Computer Networks 47 (2005) 445–487
`
`www.elsevier.com/locate/comnet
`
`Wireless mesh networks: a survey
`
`Ian F. Akyildiz a, Xudong Wang b,*, Weilin Wang b
`
`a Broadband and Wireless Networking (BWN) Lab, School of Electrical and Computer Engineering,
`Georgia Institute of Technology, Atlanta, GA 30332, USA
`b Kiyon, Inc., 4225 Executive Square, Suite 290, La Jolla, CA 92037, USA
`
`Received 1 June 2004; received in revised form 1 November 2004; accepted 20 December 2004
`Available online 1 January 2005
`
`Abstract
`
`Wireless mesh networks (WMNs) consist of mesh routers and mesh clients, where mesh routers have minimal mobi-
`lity and form the backbone of WMNs. They provide network access for both mesh and conventional clients. The inte-
`gration of WMNs with other networks such as the Internet, cellular, IEEE 802.11, IEEE 802.15, IEEE 802.16, sensor
`networks, etc., can be accomplished through the gateway and bridging functions in the mesh routers. Mesh clients can
`be either stationary or mobile, and can form a client mesh network among themselves and with mesh routers. WMNs
`are anticipated to resolve the limitations and to significantly improve the performance of ad hoc networks, wireless local
`area networks (WLANs), wireless personal area networks (WPANs), and wireless metropolitan area networks
`(WMANs). They are undergoing rapid progress and inspiring numerous deployments. WMNs will deliver wireless ser-
`vices for a large variety of applications in personal, local, campus, and metropolitan areas. Despite recent advances in
`wireless mesh networking, many research challenges remain in all protocol layers. This paper presents a detailed study
`on recent advances and open research issues in WMNs. System architectures and applications of WMNs are described,
`followed by discussing the critical factors influencing protocol design. Theoretical network capacity and the state-of-
`the-art protocols for WMNs are explored with an objective to point out a number of open research issues. Finally, test-
`beds, industrial practice, and current standard activities related to WMNs are highlighted.
`Ó 2004 Elsevier B.V. All rights reserved.
`
`Keywords: Wireless mesh networks; Ad hoc networks; Wireless sensor networks; Medium access control; Routing protocol; Transport
`protocol; Scalability; Security; Power management and control; Timing synchronization
`
`* Corresponding author. Tel.: +1 425 442 5039.
`E-mail addresses: ian@ece.gatech.edu (I.F. Akyildiz), wxudong@ieee.org (X. Wang).
`
`1389-1286/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
`doi:10.1016/j.comnet.2004.12.001
`
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`1. Introduction
`
`As various wireless networks evolve into the
`next generation to provide better services, a key
`technology, wireless mesh networks
`(WMNs),
`has emerged recently. In WMNs, nodes are com-
`prised of mesh routers and mesh clients. Each
`node operates not only as a host but also as a rou-
`ter, forwarding packets on behalf of other nodes
`that may not be within direct wireless transmission
`range of their destinations. A WMN is dynami-
`cally self-organized and self-configured, with the
`nodes in the network automatically establishing
`and maintaining mesh connectivity among them-
`selves (creating, in effect, an ad hoc network). This
`feature brings many advantages to WMNs such as
`low up-front cost, easy network maintenance,
`robustness, and reliable service coverage.
`laptops,
`Conventional nodes (e.g., desktops,
`PDAs, PocketPCs, phones, etc.) equipped with
`wireless network interface cards (NICs) can con-
`nect directly to wireless mesh routers. Customers
`without wireless NICs can access WMNs by con-
`necting to wireless mesh routers through,
`for
`example, Ethernet. Thus, WMNs will greatly help
`the users to be always-on-line anywhere anytime.
`Moreover, the gateway/bridge functionalities in
`mesh routers enable the integration of WMNs
`with various existing wireless networks such as
`cellular, wireless sensor, wireless-fidelity (Wi-Fi)
`[136], worldwide inter-operability for microwave
`access (WiMAX) [137], WiMedia [138] networks.
`Consequently, through an integrated WMN, the
`users of existing network can be provided with
`otherwise impossible services of these networks.
`WMN is a promising wireless technology for
`numerous applications [98], e.g., broadband home
`networking, community and neighborhood net-
`works, enterprise networking, building automa-
`tion, etc. It is gaining significant attention as a
`possible way for cash strapped Internet service
`providers (ISPs), carriers, and others to roll out ro-
`bust and reliable wireless broadband service access
`in a way that needs minimal up-front investments.
