks Because of the randomcharacteristics of wireless links and the variability of network conditions, we in-

troduce oscillation avoidance schemes, which consist of periodic STA scan andre-association threshold We further evaluate our dynamic association mechanismthrough elaborate simulation, which shows that the proposed dynamic associationmechanism outperforms other association mechanisms and improve network perfor-mance significantly Furthermore, our mechanism can accelerate the convergencespeed of WMNs The simulation additionally shows there exists optimal valuesfor both re-association threshold and STA scan period, corresponding to specificnetwork scenarios (e.g., network topology, scale, traffic load, etc.)

Our dynamic association mechanism characterizes the dynamic network narios in real time, hence improve network performances significantly But thereare still some further work to do in order to perfect our association mechanism, aswill be elaborated in the last chapter of future work

sce-SUMMARY

List of Figures

1.1 Taxonomy of wireless networks 2

2.1 Infrastructure/Backbone wireless mesh networks 9

2.2 Hybrid wireless mesh networks 10

5.1 System model for association in 802.11 based WMNs 34

5.2 Association in uncoordinated wireless mesh networks 46

6.1 Schematic of a mobilenode under the CMU monarch’s wireless ex-tensions to ns 50

6.2 Modified mobile node architecture supporting multiple interfaces 51

6.3 One of the random topologies of the Wireless Mesh Networks for performance evaluation 53

6.4 Channel based load detection versus cell based load detection 54

6.5 Basic performance evaluation with 8 cross flows 57

6.6 Basic performance evaluation with 8 edge flows 58

6.7 Basic performance evaluation with 8 parellel flows 58

6.8 Dynamic re-association versus static association 61

6.9 Oscillation avoidance 63

LIST OF FIGURES

List of Tables

List of Symbols

List of Symbols

List of Symbols

List of Symbols

ACW M N s

RCBackbone

List of Symbols

industrial alliances and forums are also established, such as WiMAX [1], Wi-Fi [2],[3], Bluetooth [4] and ZigBee [5], to promote the commercial application of all theproposed wireless technologies.

Table 1.1: Wireless Network Standards.

CHAPTER 1 Introduction

Actually, the most general form of wireless networks are ad-hoc networks.There are no constraints on network topology, node roles and services In ad-hoc networks, all nodes can move freely and all nodes are peers to each other

It is extraordinarily complex in this general form due to its lack of constraints.Hence many prerequisites and assumptions are introduced to simplify the studyand real application of this general wireless networks All other wireless networks insome sense could be considered as the specific examples and particular applicationoriented scenarios, which involves wireless sensor networks, commercial applications

of wireless standards (e.g., Wi-Fi, cellular networks) While most of the wirelesstechnologies applied widely today involve only one hop wireless links (connecting

to traditional wired networks) such as Wi-Fi, cellular networks and Bluetooth,multi-hop wireless networks are studied more and more due to their attractiveadvantages Therefore, wireless mesh networks (WMNs) [6], [7], [8], a simplifiedform of ad-hoc networks, are emerging as a promising wireless network technologywhich is attracting numerous academia and industrial interest All these wirelessnetworks can be categorized according to their unique characteristics as in Fig 1.1

Infrastructure-less

802.11 ZigBee Bluetooth Infrastructure-based

Infrastructure-less (MANET)

Wireless Mesh Networks Wireless Sensor Networks ZigBee

Fig 1.1: Taxonomy of wireless networks.

CHAPTER 1 Introduction

To date, the most broadly deployed wireless networks are wireless local areanetworks (WLANs), except cellular networks IEEE 802.11 WLANs are deployedwidely in campuses, communities and other public areas In this kind of WLANs,there exist two kinds of nodes, access points (APs) and end stations (STAs) STAsmust associate with APs, which are connected to the Internet directly using wiredcables, to obtain network access Because STAs’ traffic communications must gothrough APs, the association mechanism which is responsible for choosing APs isvery important in WLANs WMNs also involve association mechanisms for STAs

to choose mesh access points (MAPs) Similarly, association mechanisms are alsosignificant in WMNs

In this thesis, we propose a dynamic association mechanism in WMNs thattakes load balancing, link quality and association oscillation avoidance into con-sideration The metric introduced in this association mechanism measures the realtraffic load through channel based load detection and suits both coordinated anduncoordinated networks Because of the random characteristics of wireless linksand the variability of network conditions such as node mobility and traffic require-ments, dynamic re-association should be involved To avoid association oscillationduring re-association, we introduce oscillation avoidance schemes, which consist ofperiodic STA scan and re-association threshold The performance of this dynamicassociation mechanism is evaluated in the context of 802.11 based wireless meshnetworks

