Wireless Mesh Networks

The notes of this talk are excerpted from the lecture notes by Prof. Akyildiz at Georgia Institute of Technology

By Cunqing Hua

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References• Faccin, S.M.; Wijting, C.; Kenckt, J.; Damle, A., Mesh WLAN

networks: concept and system design, IEEE Wireless Communication, Vol 13, No. 2, 2006.

• Lee, M.J.; Jianliang Zheng; Young-Bae Ko; Shrestha, D.M., Emerging standards for wireless mesh technology, IEEE Wireless Communication, Vol 13, No. 2, 2006

• Akyildiz, I.F., Wang, X. and Wang, W., Wireless Mesh Networks: A Survey, Computer Networks Journal (Elsevier), March 2005.

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Outline

• Application Scenarios

• Network Architecture

• Characteristics

• Protocols Design

• Standardization Activities

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Wireless Mesh Networks• Wireless Mesh Networks (WMN) are the networks in

which each node can communicate directly with one or more peer nodes.

• Different from traditional wireless networks (e.g. 802.11 WLANs) requiring centralized access points to mediate the wireless connection.

• Each node operates not only as a host but also as a router, forwarding packets on behalf of other nodes that may not be within direct wireless transmission range of their destinations.

• It is dynamically self-organized and self-configured, nodes can automatically establishing and maintaining mesh connectivity among nodes

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Application Scenarios

• Broadband Home Networking• Community and Neighborhood Networking• Enterprising Networking• Metropolitan Area Networking• Transportation Systems• Building Automation• Health and Medical Systems• Security and Surveillance Systems

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Broadband Home Networking

• Current home network realized through IEEE 802.11 WLANs– Problem location of the access points. – Homes have many dead zones without service coverage. – Site survey are expensive and not practical – Installation of multiple access points is also expensive and not

convenient.– Communications between nodes under two different access

points have to go through the access hub, not an efficient solution.

WMNs can resolve all these issues in home networking!!!

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Community and Neighborhood Networking

Community networks based on cable, DSL and last-hop wireless

• All traffic must flow through Internet, this significantly reduces network resource utilization.

• Large percentage of areas in between houses is not covered by wireless services.

• Gateways may not be shared and wireless services must be set up individually, network service costs may increase.

• Each home has single path to access Internet

WMNs can mitigate these disadvantages and provide many applications such as distributed file storage, distributed file access, and video streaming.

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Enterprise Networking• IEEE 802.11 WLANs

– Isolated islands, connections among them are achieved through wired Ethernet

– Adding more backhaul access modems only increases capacity locally, but does not improve robustness to link failures, network congestion and other problems of the entire enterprise network.

• WMNs Solutions– Multiple backhaul access

modems can be shared by all nodes in the entire network

– Scalable

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Metropolitan Area Networks• WMNs provide higher transmission rate than cellular

networks, • The communication between nodes does not rely on a wired

backbone. • An economic alternative to broadband networking• Covers larger area than home, enterprise, building, or

community networks.• Higher scalability

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Transportation Systems• WMNs can extend access from stations and stops into buses,

ferries, and trains.

• Convenient passenger information services, remote monitoring of in-vehicle security video, and driver communications.

• Two key techniques are needed– High-speed mobile backhaul from a vehicle to the Internet – Mobile mesh networks within the vehicle.

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Building Automation• Various electrical devices need to be

controlled and monitored.

• Standard wired networks is very expensive

• Wi-Fi networks can reduce the cost of such networks. However, the deployment of Wi-Fis for this application is still expensive.

• Low deployment cost of BACnet (Building Automation and Control Networks) with WMNs

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Health and Medical Systems

• Monitoring and diagnosis data need to be processed and transmitted across rooms for various purposes.

• Large data volume by high resolution medical images, various periodical monitoring information

• Wi-Fi based networks must rely on the existence of Ethernet connections, cause high system cost, complexity and dead spots.