`With the capability of self-organization and self-
`configuration, WMNs can be deployed incremen-
`tally, one node at a time, as needed. As more nodes
`
`are installed, the reliability and connectivity for the
`users increase accordingly.
`Deploying a WMN is not too difficult, because
`all the required components are already available
`in the form of ad hoc network routing protocols,
`IEEE 802.11 MAC protocol, wired equivalent pri-
`vacy (WEP) security, etc. Several companies have
`already realized the potential of this technology
`and offer wireless mesh networking products. A
`few testbeds have been established in university re-
`search labs. However, to make a WMN be all it
`can be, considerable research efforts are still
`needed. For example, the available MAC and
`routing protocols applied to WMNs do not have
`enough scalability; the throughput drops signifi-
`cantly as the number of nodes or hops in a
`WMN increases. Similar problems exist in other
`networking protocols. Consequently, all existing
`protocols from the application layer to transport,
`network MAC, and physical layers need to be en-
`hanced or re-invented.
`Researchers have started to revisit the protocol
`design of existing wireless networks, especially of
`IEEE 802.11 networks, ad hoc networks, and wire-
`less sensor networks,
`from the perspective of
`WMNs. Industrial standards groups are also ac-
`tively working on new specifications for mesh net-
`working. For example, IEEE 802.11 [64,74], IEEE
`802.15 [65,79], and IEEE 802.16 [66,111,135] all
`have established sub-working groups to focus on
`new standards for WMNs.
`The remainder of the paper is organized as fol-
`lows. In Section 2, we present possible system
`architectures of WMNs. The characteristics of
`WMNs are summarized in Section 3, where a com-
`parison between WMNs and ad hoc networks is
`also conducted. In Section 4, different application
`scenarios of WMNs are addressed. Critical factors
`influencing protocol design are emphasized in Sec-
`tion 5. We discuss fundamental issues such as net-
`work capacity and optimal node density of WMNs
`in Section 6. Recent advances in protocol design
`for WMNs are investigated in Sections 7–15,
`where protocols on both data and management
`planes are covered and challenging research issues
`in all these aspects are discussed. Several testbeds
`and implementation practice of WMNs are pre-
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`not exist in these nodes. In addition, mesh clients
`usually have only one wireless interface. As a con-
`sequence, the hardware platform and the software
`for mesh clients can be much simpler than those
`for mesh routers. Mesh clients have a higher vari-
`ety of devices compared to mesh routers. They can
`be a laptop/desktop PC, pocket PC, PDA, IP
`phone, RFID reader, BACnet (building automa-
`tion and control networks) controller, and many
`other devices, as shown in Fig. 2.
`The architecture of WMNs can be classified
`into three main groups based on the functionality
`of the nodes:
`
`• Infrastructure/Backbone WMNs. The architec-
`ture is shown in Fig. 3, where dash and solid
`lines indicate wireless and wired links, respec-
`tively. This type of WMNs includes mesh rou-
`ters forming an infrastructure for clients that
`connect to them. The WMN infrastructure/
`backbone can be built using various types of
`radio technologies, in addition to the mostly
`used IEEE 802.11 technologies. The mesh rou-
`ters form a mesh of self-configuring, self-healing
`links among themselves. With gateway func-
`tionality, mesh routers can be connected to
`the Internet. This approach, also referred to as
`
`sented in Section 16. Current status of standard
`activities in WMNs is highlighted in Section 17.
`The paper is concluded in Section 18.
`
`2. Network architecture
`
`WMNs consist of two types of nodes: mesh rou-
`ters and mesh clients. Other than the routing capa-
`bility for gateway/repeater
`functions as
`in a
`conventional wireless router, a wireless mesh rou-
`ter contains additional routing functions to sup-
`port mesh networking. To further improve the
`flexibility of mesh networking, a mesh router is
`usually equipped with multiple wireless interfaces
`built on either the same or different wireless access
`technologies. Compared with a conventional wire-
`less router, a wireless mesh router can achieve the
`same coverage with much lower transmission power
`through multi-hop communications. Optionally,
`the medium access control (MAC) protocol in a
`mesh router is enhanced with better scalability in a
`multi-hop mesh environment.
`In spite of all these differences, mesh and con-
`ventional wireless routers are usually built based
`on a similar hardware platform. Mesh routers
`can be built based on dedicated computer systems
`(e.g., embedded systems) and look compact, as
`shown in Fig. 1. They can also be built based on
`general-purpose computer systems (e.g.,
`laptop/
`desktop PC).
