Wireless Mesh Networks

Myungchul Kimmckim@cs.kaist.ac.kr

- I Akyildiz and X Wang, IEEE Communications Magazine, pp. 523-530, 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