An Introduction To Wireless Mesh Networks

Published Date: 02 Nov 2017

Wireless mesh networks (WMNs) are becoming increasingly popular as they provide mobility support, flexibility, and are easy to deployable. A WMN is a set of wireless nodes. Each node can communicate with each other and also can forward packets of each other. Every node is both a host and a wireless router. Wireless routers acts as access points (APs) for wireless mobile devices. Some of them also act as gateways to the Internet through high speed wired links.

WMNs are constituted with two nodes: (1) mesh routers and (2) mesh clients. A mesh router having advanced routing functions to support multi-hop mesh networking. Using multi-hop communications, the more coverage can be achieved by a mesh router with less transmission power. To increase the features of mesh networking, a mesh router is generally equipped with (NICs) wireless network interface cards. Mesh and wireless routers are generally made based on a similar hardware platforms. The mobility of mesh routers have minimum and form the mesh backbone for mesh clients. Mesh clients can be either mobile or stationary depends upon type of network. Communication protocols for mesh clients can be light-weight. Only a single wireless interface may be used in a mesh client.

Mesh Networking Defined

To understand mesh networking, we first need to obtain an appreciation for what a mesh topology represents. If we have n nodes in a network, where the term "node" refers to a communications device that can transport data from one of its interfaces to another, then the ability of each node to communicate with every other node in the network represents a mesh network topology. We can view the structure of a mesh network by simplifying the number of nodes in the network from a value of n , which is what mathematicians like to work with, to an easy-to-visualize number, such as three, four, or five [3].

Nodes and Links

Figure 1.1 illustrates three-, four-, and five-node mesh network structures, in which each node has a communications connection to all other nodes in the network. The connection between each node is referred to as a link. If we examine the number of links associated with each network shown in Figure 1.1, it’s obvious that the number of links increases as the number of nodes increases. Although only three links are required to interconnect three nodes, six are required to interconnect four nodes, and ten are required to interconnect five nodes. If you take the time to draw six nodes and interconnect each, you would then note the need for fourteen links. What this means is that a classical mesh network structure in which each node is interconnected to every other node in the network becomes impractical as the number of nodes in the network increases. After all, when networks were first constructed, the links interconnecting nodes were dedicated or leased telephone lines. This meant that a separate physical interface was required by a node to connect to each link. That interface primarily performed parallel to serial and serial to parallel conversion, because data flows bit by bit on a serial link. Because each interface requires buffer memory and a node is a computing device with a finite amount of processing power, adding interfaces increases the amount of processing the node needs to perform until one interface too many is added that saturates the processing capability of the node. Thus, from a classical perspective, a mesh network in which every node can directly communicate with every other node has physical constraints that limit the number of nodes that can be interconnected [3].

Figure 1.1 in a true mesh network structure, each node has a connection to every other node in the network.

2. WMN Overview

In this part, we will present a technical and useful overview of WMN.

2.1 Classification of WMNs

Wireless networks can be classified based on the Point to Point (PTP), Point to Multi-Point (PTM) or Multi-Point to Multi-Point (MPM) networks.

Point-to-Point structure consisted of a link interconnecting two nodes. This network structure is simply referred to as a point-to-point link because it directly interconnects two locations.

PTM networks are scalable, but they have low adaptability and reliability.

MTM wireless networks use multiple hops to increase coverage without the need for increasing the transmission power.

Multipoint

To reduce the cost of leased lines, vendors developed poll and select software that enabled multiple terminal devices to be connected to a common communications line. Referred to as multi-drop or multipoint networking, terminals were either individually located in different geographical areas or were clustered together and connected to a common communications line by a control unit. For either situation poll or select software permits multiple terminal devices to share a common communications line.

2.2 Advantages of WMNs

Some advantages of WMNs are given as below:

Self Organizing and Self Configuring Network: Ad hoc networking enhances network performance, such as flexible network architecture, easy deployment and configuration, fault tolerance, and mesh connectivity, i.e., multipoint-to-multipoint communications. Due to these features, WMNs have low upfront investment requirement, and the network can grow gradually as needed.

Reduction of Deployment Cost: Mesh routers are wireless and they have the ability to service in multi-hop data communication. Thus, using wireless routers in large areas are cheaper compared to single hop routers or access points that they have wired connections.

Increased Reliability: In a WMN, the multiple paths from source to destination nodes provide alternate paths in case of failure. Alternate paths may be selected, in order to reduce the bottlenecks in congested area of the network too. This also allows the traffic loads to be balanced in the network. Load balancing and reducing the bottleneck via alternate routing can significantly increase network reliability in WMNs.

