Wireless Mesh Network

Members

Suman Raj Adhikari

Saroj Raj Regmi

Introduction

WMNs offer multiple redundant communications paths throughout the network. Whenever a link fails, the network automatically routes messages through alternate paths.

Even in adverse conditions devices in WMNs co-operate with each other in transmitting packets through the network.

WMNs are believed to be self-configuring and self-healing networks.

Dramatic increase in link quality just by shortening the distance between the nodes

Introduction

Dramatic increase in link quality just by shortening the distance between the nodes

Reduction in the distance by a factor of two can result at least four times more powerful signals at the receiver.

Suggests that the links are more reliable without even having to increase the power of the transmitter in individual nodes.

The integration of WMNs with other networks such as Internet, IEEE 802.11, 802.15, 802.16, sensor networks etc can be accomplished.

Types of nodes in WMNs Wireless Mesh Router

contains additional routing functions to support mesh networking. usually equipped with multiple wireless interfaces built on either the

same or different wireless access technologies improves the flexibility of mesh networking

Mesh Clients can also work as routers since they also have necessary functions for

mesh networking. gateway and bridge functions do not exist in these nodes. usually have only one wireless interface as in Laptop/desktop PC,

pocket PC, PDA, IP phone, RFID reader, etc.

WMN Architecture

Based on the functionality of the nodes the architecture of WMNs can be classified into:

Infrastructure mesh Architecture, Client mesh Architecture and Hybrid mesh architecture

Infrastructure Mesh In infrastructure mesh architecture, the mesh routers

collectively provide a wireless backbone infrastructure. Client node is passive in mesh infrastructure.

Infrastructure Mesh Via Ethernet links conventional clients with Ethernet

interfaces can be connected to mesh routers. If the conventional clients have the same radio technologies

as the mesh routers then they can communicate directly with the mesh routers

If different radio technologies are used then the clients communicate with their base station that have Ethernet connections to mesh routers

Client Mesh

Client meshing provides peer-to-peer networks among client devices. Here no such mesh router is required. Client will act like a mesh router by relaying the packets.

Hybrid Mesh

Mesh router provide the backbone of such network. With the help of network functionalities such as routing and forwarding

of data packets, clients can actively participate in the creation of the mesh.

Characteristics

Dynamic self-configuration and self-organization Adaptation Fault tolerance and robustness Low-Cost Integration and interoperability

Applications

Various companies have realized the potential of WMNs and have started offering mesh networking products for a range of application scenarios.

Though most of the products are based on commodity IEEE 802.11 hardware, the majority of the commercial systems implement their own mesh protocols for routing and network configuration.

This makes integration of mesh routers from different vendors into a single WMN very difficult.

Use of IP as a common network protocol can resolve the difficulty.

Applications

Broadband home networking Community and neighborhood networking Enterprise networking Metropolitan area networks Transportation systems Building automation

Standard

IEEE 802.11s is the most relevant emerging standards for WMN technology

Efforts are underway in several IEEE working groups, like 802.11, 802.15, 802.16, and 802.20, to define mesh standards.

Protocol Layers Some of the factors that affect the capacity and performance of

WMNs network architecture, network topology, traffic pattern, network node density, number of channels used for each node, transmission power level, and node mobility.

In order to develop the protocols we need to clearly understand the relationship between the above factors and the capacity of WMNs.

Physical Layer The wireless radios of WMNs can support multiple transmission rates

by a combination of different modulation and coding rates. Adaptive error resilience can be provided through link adaptation. Orthogonal frequency multiple access (OFDM) and ultra-wide band

(UWB) techniques are the schemes that are being used to support high-speed transmissions.

If we desire to increase the capacity and mitigate the impairment by fading, delay-spread, co-channel interference, fading, multi-antenna, systems such as antenna diversity and smart antenna have been proposed for wireless communications.

It is more difficult to develop such techniques for WMNs although these physical-layer techniques are also desired by other wireless networks.

Physical Layer Frequency-agile or cognitive radios are being developed to

dynamically capture the unoccupied spectrum to achieve better spectrum utilization and viable frequency planning for WMNs.

Since all the components of a radio, such as RF bands, channel access modes, and channel modulations, are programmable implementing cognitive radios on a software radio platform is one of the most powerful solutions.

Although physical test-beds are currently available, the software radio platform is not a mature technology yet. But as it can enable the programmability of all advanced physical layer techniques, in the future it seems to be a key technology for wireless communications

MAC Layer There are differences between the MAC in WMNs and other

wireless networks: MAC for WMNs is concerned with more than one-hop communication. MAC is distributed, needs to be collaborative, and works for multipoint-

to-multipoint communication. Network self-organization is needed for better collaboration between

neighboring nodes and nodes in multi-hop distances. Mobility is low but still affects the performance of MAC For WMNs, a MAC protocol can be designed to work on a single channel

or multiple channels simultaneously.

Routing Layer The design of the routing protocols for WMNs is still an area of

research although there are many routing protocols that are available for ad hoc networks.

However, an optimal routing protocol for WMNs must possess features like:

multiple performance metrics scalability robustness and efficient routing with mesh infrastructure.