CHAPTER 1 Introduction

The rest of this thesis is organized as follows In chapter 2, WMNs are duced, which covers network architecture, typical features, application scenariosand advantages of WMNs Of course, there exist limitations and challenges inWMNs These should also be mentioned Nowadays, many existing standards arerevisited to extend support for mesh functionalities This brings out many newspecifications, such as IEEE 802.11s and mesh in 802.16 Furthermore, some citiesover the world have deployed real WMNs, which validate the advantage and feasible

intro-of WMNs

Association mechanisms in wireless networks, especially in WLANs, are sented in chapter 3 The classic association mechanism in the IEEE 802.11 specifi-cation is based on received signal strength indication (RSSI) STAs simply choosethe APs with the highest RSSI to associate with, which is simple but non-optimal.Therefore, there has been increasing interest in this topic and many new associa-tion mechanisms are designed Furthermore, the association procedure is modeled

pre-as an pre-association game in some literature to find the theoretical optimal solutions.The association mechanisms in WMNs are introduced in chapter 4 To im-prove the performance of WMNs, many factors should be considered during theassociation procedure Wireless link quality, load balancing, cross-layer associa-tion, association time overhead, how to reflect dynamic network conditions andaccelerating network convergence speed are analyzed here

Chapter 5 elaborates on our proposed association mechanism in 802.11 basedwireless mesh networks To explain our mechanism, the system model where our as-sociation mechanism is applied is introduced first Because our mechanism is based

on a cross-layer framework, the association costs of the access link and multi-hopwireless backbone are described respectively then Finally, the detailed procedure

CHAPTER 1 Introduction

and important factors we take into account are specified

To validate the advantages of our association mechanism, we evaluate it inchapter 6 Network simulator ns 2 and topology generator GT-ITM are used to-gether as the evaluation tool We have carried out a series of experiments to verifythe performance comprehensively, which involves performance of the basic associa-tion mechanism, dynamic re-association, oscillation avoidance schemes and networkconvergence speed and so on We obtain convincing results from these experimentsand are confident that our mechanism outperforms other existing mechanisms inWMNs

Chapter 7 states the conclusion and future work

CHAPTER 1 Introduction

CHAPTER 2 Wireless Mesh Networks

further integrated with gateway functionalities that can be connected to the ternet through wired cables All these features bring many advantages to WMNs,such as rapid deployment with lower cost backbone, easy to provide coverage inhard-to-wire areas, greater range due to multi-hop forwarding, higher bandwidthdue to shorter hops, etc

As a new networking technology, WMNs can be classified into three types interms of their architecture [6]

Infrastructure/Backbone WMNs In this architecture, mesh routers form

an multi-hop wireless infrastructure/backbone for STAs/mesh clients, as shown inFig 2.1 Without routing functions, all the STAs must associate with mesh routers

to access other networks and STAs Some mesh routers are equipped with way/bridge functionalities, which can be connected to other types of traditionalnetworks, such as Wi-Fi, cellular networks, WiMax, wireless sensor networks andthe Internet, etc with wireless technologies or wired cables Hence, in this kind ofWMN architecture, existing networks can be integrated together with the multi-hopwireless infrastructure/backbone, if gateway/bridge functionalities are provided

gate-Of course, the wireless backbone can be built using various types of radio gies, in addition to the most commonly used IEEE 802.11 technology In order tointegrate different networks and optimize the channel resource assignment, meshrouters may be equipped with multiple radios with different radio technologies Forconventional clients/STAs with the same radio technologies as mesh routers, they

technolo-CHAPTER 2 Wireless Mesh Networks

can directly communicate with mesh routers If different radios are used, STAsmust communicate with their base stations that have connections to mesh routerswith gateways

Internet

Wireless mesh backbone

Mesh router with gateway Mesh router

with gateway

Mesh router with

gateway/bridge Mesh router with

gateway/bridge Mesh router with

gateway/bridge

Mesh router with gateway/bridge Access point

Wi-Fi networks

Base station Cellular networks

Base station WiMax networks

Sensor

Sink node

Sensor networks Wireless clients

Fig 2.1: Infrastructure/Backbone wireless mesh networks.