`Mesh clients also have necessary functions for
`mesh networking, and thus, can also work as a
`router. However, gateway or bridge functions do
`
`Fig. 1. Examples of mesh routers based on different embedded
`systems:
`(a) PowerPC and (b) Advanced Risc Machines
`(ARM).
`
`Fig. 2. Examples of mesh clients: (a) Laptop, (b) PDA, (c) Wi-
`Fi IP Phone and (d) Wi-Fi RFID Reader.
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`Fig. 3. Infrastructure/backbone WMNs.
`
`infrastructure meshing, provides backbone for
`conventional clients and enables integration of
`WMNs with existing wireless networks, through
`gateway/bridge functionalities in mesh routers.
`Conventional clients with Ethernet
`interface
`can be connected to mesh routers via Ethernet
`links. For conventional clients with the same
`radio technologies as mesh routers, they can
`directly communicate with mesh routers. If dif-
`ferent radio technologies are used, clients must
`communicate with the base stations that have
`Ethernet connections to mesh routers.
`Infrastructure/Backbone WMNs are the most
`commonly used type. For example, community
`and neighborhood networks can be built using
`infrastructure meshing. The mesh routers are
`placed on the roof of houses in a neighborhood,
`which serve as access points for users inside the
`homes and along the roads. Typically, two
`types of radios are used in the routers, i.e., for
`backbone communication and for user commu-
`nication, respectively. The mesh backbone com-
`munication can be established using long-range
`communication techniques
`including direc-
`tional antennas.
`
`• Client WMNs. Client meshing provides peer-to-
`peer networks among client devices. In this type
`of architecture, client nodes constitute the
`actual network to perform routing and configu-
`ration functionalities as well as providing end-
`user applications to customers. Hence, a mesh
`router is not required for these types of net-
`works. The basic architecture is shown in Fig.
`4. In Client WMNs, a packet destined to a node
`in the network hops through multiple nodes to
`reach the destination. Client WMNs are usually
`formed using one type of radios on devices.
`Moreover, the requirements on end-user devices
`is increased when compared to infrastructure
`meshing, since, in Client WMNs, the end-users
`
`Fig. 4. Client WMNs.
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`Fig. 5. Hybrid WMNs.
`
`must perform additional functions such as rout-
`ing and self-configuration.
`• Hybrid WMNs. This architecture is the combi-
`nation of infrastructure and client meshing as
`shown in Fig. 5. Mesh clients can access the net-
`work through mesh routers as well as directly
`meshing with other mesh clients. While the
`infrastructure provides connectivity to other
`networks such as the Internet, Wi-Fi, WiMAX,
`cellular, and sensor networks; the routing capa-
`bilities of clients provide improved connectivity
`and coverage inside the WMN. The hybrid
`architecture will be the most applicable case in
`our opinion.
`
`3. Characteristics
`
`The characteristics of WMNs 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-of-sight
`
`(LOS) links. To meet these requirements, the
`mesh-style multi-hopping is indispensable [85],
`which achieves higher throughput without sac-
`rificing effective radio range via shorter link dis-
`tances, less interference between the nodes, and
`more efficient frequency re-use.
`• Support for ad hoc networking, and capability of
`self-forming, self-healing, and self-organization.
`WMNs enhance network performance, because
`of flexible network architecture, easy deploy-
`ment and configuration, fault tolerance, and
`mesh connectivity,
`i.e., multipoint-to-multi-
`point communications [128]. Due to these fea-
`tures, WMNs have low upfront
`investment
`requirement, and the network can grow gradu-
`ally as needed.
`• Mobility dependence on the type of mesh nodes.
`Mesh routers usually have minimal mobility,
`while mesh clients can be stationary or mobile
`nodes.
`• Multiple types of network access. In WMNs,
`both backhaul access to the Internet and peer-
`to-peer (P2P) communications are supported
`[75]. In addition, the integration of WMNs with
`other wireless networks and providing services
`to end-users of these networks can be accom-
`plished through WMNs.
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`• Dependence of power-consumption constraints on
`the type of mesh nodes. Mesh routers usually
`do not have strict constraints on power con-
`sumption. However, mesh clients may require
`power efficient protocols. As an example, a
`mesh-capable sensor [113,114] requires its com-
`munication 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 802.11 technologies [133,69]
`must be compatible with IEEE 802.11 stan-
`dards in the sense of supporting both mesh-
`capable and conventional Wi-Fi clients. Such
`WMNs also need to be inter-operable with
`other wireless networks such as WiMAX, Zig-
`Bee [148], and cellular networks.