• However, these issues do not exist in WMNs.

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Security and Surveillance Systems

• Security surveillance systems is necessity for enterprise buildings, shopping malls, grocery stores, etc.

• Still images and videos are the major traffic flowing in the network, this application demands much higher network capacity than other applications.

• WMNs are a much more viable solution than wired networks to connect all devices.

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Network ArchitectureWMNs consist of two types of nodes: Mesh Routers and Mesh

Clients

• Mesh router – Additional routing functions to support mesh networking.

– Multiple wireless interfaces with same or different wireless access technologies.

– The gateway/bridge functionalities enable the integration of WMNs with existing wireless networks(cellular, sensornet, Wi-Fi, WiMAX).

• Mesh Clients – Conventional nodes (e.g., desktops, laptops, PDAs, PocketPCs,

phones, etc.) equipped with wireless network interface cards (NICs), and can connect directly to wireless mesh routers.

– Customers without wireless NICs can access WMNs by connecting to wireless mesh routers through, e.g., Ethernet.

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WMN Routers

Examples of mesh routers based on different embedded

systems: (a) PowerPC and (b) Advanced Risc Machines (ARM)

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WMN Clients

Examples of mesh clients: (a) Laptop, (b) PDA,

(c) Wi-Fi IP Phone and (d) Wi-Fi RFID Reader.

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WMN Architecture Classifications

• Infrastructure Meshing

• Client Mesh Networking

• Hybrid Mesh Networking

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Infrastructure Meshing• Mesh routers form an mesh infrastructure among themselves.

• Provides backbone for clients and enables integration of WMNs with existing wireless networks and Internet through gateway/bridge functionalities.

• Clients connect to mesh router with wireless link or Ethernet

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Client WMNs• Client nodes constitute peer-to-peer network, and perform routing

and configuration functionalities as well as provide end-user applications to customers, mesh routers are not required.

• Multi-hop routing.

• Client nodes have to perform additional functions such as routing and self-configuration.

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Hybrid WMNs• A combination of infrastructure and client meshing.

• Infrastructure provides connectivity to other networks such as the Internet, Wi-Fi, WiMAX, cellular, and sensor networks;

• Mesh clients can access the network through mesh routers as well as directly meshing with other mesh clients.

• The routing capabilities of clients provide better connectivity and coverage

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WMNs Characteristics• Multi-hop wireless networks

• Support for Ad Hoc networking, and capability of self-forming, self-healing, and self-organization

• Mobility dependence on the type of mesh nodes

• Multiple types of network access

• Dependence of power-consumption constraints on the type of mesh nodes

• Compatibility and interoperability with existing wireless networks

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Protocol Design

• Physical Layer• Mac Layer• Network Layer• Transport Layer• Application Layer• Network Management• Security

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Physical Layer Technologies• Orthogonal frequency multiple access (OFDM) has

significantly increased the speed of IEEE 802.11 from 11 mbps to 54 mbps.

• Ultra-wide band (UWB) can achieve much higher rate for short-distance applications.

• MIMO can increase system capacity by three times or even more.

• Frequency agile or cognitive radios can achieve much better spectrum utilization.

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Physical Layer: Research Issues

• Improve the transmission rate and the performance of physical layer techniques– OFDM, UWB– Multiple-antenna systems– Frequency agile

• Design higher layer protocols to utilize the advanced features provided by physical layers– MAC protocols for directional and smart antennas– MAC protocols for MIMO systems– Communication protocols for cognitive radios

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MAC LayerDifferences between WMNs MACs and Wireless

Networks MACs

• MACs for WMNs are concerned with more than one hop communication

• MAC must be distributed and collaborative, and must work for multipoint-to-multipoint communication.

• Network self-organization is needed for better collaboration between neighboring nodes and nodes in multi-hop distances.

• Mobility affects the performance of MAC.

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Single Channel MACs• Improving Existing MAC Protocols

– Adjust parameters of CSMA/CA– Only achieve a low end-to-end throughput.