Client WMNs In client WMNs, all STAs/client devices form a peer-to-peerwireless network and relay traffic for each other So, unlike the STAs in infras-tructure/backbone WMNs, the STAs here are not only the end user applicationsproviders, but also the traffic relays/routers Thus, a client WMN is actually thesame as traditional ad hoc network The STAs initiate and relay traffics themselves.Hybrid WMNs Hybrid WMNs are the combination of infrastructure andclient WMNs, which are depicted in Fig 2.2 In this architecture, there also exists

a multi-hop infrastructure/backbone, which can be used to integrate different types

of networks, with some of the mesh routers equipped with gateway functionalities

CHAPTER 2 Wireless Mesh Networks

While as in client WMNs, STAs in hybrid WMNs can also act as traffic routersfor other STAs This means STAs can communicate with each other without thehelp of mesh routers Anyhow, if STAs want to communicate with STAs in othernetworks, they should associate with mesh routers directly or indirectly Combiningthe advantages of both infrastructure and client WMNs, hybrid WMNs are veryflexible and will be the most applicable case While nowadays, due to the widedeployment of IEEE 802.11 based WLANs, 802.11 based WMNs are proposed as themost realistic implementation of WMNs, to maintain the backward compatibilitywith existing WLANs So, in our study of the association mechanisms in WMNs,

we take the infrastructure/backbone architecture as the association context

Internet

Wireless mesh backbone

Mesh router with gateway Mesh router

with gateway

Mesh router with

gateway/bridge Mesh router with

gateway/bridge Mesh router

Wireless mesh clients

Fig 2.2: Hybrid wireless mesh networks.

CHAPTER 2 Wireless Mesh Networks

Capabilities of self-organization, auto-configuration and self-healing.All the mesh routers self-organize to form the multi-hop mesh backbone, and theyauto-configure and self-heal themselves to maintain the mesh connectivity, in casesome mesh routers were down This self-organizing and flexible architecture enablesWMNs’ rapid deployment with lower upfront cost and gradual growth as needed.Mobility depending on the type of mesh nodes The two types ofmesh nodes in WMNs have different mobility characteristics Mesh routers in themulti-hop mesh backbone have minimal mobility, while STAs/mesh clients can bestationary or mobile This simplifies the design of the management protocols (eg.routing protocols, channel assignment mechanisms, etc.) while not constrainingthe mobility freedom of STAs

Multiple types of network access In WMNs, STAs can access the Internetthrough multi-hop mesh backbone and communicate with each other through both

integration of WMNs with other wireless networks enables STAs to access theapplications provided by other networks

Power consumption constraints depending on the type of mesh nodes

CHAPTER 2 Wireless Mesh Networks

The two types of mesh nodes in WMNs also have different power consumptionconstraints Mesh routers are usually powered by power cables and do not havestrict constraints on power consumption However, STAs (eg hand phones, PDAs,laptops, etc.) may require power efficient protocols to facility their free mobil-ity Therefore, the protocols applied in mesh routers and STAs shall be designedrespectively, because of their unique requirements

Compatibility and interoperability with existing wireless networks

To promote the realization and popularity of WMNs, they must be backward patible with existing wireless networks, such as Wi-Fi, cellular networks, Bluetooth,etc when integrating with these existing networks

The unique characteristics and advantages make WMNs a promising networktechnology in many applications Actually, the research and development of WMNsare motivated by real applications which cannot be full satisfied by traditionalwireless networks Therefore, WMNs are introduced accordingly and aimed tofulfill these real application scenarios

Residential networking Nowadays, residential networking is realized throughIEEE 802.11 WLANs While residential WLANs usually have dead zones withoutservice coverage Deploying multiple APs may solve this problem but is very ex-pensive In addition, all APs should be connected to the Internet though wiredEthernet, which makes it also inconvenient A WMN is a desirable candidate insuch a scenario The primary purposes for WMNs are to create low cost, eas-ily deployable, high performance wireless coverage throughout the home WMNsshould help to eliminate service dead-spots and areas of low quality wireless cov-

CHAPTER 2 Wireless Mesh Networks

erage throughout the home In WMNs, STAs could communicate with each otherthrough a multi-hop mesh backbone rather than going back to the wired backhaulnetwork access modem or hub Hence, network congestion due to backhaul accesscan be avoided All these enable WMNs to support bandwidth demanding appli-cations such as video transfer within home networks Consequently, WMNs suitsresidential networking well

Community networking The common solutions for network access in munities are based on cables or DSL connecting to the Internet, and the last hop

com-is wireless by connecting a wireless router to a cable or DSL modem Thcom-is kind

of solutions has several drawbacks, such as having all traffic flowing through theInternet, much area in between houses is not covered by wireless services and only

a single path may be available for a home to communicate with the Internet orneighbors, etc WMNs can mitigate these drawbacks through self-organized meshconnectivity between homes In community networking, WMNs rapidly provideconnectivity to locations where the wired infrastructure is not available or is cost

ad-vanced applications/services through ubiquitous access and reliable connectivity,compared with the single access path in traditional community networking.Metropolitan area networks Metropolitan area network is one of WMNs’typical application scenarios WMNs outperform existing metropolitan area net-works in several aspects The data rate provided by WMNs is much higher thantraditional networks such as cellular networks Multi-hop wireless mesh backbonetaking the place of wired backhaul significantly decreases the deployment cost,which makes WMNs economical alternatives to broadband networking, especially

in developing countries and cities Furthermore, WMNs scale very well due to

CHAPTER 2 Wireless Mesh Networks

the flexible wireless mesh backbone, which is very important for metropolitan areanetworks

