A Survey on Wireless Mesh Networks
IAN F. AKYILDIZ, GEORGIA INSTITUTE OF TECHNOLOGY XUDONG WANG, KIYON, INC.
IEEE Radio Communications September 2005
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Contents
Abstract
Introduction
Network Architectures
Critical Design Factors
Network Capacity
Layered Communication
PHY
MAC
Routing
Transport
Cross Layer Design
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Abstract
Wireless mesh networks (WMNs)
A key technology for next-generation wireless networking
Advantages over other wireless networks
Rapid progress and inspiring numerous applications
Many technical issues exist
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Introduction
WMNs
Self-organize
Self-configure
Automatically establishing an ad hoc network
Maintaining the mesh connectivity
WMNs are comprised of 2 types of nodes
Mesh routers
Mesh clients
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Introduction
Mesh router
Additional routing functions
Support mesh networking
Lower transmission power
Same coverage multi-hop communications
Same or different wireless access technologies
Usually equipped with multiple wireless interfaces
Minimal mobility
Mesh backbone for mesh clients
Integration various other networks
Gateway/bridge functionalities
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Introduction
Mesh client
Hardware platform & software simpler
light-weight Communication protocols
Only a single wireless interface is needed
WMNs capabilities of ad-hoc networks
Low up-front cost
Easy network maintenance
Robustness
Reliable service coverage
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Network Architecture
1. Infrastructure/Backbone WMNs
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Network Architecture
1. Infrastructure/Backbone WMNs
Mesh routers for clients
Using various types of radio technologies
Connected to the Internet
Conventional clients with an Ethernet interface can be connected to mesh routers via Ethernet links
Same radio technologies (clients, routers) -> Directly communicate with mesh routers
Different radio technologies (clients, routers)
-> Clients communicate with their BS
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Network Architecture
2. Client WMNs
Peer-to-peer networks among client devices
Mesh router is not required
Using one type of radios on devices
Same as a conventional ad hoc network
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Network Architecture
3. Hybrid WMNs
Combination of infrastructure and client meshing
Clients can access the network through mesh routers
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Network Architecture
The characteristics of WMNs
Support ad hoc networking
Capability of self-forming, self-healing, self-organization
Multi-hop wireless networks
Decreases the load (mesh clients, other end nodes)
Mesh routers have minimal mobility
Dedicated routing and configuration
Mobility of end nodes is supported
Mesh routers integrate heterogeneous networks
Different Power-consumption constraints
mesh routers, clients
Need compatibility, interoperability
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Critical Design Factors
1. Radio Techniques.
Increase capacity, flexibility approaches
Directional & smart antennas
Multiple input multiple output (MIMO) systems
Multi-radio/multi-channel systems
Advanced radio technologies
Reconfigurable radios
Frequency agile/cognitive radios
Software radios
Need design with higher-layer protocols
MAC and routing protocol
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Critical Design Factors
2. Scalability
Without support Scalability
Network performance degrades as the network size increases.
Example
Routing protocols can’t find a reliable routing path
Transport protocols loose connections
MAC protocols significant throughput reduction
Ensure the scalability
All protocols need to be scalable
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Critical Design Factors
3. Mesh Connectivity
Many advantages of WMNs
Ensure reliable mesh connectivity
Require Network self-organization & topology control algorithms
Topology-aware MAC & routing protocols
Improve performance
4. Broadband and QoS
Applications
Broadband services & Heterogeneous QoS requirements
Protocol consider
End-to-end transmission delay, fairness, delay jitter,
aggregate and per-node through-put, and packet loss ratios
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Critical Design Factors
5. Security
Security schemes are still not fully applicable
6. Ease of Use
Enable the network to be as autonomous as possible
Consider Protocols designed
Require network management tools
Maintain the operation, monitor the performance, configure the parameters
7. Compatibility & Inter-operability
Require backward compatible
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Network Capacity
Researchs
Using the similarities between WMNs and ad hoc networks
Limitation
Do not consider different medium access control, power control, routing protocols
New analytical results need!
Layered Communication Protocol for WMNs
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Layered Communication Protocols - Physical Layer
Advanced Physical-Layer Techniques
Multiple transmission rates
Different modulation & Coding rates Combination
Link adaptation Adaptive error resilience
high-speed transmissions
OFDM
UWB techniques
Increase capacity & mitigate the impairment
Antenna diversity
Smart antenna
MIMO systems
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Layered Communication Protocols - Physical Layer
Software radio platform
Programmable Channel access modes, Channel modulations
Not a mature technology yet
Open Research Issues.
