A Survey on Wireless Mesh Networks

1. A Survey on Wireless Mesh Networks IAN F. AKYILDIZ, GEORGIA INSTITUTE OF TECHNOLOGY XUDONG WANG, KIYON, INC. IEEE Radio Communications September 2005

2. Contents • Abstract • Introduction • Network Architectures • Critical Design Factors • Network Capacity • Layered Communication • PHY • MAC • Routing • Transport • Cross Layer Design

3. 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

4. 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

5. 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

6. 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

7. Network Architecture • 1. Infrastructure/Backbone WMNs

8. 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

9. 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

10. Network Architecture • 3. Hybrid WMNs • Combination of infrastructure and client meshing • Clients can access the network through mesh routers

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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!

17. Layered Communication Protocol for WMNs

18. 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

19. 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

20. 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

21. 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

22. 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.

23. 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

24. Layered Communication Protocols- MAC Layer • Open Research Issues • Scalable MAC • MAC/Physical Cross-Layer Design • Network Integration in the MAC Layer

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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