`
`Based on their characteristics, WMNs are gen-
`erally considered as a type of ad-hoc networks
`due to the lack of wired infrastructure that exists
`in cellular or Wi-Fi networks through deployment
`of base stations or access points. While ad hoc net-
`working techniques are required by WMNs, the
`additional capabilities necessitate more sophisti-
`cated algorithms and design principles for the real-
`ization of WMNs. More specifically, instead of
`being a type of ad-hoc networking, WMNs aim
`to diversify the capabilities of ad hoc networks.
`Consequently, ad hoc networks can actually be
`considered as a subset of WMNs. To illustrate this
`point, the differences between WMNs and ad hoc
`networks are outlined below. In this comparison,
`the hybrid architecture is considered, since it
`comprises all the advantages of WMNs.
`
`• Wireless infrastructure/backbone. As discussed
`before, WMNs consist of a wireless backbone
`with mesh routers. The wireless backbone pro-
`vides large coverage, connectivity, and robust-
`ness in the wireless domain. However,
`the
`connectivity in ad hoc networks depends on
`the individual contributions of end-users which
`may not be reliable.
`
`• Integration. WMNs support conventional cli-
`ents that use the same radio technologies as a
`mesh router. This is accomplished through a
`host-routing function available in mesh rou-
`ters. WMNs also enable integration of various
`existing networks such as Wi-Fi, the Inter-
`net, cellular and sensor networks through gate-
`way/bridge functionalities in the mesh routers.
`Consequently, users in one network are pro-
`vided with services in other networks, through
`the use of the wireless infrastructure. The inte-
`grated wireless networks
`through WMNs
`resembles the Internet backbone, since the
`physical location of network nodes becomes less
`important
`than the capacity and network
`topology.
`• Dedicated routing and configuration. In ad hoc
`networks, end-user devices also perform routing
`and configuration functionalities for all other
`nodes. However, WMNs contain mesh routers
`for these functionalities. Hence, the load on
`end-user devices
`is
`significantly decreased,
`which provides
`lower
`energy consumption
`and high-end application capabilities to possi-
`bly mobile and energy constrained end-users.
`Moreover, the end-user requirements are lim-
`ited which decreases the cost of devices that
`can be used in WMNs.
`• Multiple radios. As discussed before, mesh rou-
`ters can be equipped with multiple radios to
`perform routing and access functionalities. This
`enables separation of two main types of traffic
`in the wireless domain. While routing and
`configuration are performed between mesh rou-
`ters, the access to the network by end users
`can be carried out on a different radio. This sig-
`nificantly improves the capacity of the net-
`work. On the other hand, in ad hoc networks,
`these functionalities are performed in the same
`channel, and as a result,
`the performance
`decreases.
`• Mobility. Since ad hoc networks provide
`routing using the end-user devices, the net-
`work topology and connectivity depend on
`the movement of users. This imposes addi-
`tional
`challenges on routing protocols as
`well
`as
`on
`network
`configuration
`and
`deployment.
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`4. Application scenarios
`
`Research and development of WMNs is moti-
`vated by several applications which clearly demon-
`strate 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, stan-
`dard IEEE 802.11, etc. In this section, we discuss
`these applications.
`
`• Broadband home networking. Currently broad-
`band home networking is realized through
`IEEE 802.11 WLANs. An obvious problem is
`the location of the access points. Without a site
`survey, a home (even a small one) usually has
`many dead zones without service coverage.
`Solutions based on site survey are expensive
`and not practical for home networking, while
`installation of multiple access points is also
`expensive and not convenient because of Ether-
`net wiring from access points to backhaul net-
`work access modem or hub. Moreover,
`communications between end nodes under two
`different access points have to go all the way
`back to the access hub. This is obviously not
`an efficient solution, especially for broadband
`networking. Mesh networking, as shown in
`Fig. 6, can resolve all these issues in home
`networking.
`The access points must be replaced by wireless
`mesh routers with mesh connectivity established
`
`Fig. 6. WMNs for broadband home networking.