Complexity of OFDM, UWB and cost
Best utilize
Higher-layer protocols, MAC protocols need to work interactively with the physical layer
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Layered Communication Protocols - MAC Layer
MAC Differences (WMNs, classical wireless networks)
Concerned with more than one-hop communication
Distributed MAC
Needs to be collaborative
Works for multipoint-to-multipoint communication
Network self-organization is needed for better collaboration
Low Mobility
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Layered Communication Protocols - MAC Layer
Single-channel MAC
Modifying Existing MAC Protocols
Adjusting parameters of CSMA/CA
Cannot reduce the probability of contentions
Cross-layer Design
Directional antenna-based MACs
Eliminates exposed nodes
Directional transmission -> More hidden nodes produce
Difficulties -> Cost, system complexity, practicality of fast steerable directional antennas
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Layered Communication Protocols - MAC Layer
MACs with power control.
Reduces exposed nodes, especially in a dense network
Low transmission power Improve the spectrum spatial reuse factor
Lower transmission power Reduce the possibility of detecting a potential interfering node Hidden nodes issue become worse
Proposing Innovative MAC Protocols.
Poor scalability in a multi-hop network CSMA/CA are not an efficient solution
Revisiting the design of MAC protocols based on TDMA or CDMA is indispensable
Problems
Complexity & Cost
Compatibility of TDMA (or CDMA) MAC with existing MAC protocols.
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Layered Communication Protocols - MAC Layer
Multi-Channel MAC.
Multi-Channel Single-Transceiver MAC
Low cost & compatibility One transceiver on a radio
Only one transceiver is available Only one channel is active at a time in each network node
Multi-Channel Multi-Transceiver MAC
Multiple parallel RF front-end chips & baseband
processing modules Support several simultaneous channels
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Layered Communication Protocols - MAC Layer
Open Research Issues
Scalable MAC
MAC/Physical Cross-Layer Design
Network Integration in the MAC Layer
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Layered Communication Protocols - Routing Layer
Optimal routing protocol for WMNs
Multiple Performance Metrics
Minimum hop-count ineffective
Scalability
Setting up or maintaining a routing path take a long time Critical to scalability routing protocol
Robustness
Robust to link failures or congestion
Perform load balancing
Efficient Routing with Mesh Infrastructure
Minimal mobility and no power consumption constraints Simpler routing protocols
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Layered Communication Protocols - Routing Layer
Open Research Issues
Scalability
Better Performance Metrics
New performance metrics need to be developed
Routing/MAC Cross-Layer Design
Interact with the MAC layer in order to improve performance
Efficient Mesh Routing
Much simpler and more efficient routing protocol need
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Layered Communication Protocols - Transport Layer
Reliable Data Transport
TCP variants
Non-Congestion Packet Loss
Classical TCPs do not differentiate congestion & non-congestion losses
Unknown Link Failure
Wireless channels & mobility link failure happen
To enhance TCP performance, link failure needs to be detected
Network Asymmetry
Large RTT Variations
Mobility Large RTT variations Degrade the TCP performance
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Layered Communication Protocols - Transport Layer
New transport protocols
Better performance than the TCP variants
Integrated many other wireless networks transport protocols need to be compatible with TCPs
New transport protocols is not compatible
Real-Time Delivery
Require Rate control protocol (RCP)
To support end-to-end delivery of realtime traffic
Work with UDP
No schemes are available
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Layered Communication Protocols - Transport Layer
Open Research Issues
Reliable Data Transport
Cross-layer Solution to Network Asymmetry
All problems of TCP performance degradation are actually related to protocols in the lower layers
Adaptive TCP
Integrating various wireless network compatible is important adaptive TCP
Real-time transport
Entirely new RCPs need
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Layered Communication Protocols - Cross Layer Design
Approachs
1.Taking into account parameters in other protocol layers
keeps the transparency between protocol layers
2.To merge several protocols into one component
achieve much better performance through closer interaction between protocols
Cross-layer designs risks
Protocol-layer abstraction loss
Incompatibility with existing protocols
Unforeseen impact on the future design