Transportation systems Currently, limited IEEE 802.11 or 802.16 networkaccess is available at stations, stops and buses in some cities WMNs can extendnetwork access into buses, trains, even cars Thus, convenient passenger informa-tion services, remote monitoring of in-vehicle security video and driver communi-cations can be supported To enable such WMNs in transportation systems, highspeed mobile backhaul from a vehicle (bus, train or car) to the Internet are needed.Public safety usage case Public safety mesh networks provide wirelessnetwork access to emergency and municipal safety personnel such as fire fighters,policemen and emergency workers responding to an incident The network may

be used for video surveillance, tracking emergency workers with sensors, voice anddata communication between emergency workers, uploading images, downloadinginformation, etc

Military usage Military usage of WMNs has much more requirements Itinvolves more node mobility, a heavy reliance on fully automated network manage-ment, self-healing property and power constraint

The distinct features and advantages of WMNs bring many issues and lenges to be solved when building a large scale high performance wireless meshnetworks The issues and challenges exist at all layers of the ISO model Here itfollows a bottom-up layered approach to elaborate these issues and challenges

chal-A Physical Layer

CHAPTER 2 Wireless Mesh Networks

To increase the capacity of WMNs, challenges at the physical layer are similar

to that in other wireless networks All advanced physical layer techniques can beused in WMNs Schemes such as Frequency Hopping Spread Spectrum (FHSS), Or-thogonal Frequency Division Multiplexing (OFDM) and Ultra-Wide Band (UWB)are commonly applied to increase the reliability of the high speed transmission

In order to mitigate the wireless interference, multi-channel, multi-radio, MIMOand directional antennae can be considered Besides, several other characteristicsshall be taken into account, which are mobility, link adaption, variable transmissionpower, multiple transceivers and link quality feedback, etc

B MAC Layer

Due to the advanced underlying physical layer techniques, designing an ficient MAC protocol is a challenging task Furthermore, the distinct features ofWMNs such as multi-hop, self-organization and mobility make MAC design an eventougher problem MAC protocols in WMNs can be single-channel or multi-channel

ef-In the multi-channel case, how to assign channels to nodes and transceivers ciently so as to maximize the network capacity and minimize the interference iscritical When directional or smart antennae are used, cross-layer design is re-quired In addition, WMNs consist of hundreds of nodes which are distributed in arelatively wide area So, the deployed MAC protocol must be scalable, which im-plies a distributed MAC protocol may be better During the design of an efficientMAC protocol, self-organization must be supported by the MAC protocol, andproblems in network layer should also be considered, because the formed topologymay impact the routing algorithm

effi-C Network Layer

Although WMNs and ad hoc networks are both multi-hopped, the traffic

re-CHAPTER 2 Wireless Mesh Networks

quirements of them are different In WMNs, most of the traffic is between gatewaysand mesh clients, while in ad hoc networks, traffic is flowing between arbitrary pair

of nodes Additionally, nodes mobility situations in WMNs and ad hoc are verydifferent Hence, the routing algorithms proposed for ad hoc networks may notwork well in WMNs Specific customized routing protocol shall outperform general

ad hoc routing protocols Furthermore, conventional routing metrics (e.g., hopcount) may be inefficient in WMNs Some new routing metrics (e.g., link quality,loss rate, etc.) should be considered Besides, fairness may be another concern

in routing design in WMNs Because users relaying traffic for the source clientalong the route between source client and gateway may starve the source client bysending their own data, which may be more serious if all nodes just use a single for-warding queue Of course, scalability, robustness, reliability and flexibility should

be kept in mind also Multi-radio routing, multi-path routing, hierarchical routingand geographic routing are all the open research issues in WMNs

D Transport Layer

Today, no specific transport protocol has been proposed for WMNs Due tothe distinct characteristics of WMNs, the current widely deployed TCP transportprotocol in Internet can not be used in WMNs directly TCP is designed specificallyfor wired networks, in which the packet losses are mostly caused by buffer overflow

in routers While this prerequisite is not true in WMNs In WMNs, packet lossesmay be caused by poor wireless links, medium access contention or user mobility