`
`among them. Therefore, the communication be-
`tween these nodes becomes much more flexible
`and more robust to network faults and link fail-
`ures. Dead zones can be eliminated by adding
`mesh routers, changing locations of mesh rou-
`ters, or automatically adjusting power levels of
`mesh routers. Communication within home net-
`works can be realized through mesh networking
`without going back to the access hub all the
`time. Thus, network congestion due to back-
`haul access can be avoided. In this application,
`wireless mesh routers have no constraints on
`power consumptions and mobility. Thus, proto-
`cols proposed for mobile ad hoc networks [34]
`and wireless sensor networks [8,9] are too cum-
`bersome to achieve satisfactory performance in
`this application. On the other hand, Wi-FiÕs are
`not capable of supporting ad hoc multi-hop net-
`working. As a consequence, WMNs are well-
`suited for broadband home networking.
`• Community and neighborhood networking. In a
`community, the common architecture for net-
`work 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:
`the information must be shared
`– Even if
`within a community or neighborhood, all
`traffic must flow through Internet. This sig-
`nificantly
`reduces
`network
`resource
`utilization.
`– 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.
`the above disadvantages
`WMNs mitigate
`through flexible mesh connectivities between
`homes, as shown in Fig. 7. WMNs can also en-
`able many applications such as distributed file
`storage, distributed file access, and video
`streaming.
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`Fig. 7. WMNs for community networking.
`
`• Enterprise networking. This can be a small net-
`work within an office or a medium-size network
`for all offices in an entire building, or a large
`scale network among offices in multiple build-
`ings. Currently, standard IEEE 802.11 wireless
`networks are widely used in various offices.
`However, these wireless networks are still iso-
`lated islands. Connections among them have
`to be achieved through wired Ethernet connec-
`tions, which is the key reason for the high cost
`of enterprise networks. In addition, adding
`more backhaul access modems only increases
`capacity locally, but does not improve robust-
`ness to link failures, network congestion and
`other problems of the entire enterprise network.
`If the access points are replaced by mesh rou-
`ters, as shown in Fig. 8, Ethernet wires can be
`eliminated. Multiple backhaul access modems
`can be shared by all nodes in the entire network,
`and thus, improve the robustness and resource
`
`utilization of enterprise networks. WMNs can
`grow easily as the size of enterprise expands.
`WMNs for enterprise networking are much
`more complicated than at home because more
`nodes and more complicated network topolo-
`gies are involved. The service model of enter-
`prise networking can be applied to many other
`public and commercial service networking sce-
`narios such as airports, hotels, shopping malls,
`convention centers, sport centers, etc.
`• Metropolitan area networks. WMNs in metro-
`politan area have several advantages. The phys-
`ical-layer transmission rate of a node in WMNs
`is much higher than that in any cellular net-
`works. For example, an IEEE 802.11g 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 net-
`works, wireless mesh MAN is an economic
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`Fig. 8. WMNs for enterprise networking.
`
`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 Fig. 9. Thus, the require-
`ment on the network scalability by wireless
`mesh MAN is much higher than that by other
`applications.
`• Transportation systems.
`limiting
`Instead of
`IEEE 802.11 or 802.16 access to stations and
`stops, mesh networking technology can extend
`access into buses, ferries, and trains. Thus, con-
`venient passenger information services, remote
`monitoring of in-vehicle security video, and dri-
`ver communications can be supported. To
`enable such mesh networking for a transporta-
`tion 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
`Fig. 10.
`
`• Building automation. In a building, various elec-
`trical 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. Recently
`Wi-Fi based networks have been adopted to
`reduce the cost of such networks. However, this
`effort has not achieved satisfactory performance
`yet, because deployment of Wi-FiÕs for this
`application is still rather expensive due to wir-
`ing of Ethernet. If BACnet (building automa-
`tion and control networks) access points are
`replaced by mesh routers, as shown in Fig. 11,
`the deployment
`cost will be
`significantly
`reduced. The deployment process is also much
`simpler due to the mesh connectivity among
`wireless routers.
`• Health and medical systems. In a hospital or
`medical center, monitoring and diagnosis data
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`Fig. 9. WMNs for metropolitan area networks.
`
`Fig. 10. WMNs for transportation systems.
`
`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. Wi-Fi based networks must rely on
`the existence of Ethernet connections, which
`may cause high system cost and complexity
`but without
`the abilities to eliminate dead
`spots. However, these issues do not exist in
`WMNs.
`
`• 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. Since still images and videos are the
`major traffic flowing in the network, this appli-
`cation demands much higher network capacity
`than other applications.
`
`In addition to the above applications, WMNs
`can also be applied to Spontaneous (Emergency/
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`capacity and flexibility of wireless systems. Typ-
`ical examples include directional and smart
`antennas [117,124], MIMO systems [139,126],
`and multi-radio/multi-channel systems [122,3].
