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 Difficulty in maintenance and management