Wireless Mesh Networks

1
Wireless Mesh Networks
By Cunqing Hua

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

2
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.

3
Outline

• Application Scenarios
• Network Architecture
• Characteristics
• Protocols Design
• Standardization Activities

4
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

5
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

6
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!!!
7
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.
8
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

9
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

10
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.

11
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

12
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.

13
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.

14
Network Architecture

• WMNs 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.

15
WMN Routers
Examples of mesh routers based on different
embedded systems (a) PowerPC and (b)
Advanced Risc Machines (ARM)
16
WMN Clients
Examples of mesh clients (a) Laptop, (b) PDA,
(c) Wi-Fi IP Phone and (d) Wi-Fi RFID Reader.
17
WMN Architecture Classifications

• Infrastructure Meshing
• Client Mesh Networking
• Hybrid Mesh Networking

18
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

19
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.

20
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

21
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

22
Protocol Design

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

23
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.

24
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

25
MAC Layer

• Differences 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.

26
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

27
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.

28
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.

29
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.

30
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.

31
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)

32
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

Z
X
Y
33
DSDV Protocol

• Node X takes the following steps
• If S(X) gt 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) lt S(Y), then X sets Y as the next hop to
  Z, and S(X) is updated to equal S(Y)

Z
X
Y
34
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

35
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.

36
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.

37
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.

38
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.

39
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.

40
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.

41
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.

42
WMNs Standards

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

Range
WiMAX
50Km
100m
WPAN
Wi-Fi
Data Rate
10Mb
100Mb
1Mb
100kb
43
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 211 GHz, allowing
  non-line-of-sight (NLO) communications, spanning
  up to a 50 km range.
• Supporting multihop communications.

44
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

45
WMNs Standards

• 802.11s MCF Sublayer