So, TCP cannot be used in WMNs New transport protocols shall be designedspecifically for WMNs, or an adaptive TCP may work well in WMNs

E Other Challenges

Additionally, other challenges such as security, authentication and privacy

CHAPTER 2 Wireless Mesh Networks

should not be neglected

is extended and multi-hop wireless backbone takes the place of traditional wiredbackbone All these are introduced in IEEE 802.11s [10], [11] In IEEE 802.11smesh networks, there exists three kinds of nodes: mesh access points (MAPs), meshpoints (MPs) and STAs MAPs and MPs self-organize to form the multi-hop wire-less backbone and relay end-to-end traffic from and to STAs Whereas MPs justfunction as traffic relay, MAPs also provide wireless access links to STAs STAs areunaware of the backbone connectivity and not involved in routing procedure Theyassociate with MAPs to obtain network access Some of the MAPs are integratedwith gateway functionalities and named as mesh portals They can connect tothe Internet and other types of networks In order to determine the best path be-

CHAPTER 2 Wireless Mesh Networks

tween STAs or between STA and other networks (e.g., the Internet), IEEE 802.11sproposes Hybrid Wireless Mesh routing protocol (HWMP) as the default routingprotocol HWMP combines the flexibility of on-demand route discovery with theefficiency of proactive routing to a mesh portal When determining the best route,HWMP applies a simple metric based on airtime as default, with support for othermetrics Unlike in traditional ad hoc networks, where all nodes are involved inrouting procedure, in IEEE 802.11s mesh networks, to find the route to the desti-nation STA, the associated MAP of the source STA must get knowledge of whichMAP is associated by the destination STA To handle this association information,Local Association Base (LAB) and Global Association Base (GAB) are introduced

to MAPs Referring to the LAB and GAB, MAPs can find the destination MAP

to which the final STA is associated with Due to the aforementioned unique acteristics of 802.11 based wireless mesh networks, some conventional managementprocedures especially the association mechanism that affects the network perfor-mance significantly should be reconsidered

Although IEEE 802.11s standard is relatively mature within all emerging dards which support mesh networking, there are other standards for WMNs.IEEE 802.15.5 in WPAN [12] In November 2003 the IEEE 802.15.5 MeshNetwork Task Group was formed to determine the necessary mechanisms that must

stan-be presented in the PHY and MAC layers of WPANs to enable mesh networking.The use of mesh networking in WPAN environments is motivated by the power lim-itations of mobile devices Specifically, applying multi-hop mesh communicationsincreases the coverage of WPANs and allows shorter links to be used, which pro-

CHAPTER 2 Wireless Mesh Networks

vides both higher throughput and lower transmission power Actually, the currentIEEE 802.15.1 (Bluetooth) and IEEE 802.15.4 (Zigbee) standards already partiallysupport mesh networking, although they are not exactly as “mesh” as aforemen-tioned In IEEE 802.15.1, it supports a cluster architecture which is called piconets.All the piconets (a quasi-cluster) can be inter-connected to form a mesh network.While in IEEE 802.15.4, it supports three topologies, star, tree and mesh But thismesh topology is flat, not like WMNs’ which can be hierarchical

IEEE 802.16 in WMAN [1] In IEEE 802.16, WiMax supports mesh ation mode, besides the basic point-to-multipoint (PMP) mode Unlike the MACprotocols in other wireless networks, WiMax applies a time division multiple access(TDMA) based MAC to support mesh networking In mesh mode, all subscriberstations (SSs) may have direct links with other SSs, and the data traffic can berouted through other SSs and occur directly between SSs Due to the TDMA basedMAC, link scheduling mechanisms should be provided There exist two kinds oflink scheduling in WiMax, centralized and distributed algorithms Although thedefinitive standards have already been released, the protocols and mechanisms inWiMax are still under study

oper-IEEE 802.20 [13] In December 2002, oper-IEEE 802.20, the Mobile BroadbandWireless Access (MBWA) Working Group was established IEEE 802.20 systemsare intended to provide ubiquitous mobile broadband wireless network access in acellular architecture, supporting the mesh networking in both indoor and outdoorscenarios

CHAPTER 2 Wireless Mesh Networks

Although there exist many issues and problems to bring WMNs into tion, the unique characteristics and advantages of WMNs have been attractingmany companies to commercialize WMNs applications, such as Strix, Nortel, etc.There are also some cities across the world which have already deployed city wideWMNs to facilitate public network access Take Oulu, the largest city in northernFinland for example, the people in Oulu Finland have free access to the Internetthrough wireless services almost everywhere in the city It’s due to the city wideoutdoor WMNs using Strix systems’ technology