Scalability: In wireless networks, when nodes are increased, the network performance will be affected. But, in WMNs, increasing the number of nodes will increase transmission capacity for better load balancing and alternate routes. Usually, the local packets (generated in clients of mesh router) run faster compared to packets (generated in two or more hops away) from the neighbours.

Interoperability: WMN has a hybrid multi-point to multi-point architecture which is compatible with existing standards such as: WiMAX, Cellular, Wi-Fi, Bluetooth, Sensor, MANET, etc. All technologies mentioned above, are able or will be able soon to configured a WMN and communicate with each others. Most of necessary improvements needed in any type of networks to enable them communicate with others; can augment the current standards to maintain interoperability.

3. Wireless Mesh Network architecture:

WMNs generally having two types of nodes: mesh routers and mesh clients. A wireless mesh router contains additional routing functions to support mesh networking. A mesh router is usually equipped with multiple wireless interfaces built on either the same or different wireless access technologies. Mesh clients also have the additional functions for mesh networking, and thus can also work as a router in WMN.

Fig. Wireless Mesh Network

3.1 Type of nodes in WMNs

WMN clients are the devices used by end-users: such as: laptops, PDAs, smart phones, etc. Client nodes constitute the actual network to perform routing and configuration functionalities as well as providing end user applications to customers. Therefore, a mesh router is not required for these types of networks. Such devices are considered to be mobile; they have limited power, they may have routing capability. These devices may or may not be always connected to the network.

Fig. Examples of mesh clients: (i) laptop, (ii) PDA, (iii) Wi-Fi IP phone, (iv) Wi-Fi

RFID reader

WMN routers The mesh routers form a mesh of self-configuring, self-healing links among themselves. With gateway functionality, mesh routers can be connected to the Internet. This approach, also referred to as infrastructure meshing, provides backbone for conventional clients and enables the integration of WMNs with existing wireless networks, through gateway/bridge functionalities in mesh routers. Transmission power consumption in mesh routers is low, for multi-hop communications strategy.The Medium Access Control (MAC) protocol in a mesh router supports multiple-channels and multiple interfaces to enable scalability in a multi-hop mesh environment.

Fig. Examples of mesh routers based on different embedded systems: (a) PowerPC, (b) ARM

WMN gateways are routers with direct access to the wired infrastructure/Internet. Because the gateways in WMNs have multiple interfaces to connect to both wired and wireless networks, hence they are expensive. Therefore, there are a few number of WMN gateways in the network. Moreover, these WMN gateways play a great role on the performance of the network.

3.2 Applications of WMNs

There are some applications which are directed and fully supported by Wireless Mesh Networks [4]:

Broadband Wireless Access: Currently, Broadband access has an important role in information economy. It provides services for real time applications such as: video telephony, online-gaming, video on demand, and Tele-communications. Each new application has a significant impact on quality of life. For example, Telecommuting can reduce daily travelling of individuals. It leads to increased productivity for the time saving. It also reduces traffic on the streets, thus it has a positive impact on the environment. In urban and sub-urban areas, wired access (Cable and DSL) is the first choice if the population density be reasonably high. Rural areas have limited coverage using wireless technologies like satellite and cellular networks. Satellite access has two drawbacks: expensive technology and high latency due to the distance between the end client and the satellite. In the case of cellular networks the towers are expensive to install and operate. Lack of service providers and the higher cost of the service itself make lower usage of broadband access. Deployment and operations of WMNs, free broadband access to city residents are also become possible. Many such networks already exist, and more are on the way.

Industrial Applications: Building automation: In a building, there are many devices need to be monitored and controlled like electrical devices including power, light, air conditioner, elevator, etc. Today, the wired networks are taking care of such environment. This is very expensive due to the complexity in deployment and maintenance of a wired network. Currently, Wi-Fi networks are another option to reduce the cost of such networks. But, this solution has not achieved satisfactory performance yet for expensive wiring of Ethernet which is needed for Wi-Fi Access Points (APs). Replacing APs by mesh routers will solve the problem. The deployment process will be much simpler and also the deployment cost will be significantly

Healthcare: In a hospital or medical centre, monitoring and updating patient information like medical history, test results, insurance information, etc., need to be processed and transmitted from room to room. The ability to connect to the network is crucial to ensure data access in every operating room, office, and lab. In many hospitals data transmission is usually broadband due to large amount of data, for instance: high resolution medical images and periodical monitoring information. WMN provides unlimited network access to any fixed medical devices. It does not need to use existed Ethernet connections, so that, it will eliminate dead spots and it also cause low system cost and simplicity which cannot be find in traditional wired networks.