• Cross-layer design with advanced physical layer techniques– MAC based on directional antenna can eliminate exposed

nodes, but may introduce more hidden nodes– MAC with power control can reduce exposed nodes,

improve spatial-reuse, but hidden nodes still exist

• Proposing Innovative MAC Protocols– Revisiting MAC protocols based on TDMA or CDMA– Design complexity and cost. – Compatibility with existing MAC protocols

Not scalable, available bandwidth ~(1/2)^n

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Multi-Channel MACs• Multi-Channel Single-Transceiver MAC

– Only one channel is active in each node, different nodes can use different channels.

– Need to coordinate transmissions between nodes

• Multi-Channel Multi-Transceiver MACs– Multiple parallel RF front-end chips and baseband

processing.– One MAC layer module to coordinate multiple channels.

• Multi-Radio MACs– Multiple radios, each with its own MAC and physical layers. – Communications in these radios are totally independent.– A virtual MAC protocol to coordinate communications in all

channels.

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MAC Layer : Research Issues

• Scalable Single-Channel MACs– Distributed and collaborative schemes to ensure

scalability.

• Scalable Multi-Channel MACs– Overall performance improvement in multiple channel

• Network Integration in the MAC Layer– Advanced bridging functions in the MAC layer so that

different wireless radios can seamlessly work together.– Reconfigurable/software radios may be the ultimate

solution to these bridging functions.

• MAC Protocol Implementation– Modifying functions in the firmware or hardware is much

more complicated and costly.– New architecture such that MAC functions can be

completely implemented in the software.

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Routing Layer

Features of routing protocol for WMNs:• Multiple Performance Metrics

– Hop-count is not an effective routing metric.

– Other performance metrics, e.g., link quality and

round trip time (RTT), must be considered.

• Scalability– Routing setup in large network is time consuming. – Node states on the path may change.

– Scalability of routing protocol is critical in WMNs.

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Routing Layer• Robustness

– WMNs must be robust to link failures or congestion. – Routing protocols need to be fault tolerant with link failures

and can achieve load balancing.

• Adaptive Support of Both Mesh Routers and Mesh Clients– Mesh routers : minimal mobility, no constraint of power

consumption, routing is simpler

– Mesh clients : mobility, power efficiency, routing is complicated

– Need to design a routing protocol that can adaptively support both mesh routers and mesh clients.

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Destination-Sequenced Distance-Vector (DSDV)

• Proactive Protocols• Each node maintains a routing table which stores

– next hop towards each destination– a cost metric for the path to each destination– a destination sequence number that is created by the

destination itself– Sequence numbers used to avoid formation of loops

• Each node periodically forwards the routing table to its neighbors– Each node increments and appends its sequence number when

sending its local routing table– This sequence number will be attached to route entries created

for this node• DSDV in WMNs

– Supporting multidimensional cost metrics (QoS, power efficiency, security, etc)

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DSDV Protocol• Assume that node X receives routing

information from Y about a route to node Z

• Let S(X) and S(Y) denote the destination sequence number for node Z as stored at node X, and as sent by node Y with its routing table to node X, respectively

X Y Z

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DSDV Protocol

X Y Z

• Node X takes the following steps:

– If S(X) > S(Y), then X ignores the routing information received from Y

– If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z

– If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)

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Routing Layer- Research Issues

• Scalability– Hierarchical routing protocols can only partially

solve this problem

– Geographic routing relies positioning technologies.

– New scalable routing protocols need to be developed.

• Better Performance Metrics– New performance metrics need to be developed.

– Need to integrate multiple performance metrics into a routing protocol

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Routing Layer - Research Issues

• Routing/MAC Cross-Layer Design– Needs to interact with the MAC layer, e.g. adopting multiple

performance metrics from MAC layer.

– Merely exchanging parameters between them is not enough, merging certain functions of MAC and routing protocols is a promising approach.

