a survey on wireless mesh networks ian f. akyildiz, georgia institute of technology xudong wang,...
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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 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!
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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 Difficulty in maintenance and management