The routing protocols for ad hoc networks is equipped with some of these features, but non of them possesses all of the above.

Routing Protocols Multi Radio Routing

In a multi-radio link quality source routing (MR-LQSR) a new performance metric , weighted cumulative expected transmission time (WCETT) , is incorporated.

Both link quality metric and the minimum hop count are accounted for in WCETT which provides a good tradeoff between delay and throughput.

Routing Protocols Multi-Path Routing:

Better load balancing and providing high fault tolerance are the two main objectives of using multi-path routing.

Multiple paths are selected between source and the destination This provides better fault tolerance as when a link is broken another path

can be chosen. This enhances efficiency since without waiting to set up a new routing

path, the end-to-end delay, throughput, and fault tolerance can be improved.

Complexity is the major hurdle of multi-path routing. Also, the availability of node disjoint routes between source and

destination determines the improvement, given an performance metric.

Routing Protocols Hierarchical Routing:

The hierarchical routing protocols tend to achieve better performance when the node density is high.

This is because of less overhead, shorter average routing path, and quicker set-up procedure of routing path etc.

Maintaining hierarchy definitely adds to the complexity and this may compromise the performance of the routing protocol.

Routing Protocols Geographical routing

This kind of routing scheme forwards packets only by using the position information of the nodes in the vicinity and in the destination node unlike the topology based schemes.

This suggests that there is less impact on the geographic routing due to a topology change than the other routing protocols.

The algorithm used in geographical routing (single-path greedy routings) suffers from that a delivery is not guaranteed even if a path exists between a source and the destination as the packet forwarding decision is made based on the location information of the current forwarding node, its neighbors, and the destination node.

Planar-graph based geographic routing algorithms have been proposed which guarantees the delivery. Theses algorithms suffer form a major drawback: a higher communication overhead.

Transport Layer A large number of transport protocols are available for ad

hoc networks and WMNs depend on those transport layer protocols

Till date, there is no transport protocol that has been proposed specifically for WMNs.

We know that ad hoc network is also not mature. It also has various unresolved issues. This suggests further research in this area.

Application Layer There are numerous applications that are supported by

WMNs and they are categorized into various classes. Internet access Distributed Information Storage and Sharing Information Exchange across multiple wireless networks

The areas of research in application areas include these classes.

Improving existing application layer protocols, proposing new application-layer protocols for distributed information sharing, and developing innovative applications for WMNs are the areas of more research.

Issues Issue in Network performance: Radio techniques,mesh connectivity, compatibility and

inter-operability etc. are the factors that influence the performance of WMNs

Security Issues: Security schemes for WMNs are still not ready. Due to the

distributed nature of WMN, there is no centralized authority that can be fully trusted and can distribute a public key.

Other Issue: Capacity, scalability, and QoS are considered as the major

weakness of current WMN technology

Conclusion

The nodes in a WMN automatically detect neighbor nodes and establish and maintain network connectivity in an ad hoc fashion.

The self-configuring nature facilitates the easy and rapid deployments of WMNs.

They dynamically adapt to the changing environments and can self heal in case of link or node failure.

Being inherently redundant they have a high level of fault tolerance and are hence robust.

The low cost of the hardware based on IEEE 802.11 standards is one of the prominent reasons in the increasing interests in both research and product development.

Conclusion

Integration and interoperation with other networks, and bridges to legacy networks can be easily accommodated due to the fact that most WMNs are based on Internet Protocol standards.

Based on existing technologies, some companies already have products for sale, while other companies are still not convinced to the point of production while others have started to deploy WMNs in various application scenarios.

Whatever the case be, it we look at the researchs and case studies, what we can find is that the performance of WMNs is still far below what is expected. There are various open issues that need to be resolved.

Reference

E.M. Royer, a. T. (April 1999). A Review of Current Routing Protocols for ad Hoc Mobile Wireless Networks'. IEEE Personal Communications Magazine .

Perkins, C. (2001). Ad-Hoc Networking. MA: Addison Wesley Professional, Reading.

Wang, Xudong, Kiyon, INC (n.d.). A Survey on Wireless Mesh Networks. Georgia Institute of Technology

Yan Zhang, J. L. (2007). Wireless Mesh Networking, Architectures, Protocols, and Standards. FL: Auerbach Publications

Questions

Wireless Mesh Network is a radical network form of the ever evolving wireless network. The Ad-hoc network is also a form of the wireless network. Compare and contrast the wireless mesh network and the Ad-hoc network that are the categories in the multi-hop wireless network.

Explain the characteristics of wireless mesh network and some of the factors of the mesh networks influencing the performance. Are these wireless network reliable and scalable?

Can you explain some of the management functions needed to maintain the operations of wireless mesh networks?

Ad-hoc VS WMNsThere are various factors which can differentiate wireless mesh

network from Ad-hoc Network. The major differences between these two types of network are the network topology and mobility of node Network topology: Ad- hoc network are called as the

infrastructure less network with a highly dynamic topology where as WMN have a relatively static network topology

Mobility of relay nodes: In Ad-hoc network the relay nodes have a higher degree of mobility i.e. relay nodes aren't fixed whereas in WMN's the degree of mobility of relay node is much lower than in Ad-hoc netowrks.