– For multi-radio or multi-channel routing, the channel/radio selection in the MAC layer can help the path selection in the routing layer.

• Hybrid Routing– Mesh routers and mesh clients have different constraints in power

efficiency and mobility.

– Need to adaptively support mesh routers and mesh clients.

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Transport Layer: Research Issues

• Cross-layer Solution to Network Asymmetry– Routing protocol can select an optimal path for both data and

ACK packets.– MAC layer and error control may need to treat TCP data and

ACK packets differently.

• Adaptive TCP– WMNs will be integrated with the Internet and various

wireless networks such as IEEE 802.11, 802.16, 802.15, etc.

– Same TCP is not effective for all networks.

– Applying different TCPs in different networks is a complicated and costly approach, and cannot achieve satisfactory performance.

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Application Layer

Applications supported by WMNs:

• Internet Access– Advantages of WMNs: low cost, higher speed, and easy

installation.

• Distributed Information Storage and Sharing– Data sharing between nodes within WMNs– Query/retrieve information located in distributed database

servers.

• Information Exchange across Multiple Wireless Networks.– Cellular phone talks Wi-Fi phone through WMNs, – Wi-Fi user monitors the status of wireless sensor networks.

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Application Layer: Research Issues

• Improve Existing Application Layer Protocols.– Lower layers protocols cannot provide perfect support for the

application layer. – E.g., packet loss and packet delay with a large jitter may fail

many Internet applications– Existing application layer protocols need to be improved.

• New Application Layer Protocols for Distributed Information Sharing.– P2P protocols on the Internet may not perform well in WMNs, – New application layer protocols need to be developed.

• Develop Innovative Applications for WMNs– Applications cannot achieve best performance without WMNs.– Enable WMNs to be a unique networking solution instead of

just another option of wireless networking.

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Network Management Protocols

• Mobility Management– Distributed scheme for WMNs can be simpler because

the existence of backbone nodes – Take advantages of the network backbone to design a

light-weight distributed mobility management scheme for WMNs.

– Location service is a desired feature by WMNs.

• Power Management– For mesh routers, power management aims to control

connectivity, interference, spectrum spatial-reuse, and topology.

– For mesh clients, protocols should be power efficient.

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Network Management Protocols

• Network Monitoring– Report statistics in the MIB to one or several servers.

– Data processing algorithms analyze these statistical data and determine potential abnormality.

– To reduce overhead, schemes for efficient transmission of network monitoring information are expected.

– To accurately detect abnormal operation and quickly derive network topology of WMNs, effective data processing algorithms need to be developed.

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Security• WMNs lack efficient and scalable security

solutions – Distributed network architecture– Vulnerability of channels and nodes in the shared

wireless medium – Dynamic change of network topology.

• Two strategies – Embedding security mechanism into network protocols– Developing security monitoring response systems– How to design and implement a practical security

system, including cross-layer secure network protocols and various intrusion detection algorithms, is a challenging research topic.

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WMNs Standards• WPAN: Bluetooth, Zigbee• WiFi: 802.11a, b, g, n• WiMAX: 802.16

Data Rate

Range

WPAN

WiMAX

Wi-Fi

10Mb 100Mb100kb1Mb

50Km

100m

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WMNs Standards• IEEE 802.16a WMAN Mesh

– “mesh mode” in addition to the point-to-multipoint(PMP) mode defined in IEEE 802.16.

– Operating in the licensed and unlicensed lower frequencies of 2–11 GHz, allowing non-line-of-sight (NLO) communications, spanning up to a 50 km range.

– Supporting multihop communications.

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WMNs Standards• 802.11s WLAN Mesh

– Multi-hop capability added to 802.11g/a/b– Auto configure on power up– Multi-channel multi-radio operation– Topology discovery– MAC Path selection protocol– Modified forwarding for QOS and mesh control

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WMNs Standards

• 802.11s MCF Sublayer