`To date, MIMO has become one of the key
`technologies for IEEE 802.11n [64], the high
`speed extension of Wi-Fi. Multi-radio chipsets
`and their development platforms are available
`on the market [44].
`To further improve the performance of a wire-
`less radio and control by higher layer protocols,
`more advanced radio technologies such as
`reconfigurable radios, frequency agile/cognitive
`radios [97,89], and even software radios [102]
`have been used in wireless communication.
`Although these radio technologies are still in
`their infancy, they are expected to be the future
`platform for wireless networks due to their
`capability of dynamically controlling the radios.
`These advanced wireless radio technologies all
`require a revolutionary design in higher layer
`protocols, especially MAC and routing proto-
`cols. For example, when directional antennas
`are applied to IEEE 802.11 networks, a routing
`protocol needs to take into account the selec-
`tion of directional antenna sectors. Directional
`antennas can reduce exposed nodes, but they
`also generate more hidden nodes. Thus, MAC
`protocols need to be re-designed to resolve this
`issue. As for MIMO systems, new MAC proto-
`cols are also necessary [126]. When software
`radios are considered, much more powerful
`MAC protocols, such as programmable MAC,
`need to be developed.
`• Scalability. Multi-hop communication is com-
`mon in WMNs. For multi-hop networking, it
`is well known that communication protocols
`suffer from scalability issues [62,72], i.e., when
`the size of network increases, the network per-
`formance degrades significantly. Routing proto-
`cols may not be able to find a reliable routing
`path, transport protocols may loose connec-
`tions, and MAC protocols may experience sig-
`nificant
`throughput reduction. As a typical
`example, current IEEE 802.11 MAC protocol
`and its derivatives cannot achieve a reasonable
`throughput as the number of hops increases to
`4 or higher (for 802.11b, the TCP throughput
`
`Fig. 11. WMNs for building automation.
`
`Disaster) Networking and P2P Communications.
`For example, wireless networks for an emergency
`response team and firefighters do not have in-
`advance knowledge of where the network should
`be deployed. By simply placing wireless mesh
`routers in desired locations, a WMN can be
`quickly established. For a group of people holding
`devices with wireless networking capability, e.g.,
`laptops and PDAs, P2P communication anytime
`anywhere is an efficient solution for information
`sharing. WMNs are able to meet this demand.
`These Applications illustrate that WMNs are a
`superset of ad hoc networks, and thus can accom-
`plish all functions provided by ad hoc networking.
`
`5. Critical factors influencing network performance
`
`Before a network is designed, deployed, and
`operated, factors that critically influence its perfor-
`mance need to be considered. For WMNs, the crit-
`ical factors are summarized as follows:
`
`• Radio techniques. Driven by the rapid progress
`of semiconductor, RF technologies, and com-
`munication theory, wireless radios have under-
`gone a significant revolution. Currently many
`approaches have been proposed to increase
`
`Sonos Ex. 1021, p. 11
` Sonos v. Google
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`I.F. Akyildiz et al. / Computer Networks 47 (2005) 445–487
`
`is lower than 1.0 Mbps). The reason for low sca-
`lability is that the end-to-end reliability sharply
`drops as the scale of the network increases. In
`WMNs, due to its ad hoc architecture, the cen-
`tralized multiple access schemes such as TDMA
`and CDMA are difficult to implement due to
`their complexities and a general requirement
`on timing synchronization for TDMA (and
`code management for CDMA). When a distrib-
`uted multi-hop network is considered, accurate
`timing synchronization within the global net-
`work is difficult to achieve [62]. Thus, distrib-
`uted multiple access schemes such as CSMA/
`CA are more favorable. However, CSMA/CA
`has very low frequency spatial-reuse efficiency
`[2], which significantly limits the scalability
`of CSMA/CA-based multi-hop networks. To
`improve the scalability of WMNs, designing a
`hybrid multiple access scheme with CSMA/CA
`and TDMA or CDMA is an interesting and
`challenging research issue.
`• Mesh connectivity. Many advantages of WMNs
`originate from mesh connectivity which is a crit-
`ical requirement on protocol design, especially
`for MAC and routing protocols. Network self-
`organization and topology control algorithms
`are generally needed. Topology-aware MAC
`and routing protocols can significantly improve
`the performance of WMNs.
`• Broadband and QoS. Different from other ad hoc
`networks, most applications of WMNs are
`broadband services with various QoS require-
`ments. Thus, in addition to end-to-end transmis-
`sion delay and fairness, more performance
`metrics such