Transportation systems: Internet access is limited to stations and stops using IEEE 802.11 and IEEE 802.16. To extend access into buses, plains, ferries, and trains, WMN technology can help. Thus, passengers on-board can access to the net while travelling from one place to another. Other services such as remote monitoring in-vehicle, driver communications and security cameras can be supported too.

Hospitality: In hotels and resorts, one of their services is high-speed Internet connectivity which is free. Wireless mesh networks are easy to set-up, lower in cost, and without having to change the existing structures or disrupt business for both indoor and outdoor.

Warehouses: One way to keep track of stock in warehouses is using handheld scanners. It needs connectivity throughout the area. Wireless mesh networks can ensure connectivity in modern warehouses and shipping logistics with little cost and effort.

Temporary Venues: Construction sites can enjoy the easy set-up and removal of wireless mesh networks. Architects and engineers can stay connected and using camera to communicate and talk to each other on spot. It provides them to see the real picture of the project progresses. Other temporary venues such as: political rallies, street fairs, and outdoor concerts can set-up and remove wireless mesh networks in minutes.

4. Issues and Challenges in WMNs

Issues and Challenges in wireless mesh networks at different layers, which are briefly discussed here:

4.1 Physical Layer Issues

The challenges of the physical layer of WMNs are not basically different from other wireless technologies. At least, the physical layer of a WMN should be reliable. The undesirable effects of fading and interference are well understood and several (typically spread spectrum) solutions (Frequency Hopping Spread Spectrum (FHSS), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiplexing (OFDM) and Ultra-Wide Band (UWB)) are routinely employed to increase the reliability of the radio transmission. Since the MAC protocol of WMNs is commonly contention based, resistance to interference is more important than in the case of cellular systems and 802.16 that enjoy practically collision free MAC protocols. Beyond basic reliability requirement, several other characteristics such as Mobility, Link Adaptation, Variable Transmission Power, Multiple Transceivers, Directional Antennas, Transceiver Performance, can make a significant difference for the performance of WMNs [5].

4.2 MAC (Medium Access Layer) Issues

In WMNs, improvements to the traditional contention based protocols are usually not sufficient to improve allocation efficiently and fairness. Traditional MAC protocols are limited. Thus advantage of newer underlying models is limited. Multiple radios and multiple channels bring new problems of channel assignment and medium access for instance. Multiple Input and Multiple Output (MIMO) radios have been proposed to increase the capacity of WMNs to mitigate unfair access and under utilization. However, current MAC protocol cannot take advantage of this underlying technological improvement [4].

4.3 Transport Layer Issues

WMNs have some challenges and problems at the transport layer. The transport protocols should efficiently utilize available network resources and allocate them fairly. However, fairness problem in wireless mesh networks is inherently due to the interdependencies among neighbouring wireless links [4].

4.4 Network Layer Issues

At the network layer, distinct characteristics and traffic flow direction, is highly skewed between the client and the gateway. In order to take the advantage of this, WMNs need the new and improved protocols [4].

4.5 Topological and Deployment Issues

The key purpose of a WMN is to equip the end users with high speed Internet access. To achieve this, the design of the network architecture should be addressed carefully. This is a basic issue, and providing Quality of Service (QoS) for end users and determining the network performance is critical for a WMN. Planning a WMN includes determining the number of gateways, optimal placement of gateways, utilizing bandwidth, and minimizing the deployment cost. There are typically two types of deployment: structured deployment and organic deployment. In structured deployment, services will be provided in a new area, thus, it has the flexibility of choosing the topology. This flexibility may translate into improved network performance by capturing the regularity of the deployed mesh network. On the other hand, in organic deployment, the mesh network will be deployed organically over existing infrastructure. Thus, there are limited options of topology for the network architect to choose [4].

5. Conclusions and Directions for Further Research

WMNs (multi-hop) Technology is facing many problems while it has many great advantages which make it a most likely technology of today. This paper presents the quick and technical brief overview of concept, technology, and architecture for wireless mesh networks. There are issues and challenges in WMNs like: fairness, power management, node mobility management, secure routing, connectivity with the Internet and connectivity with other type of networks, service levels, etc., at different layers of WMNs. Since many of these aspects have been presented in literature, we direct the readers to the most recent and critical challenge on routing and deployment issues. To ensure fast and enhance the performance of routing of packets on WMNs, we will develop a routing protocol based on routing algorithm and routing matrices in our further research work.