Wireless Mesh Networks Myungchul Kim

- I Akyildiz and X Wang, IEEE Communications Magazine, pp , Sept 2005 Introduction –Mesh routers and mesh clients –Multi-hop communications with much lower transmission power –Mesh router with multiple wireless interfaces –Mesh client with a single wireless interface –Customers without wireless NICs can access WMN through for example, Ethernet –Advantages: low up-front cost, easy network maintenance, robustness, reliable service coverage, etc. –The available MAC and routing protocols are not scalable; throughput drops significantly as the number of nodes or hops in WMNs increases. A Survey on WMNs

Network architecture -Infrastructure/backbone WMNs -Fig 1. Network architecture and critical design factors

-Client WMNs -Actually the same as a conventional ad hoc network -Routing and self-configuration -Hybrid WMNs -Fig 2 Network architecture and critical design factors

-The characteristics of WMN -Supports ad hoc networking: self-forming, self-healing and self- organization -Multi-hop wireless networks -Minimal mobility and dedicated routing and configuration -Mobility of end nodes -Mesh routers integrate heterogeneous networks, wireless and wired -Power-consumption constraints are different -Compatible and interoperable with other wireless networks Network architecture and critical design factors

Critical design factors -Radio techniques -Directional and smart antennas, multiple input and multiple output (MIMO) systems, and multi-radio/multi-channel systems -Reconfigurable radios, frequency agile/cognitive radios, software radios -Require a revolutionary design in higher-layer protocols such as MAC and routing protocols -Scalability -Mesh connectivity: network self-organization and topology control algorithms are needed -Broadband and QoS -Security: no centralized trusted authority to distribute a public key in a WMN -Ease of use: autonomous -Compatibility and interoperability Network architecture and critical design factors

Network capacity -The guideline to improve the capacity of ad hoc networks: a node should only communicate with nearby nodes [1]. -Throughput capacity can be increased by deploying relaying nodes. -Nodes need to be grouped into clusters. -Utilizing the node mobility [2]-> transmission delay and buffer -Limitation: networking protocols (MAC, power control, routing) have not been captured and the actual scale of WMNs is not considered Layered communication protocol –Physical layer Advanced physical-layer techniques: cognitive radios dynamically capturing the unoccupied spectrum Software radio enable the programmability of all advanced physical layer techniques Open research issues: improve the transmission rate and the performance of physical-layer techniques and utilize the cross layer design between MAC and the physical layer Advances and research challenges

–MAC layer in WMN More than one-hop communications Distributed for multipoint-to-multipoint communication Network self-organization Mobility is low Single-channel MAC and Multi-channel MAC Single-channel MAC –Modifying existing MAC protocols –Cross-layer design: directional antenna-based MACs and MACs with power control -> hidden terminal problem –Proposing innovative MAC protocols: how to design a distributed TDMA MAC protocols overlaying CSMA.CA? Advances and research challenges

Multi-channel MAC –Multi-channel single-transceiver MAC –Multi-channel multi-transceiver MAC: one MAC layer –Multi-radio MAC –Open research issues: scalable MAC, MAC/physical cross-layer design, heterogeneous network integration in the MAC layer –Routing layer Optimal routing protocol features –Multiple performance metrics: how about hop-count? –Scalability –Robustness: link failures or congestion, load balancing –Efficient routing with mesh infra considering the low mobility and no constraints on power consumption in mesh routers Advances and research challenges

–Routing layer (con’t) Routing protocols with various performance metrics –Link quality source routing (LQSR) –Link quality metrics Expected transmission count (ETX) Per-hop RTT Per-hop packet pair Multi-radio routing –Weighted cumulative expected transmission time (WCETT) Link quality metric and the minimum hop-count Multi-path routing –Better load balancing and high fault tolerance Hierarchical routing –Clusters Advances and research challenges

–Routing layer (con’t) Geographic routing –Position information of nodes in the vicinity and the destination node –Delivery is not guaranteed even if a path exists between source and destination Open research issues –Scalability –Better performance metrics –Routing/MAC cross-layer design –Efficient mesh routing protocols than an ad hoc one Advances and research challenges

–Transport layer Reliable data transport –Non-congestion packet loss –Unknown link failure due to wireless channels and mobility in mesh clients –Network asymmetry: TCP is critically dependent on ACK –Large RTT variations Ad hoc transport protocol (ATP) –For MANET –Rate-based and quick-start –Congestion detection is a delay-based approach -> congestion cause –No retransmission timeout –Congestion control and reliability are decoupled Real-time delivery –A rate control protocol (RCP) is needed to work with UDP Advances and research challenges

–Transport layer (con’t) Open research issues –Cross-layer solution to network asymmetry –Adaptive TCP on various wireless networks –Application layer Internet access Distributed information storage and sharing Information exchange across multiple wireless networks Research directions –Improve existing applications layer protocols –Propose new application-layer protocols for distributed information sharing –Develop innovative applications for WMNs Advances and research challenges

–Network management Mobility management –Distributed mobility management Power management Network monitoring –Security Factors such as distributed network architecture, vulnerability of channels and nodes in the shared wireless medium, and the dynamic change of network topology An AAA centralized server? No central authroity for managing security keys An open issue: A distributed authentication and authorization sheme with secure key management Advances and research challenges

–Cross-layer design One approach is to improve the performance of a protocol layer by taking into account parameters in other protocol layers. Another approach is to merge several protocols into one component. Advances and research challenges