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!

Layered Communication Protocol for WMNs 17

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