Research and development of WMNs is motivated by several applications which clearly demonstrate the promising market, but, at the same time, these applications cannot be supported directly by other wireless networks such as cellular systems, ad hoc networks, wireless sensor networks, standard IEEE 802.11, etc.
In this section, we discuss these applications [1, 9]:
• Broadband home networking:
Currently broadband home networking is realized through IEEE 802.11WLANs. An obvious problem is the location of the access points.
Without a site survey, a home (even a small one) usually has many dead zones without service coverage. Solutions based on site survey are expensive and not practical for home networking, while installation of multiple access points is also expensive and not convenient because of Ethernet wiring from access points to backhaul network access modem or hub. Moreover, communications between end nodes under two different access points have to go all the way back to the access hub. This is obviously not an efficient solution, especially for broadband networking. Mesh networking, as shown in Figure 1.7, can resolve all these issues in home networking. The access points must be replaced by wireless mesh routers with mesh connectivity established among them.
Therefore, the communication between these nodes becomes much more flexible and more robust to network faults and link failures. Dead zones can be eliminated by adding mesh routers, changing locations of mesh routers, or automatically adjusting power levels of mesh routers. Communication within home networks can be realized through mesh networking without going back to the access hub all the time. Thus, network congestion due to backhaul access can be avoided. In this application, wireless mesh routers have no constraints on power consumptions and mobility. Thus, protocols proposed for mobile ad hoc networks and wireless sensor networks are too cumbersome to achieve satisfactory performance in this application. On the other hand, Wi-Fis are not capable of supporting ad hoc multihop networking.
As a consequence, WMNs are well suited for broadband home networking.
Figure 1.7 WMNs for broadband home networking
• Community and neighborhood networking:
In a community, the common architecture for network access is based on cable or digital subscriber line (DSL) connected to the Internet, and the last hop is wireless by connecting a wireless router to a cable or DSL modem. This type of network access has several drawbacks.
– Even if the information must be shared within a community or neighborhood, all traffic must flow through the Internet. This significantly reduces network resource utilization.
– A large percentage of areas in between houses is not covered by wireless services.
– An expensive but high-bandwidth gateway between multiple homes or neighborhoods may not be shared, and wireless services must be set up individually. As a result, network service costs may increase.
– Only a single path may be available for one home to access the Internet or communicate with neighbors.
WMNs mitigate the above disadvantages through flexible mesh connectivities between homes, as shown in Figure 1.8. WMNs can also enable many applications such as distributed file storage, distributed file access, and video streaming.
Figure 1.8 WMNs for community networking
• Enterprise networking:
This can be a small network within an office or a medium-size network for all offices in an entire building, or a large-scale network among offices in multiple buildings. Currently standard IEEE 802.11 wireless networks are widely used in various offices.
However, these wireless networks are still isolated islands.
Connections among them have to be achieved through wired Ethernet connections, which is the key reason for the high cost of enterprise networks. In addition, adding more backhaul access modems only increases capacity locally, but it does not improve robustness to link failures, network congestion, and other problems of the entire enterprise network. If the access points are replaced by mesh routers, as shown in
Figure 1.9, Ethernet wires can be eliminated. Multiple backhaul access modems can be shared by all nodes in the entire network, and thus improve the robustness and resource utilization of enterprise networks. WMNs can grow easily as the size of enterprise expands.
WMNs for enterprise networking are much more complicated than at home because more nodes and more complicated network topologies are involved. The service model of enterprise networking can be applied to many other public and commercial service networking scenarios such as airports, hotels, shopping malls, convention centers, sport centers, etc.
Figure 1.9 WMNs for enterprise networking
• Metropolitan area networks (MAN):
WMNs in a metropolitan area have several advantages. The physical-layer transmission rate of a node in WMNs is much higher than that in any cellular systems. For example, an IEEE 802.11g node can transmit at a rate of 54 Mbps.Moreover, the communication between nodes inWMNs does not rely on a wired
backbone. Compared to wired networks, e.g., cable or optical networks, wireless mesh MAN is an economic alternative to broadband networking, especially in underdeveloped regions. The wireless mesh MAN covers a potentially much larger area than home, enterprise, building, or community networks, as shown Figure 1.10.
Thus, the requirement on the network scalability by wireless mesh MANs is much higher than that by other applications.
Figure 1.10 WMNs for metropolitan area networks
• Transportation systems:
Instead of limiting IEEE 802.11 or 802.16 access to stations and stops, mesh networking technology can extend access into buses, ferries, and trains. Thus, convenient passenger information services, remote monitoring of invehicle security video, and driver communications can be supported. To enable such mesh networking for a transportation system, two key techniques are needed: the highspeed mobile backhaul from a vehicle (car, bus,
or train) to the Internet, and mobile mesh networks within the vehicle, as shown in Figure 1.11.
Figure 1.11 WMNs for transportation systems
• Building automation:
In a building, various electrical devices including power, light, elevator, air conditioner, etc., need to be controlled and monitored.
Currently, this task is accomplished through standard wired networks, which is very expensive due to the complexity in deployment and maintenance of a wired network. Recently, Wi-Fi-based networks have been adopted to reduce the cost of such networks.
However, this effort has not achieved satisfactory performance yet, because the deployment of Wi-Fi for this application is still rather expensive due to the wiring of Ethernet. If BACnet (Building Automation and Control networks) access points are replaced by mesh routers, as shown in Figure 1.12, the deployment cost will be significantly reduced. The deployment process is also much simpler due to the mesh connectivity among wireless routers.
Figure 1.12 WMNs for building automation
• Health and medical systems:
In a hospital or medical center, monitoring and diagnosis data need to be processed and transmitted from one room to another for various purposes. Data transmission is usually broadband, since high resolution medical images and various periodical monitoring information can easily produce a constant and large volume of data. Traditional wired networks can only provide limited network access to certain fixed medical devices. Wi-Fi-based networks must rely on the existence of Ethernet connections, which may cause high system cost and complexity but without the abilities to eliminate dead spots. However, these issues do not exist in WMNs.
• Security surveillance systems:
As security is turning out to be a very high concern, security surveillance systems become a necessity for enterprise buildings,
shopping malls, grocery stores, etc. In order to deploy such systems at locations as needed, WMNs are a much more viable solution than wired networks to connect all devices.
Since still images and videos are the major traffic flowing in the network, this application demands much higher network capacity than other applications.
In addition to the above applications, WMNs can also be applied to spontaneous (emergency/disaster) networking and P2P communications. For example, wireless networks for an emergency response team and firefighters do not have in-advance knowledge of where the network should be deployed. By simply placing wireless mesh routers in desired locations, a WMN can be quickly established. For a group of people holding devices with wireless networking capability, e.g., laptops and PDAs, P2P communication anytime anywhere is an efficient solution for information sharing. WMNs are able to meet this demand. These applications illustrate that WMNs are a superset of ad hoc networks, and thus, can accomplish all functions provided by ad hoc networking.