Ad-hoc VS WMNs Deployment: Some planning is required to deploy the WMNs

whereas Ad- hoc network is easy to deploy. Infrastructure requirement: Ad-hoc network is infrastructure

less whereas WMN has partial or fully fixed infrastructure. Energy constraints: In Ad-hoc network energy constraints is

high whereas it is low in the WMNs.

Characteristics of WMNs Dynamic self-configuration and self-organization:

The nodes in a WMN automatically detect neighbor nodes and establish and maintain network connectivity in an ad hoc fashion.

Typically implemented at the network layer with the use of ad hoc routing protocols

This self-configuring nature facilitates the easy and rapid deployments of WMNs.

Adaptation: WMNs dynamically adapt to the changing environments and can self

heal in case of link or node failure.

Fault tolerance and robustness: Mesh networks are inherently redundant and hence have a high level of

fault tolerance and robustness.

Characteristics of WMNs Low-Cost:

The low cost of the hardware based on IEEE 802.11 standards is one of the prominent reasons in the increasing interests in both research and product development.

Integration and interoperability: Integration and interoperation with other networks, and bridges to

legacy networks can be easily accommodated due to the fact that most WMNs are based on Internet Protocol standards.

Factors Influencing performance Radio techniques:

There are various approaches that have been proposed inorder to increase the capacity and flexibility of the wireless systems.

E.G. Directional antennas and smart antennas, MIMO systems, and multi-radio/multi-channel systems.

Reconfigurable radios, frequency agile/cognitive radios, and even software are some of the more advanced technologies that further improve the performance of a wireless radio.

All these advanced radio technologies are not mature and require better design in higher layer protocols such as MAC and routing protocols.

Factors Influencing performance Scalability:

Without the support of scalability, which is the critical requirement of WMNs, the network performance degrades as the network size increases.

If that is the case, effects such as routing protocols may not be able to find a reliable routing path, transport protocols may loose connections, and MAC protocols may experience significant throughput reduction may be prominent.

All the protocols (MAC layer to application layer) needs to be scalable in order to ensure scalability in WMNs

Factors Influencing performance Security:

There are many different security solutions proposed for wireless LANs but the security schemes for WMNs are still not ready.

Due to the distributed nature of WMN, there is no centralized authority that can be fully trusted and can distribute a public key.

Still, the security schemes that are proposed for ad hoc wireless networks can be adopted for WMNs.

The architecture for ad hoc network is different, causing differences in security mechanisms.

If ad hoc networks and WMNs were the same, still since, ad hoc networks are still in its developmental stage, the security solution it has is also not mature enough.

This suggests us to keep on looking for more secure and complete mechanisms

Factors Influencing performance Broadband and QoS

Most applications of WMNs are broadband services with heterogeneous QoS requirements.

This is the reason why communication protocols must consider various performance metrics.

Such performance metrics are: delay jitter, aggregate and per-node throughput, packet loss ratios along with end-to-end transmission delay, and fairness.

Factors Influencing performance Ease of Use:

The network management tools that efficiently maintain the operation, monitor the performance, and configure the different parameters of WMNs needs to be developed.

For the better performance of the network, routing protocol should be designed in such a way that it is independent of the power management, self organization behavior, and robustness in the link failure and in providing the fast user authentication services.

The above mentioned management tools along with the autonomous mechanisms in networking protocols helps in the rapid deployment of WMNS.

Factors Influencing performance Compatibility and Inter-operability

There are two types of clients: conventional clients and mesh clients. WMNs need to be backward compatible with conventional client nodes

i.e. it is a requirement to support both these clients. This asks for more from the mesh routers. Mesh routers must be capable

of the integration of heterogeneous wireless networks.

Reliability and Scalability? Reliability from self configuration and self organization characteristics

of the network. WMN automatically detect neighbor nodes and establish and maintain network connectivity in an ad hoc fashion, typically implemented at the network layer through the use of ad hoc routing protocols.

High level of fault tolerance makes it more reliable, since it has multiple path for the delivery of data.

This network is scalable for a small network but may be problematic while setting routing path for larger networks as it may take longer time to set up the routing path and also the end to end delay become larger.

Research is still going on to have the better routing protocol that gives better scalability to the network.

Management Functions Mobility Management:

Location management handles the location and call delivery. Thus distributed mobility management is needed for WMN's.

Location information can enhance the performance of MAC and routing protocols , and it can help to develop better location related applications.

Proposing accurate or efficient algorithms for location service is still an open research topic.

Mobility management is closely related to multiple layers of network protocols, so developing multi-layer mobility management schemes is another areas of research.

Management Functions Power Management

Power management controls connectivity, interference, spectrum-spatial reuse, and topology.

Mesh routers do not have a constraint on power consumption. Mesh clients desire protocols to be power-efficient.

Network Monitoring The statistics in the MIB of mesh routers need to be reported to one or several

servers in order to continuously monitor network performance. Based on the statistical information collected from the MIB, data processing

algorithms can accomplish many other functions such as network topology monitoring.