Wireless Mesh Network (WMN)

A wireless mesh network is an internet protocol (IP) communications network consisting of wireless nodes organized in a logical mesh topology which can be implemented with various wireless technologies including 802.11, 802.15, 802.16, cellular technologies or combinations of more than one type (Airberry, 2012). It is also one of the latest WLAN technologies for providing large network coverage with low deployment cost, as well as for increasing network flexibility and robustness (Vanhatupa et al., 2008; Huang et al. 2008; Hou

et al. 2008; Baiamonte et al. 2008). WMN are anticipated to resolve the limitations and to

significantly improve the performance of ad hoc networks, wireless LAN, wireless PAN and wireless MAN (Akyildiz et al., 2005). While providing predictable Quality of Service (QoS) for users, consideration should also be given to the deployment cost, service area, number of users, and resource utilization. Similarly, internet providers use mesh networks as backbone connections to offer their customers Internet access (Airberry, 2012; Jun and Sichitiu, 2008; Hossain and Leung, 2008; Conti et al. 2008). According to Hossain and Leung (2008), since the concept can be used for different wireless access technologies such as IEEE 802.11, 802.15, 802.16-based wireless LAN, wireless PAN and wireless MAN technologies, the potential application scenarios for wireless mesh networks include backhaul support for cellular networks, home networks, enterprise networks, community networks, and intelligent transport system networks. Hence, Wireless coverage of a large area can be achieved in a lot of ways, but none as efficient as utilizing modern day wireless mesh technology. According to Airberry (2012), originally mobile wireless solution for military applications in the US, have been used since 2000 mainly as civil network solutions for whole streets to connect private households to broadband Internet via WLAN, which gained rapidly in popularity. As a result, mesh networks found its way into industry applications such as network measuring devices and meters, in company and city networks, in mobile applications between vehicles, and based on other technologies, such as Zigbee, used in sensor networks where cabled networks would be too complex, uneconomical or simply impossible. Consequently, Hashemian (2011) noted that wireless sensors are becoming very popular in industrial processes for measurement and control, condition monitoring, predictive maintenance, and management of operational transients and accidents. Accordingly, many sensor manufacturers have teamed up with companies who make wireless transmitters, receivers, and network equipment to provide industrial facilities with integrated networks of wireless sensors that can be used to measure process temperature, pressure, vibration, humidity, and other parameters to improve process safety and efficiency, increase output, and optimize maintenance activities. Therefore, power generation utilities have begun to use wireless technologies in their fossil, co-generation, and nuclear power plants. Hence, in NPP, redundancy is an important aspect of defence against mishaps and wireless sensors provide an easy, cost-effective path to redundancy without compromising safety. However, a process

parameter may be measured with both wired and wireless sensors. But the wired sensors can be designated as the primary element and used all the time, while the wireless sensors, as the back-up element and used only when the wired sensor is unavailable (Mok et al. 2010). The advantage of this for instance is in the case of a loss-of coolant accident (LOCA), where wires may become wet and provide poor signals while wireless sensors could be made to be more immune to water damage and provide more reliable signals for post- accident monitoring of the plant. This however offers not only redundancy but also diversity. The technology has therefore matured to the point that it can now be safely applied in industrial control, monitor, and asset management applications being cost-effective alternative communication path for many legacy control systems, enabling access to the intelligent information in field devices. Hence, it provides simple and reliable way to deploy new points of measurement and control without the wiring costs and without having to completely change existing systems as it provides an infrastructure for both central as well as mobile users to access their process and process equipments. The comparison between various wireless technologies is as shown below on Table. 6.3 (Korsah et al. 2009):

Table 6.3: Comparison of Wireless Technology (Sidhu et al. 2007)

Technology

WiFi – 802.11n

ZigBee

WiMAX

Application Wireless LAN, Internet Sensor Networks Metro Area Broadband Internet connectivity

Typical Range 100m 70-100m 50km

Data Rate 108 – 600Mbps 250Kbps 75Mbps

Modulation DSSS DSSS QAM

Network IP & P2P Mesh IP

IT Network Connectivity YES NO YES

Network Topology Infrastructure (Ad-hoc also possible)

Ad-hoc Infrastructure

Access Protocol CSMA/CA CSMA/CA Request /Grant

Key Attributes Wider Bandwidth, Flexibility

Cost, Power

Throughput, Coverage

There are several on-going research efforts to improve the capacity of WMN by exploiting alternative approaches such as multiple radio interfaces, directional antennas, multiple-input multiple-output (MIMO) techniques and modified medium access control (MAC) protocols (Conti et al. 2008). Accordingly, by using directional transmission, the interference between network nodes can be mitigated which will eventually improve the capacity of the network and also improve energy efficiency. However, this brings challenges to the MAC protocol design. The MIMO technique consists of using multiple antennas which potentially increases the system’s capacity. MIMO deploys simultaneous transmissions and transmit/receive

diversity (receive diversity is when the same information is received by different antennas while transmit diversity is when the same information is sent from multiple transmit

antennas). However, an efficient MAC protocol exploiting MIMO characteristics is needed to achieve significant throughput improvement. As far as the MAC protocols are concerned, scalability is still a very challenging issue for designing an efficient MAC protocol for WMN. Most of the existing MAC protocols solve the problem partially, but raise other problems such as throughput, capacity or fairness. Moreover, a MAC protocol for WMN must consider both scalability and heterogeneity between different network nodes (i.e. mesh routers, mesh clients).

6.2.2.5.1 Advantages of WMN

The following are the advantages of WMN (Akyildiz et al. 2005; Gungor et al. 2008; Huang et

al. 2008; Hou et al. 2008):

Low up-front cost: Using fewer wires means it costs less to set up a network, particularly for large areas of coverage.

Reliable service coverage: Wireless mesh nodes are easy to install and uninstall, making the network extremely adaptable and expandable as more or less coverage is needed.

Easy network maintenance: Mesh networks are "self configuring;" the network automatically incorporates a new node into the existing structure without needing any adjustments by a network administrator.

Robustness: They are useful for Non-Line-of-Sight (NLoS) network configurations where wireless signals are intermittently blocked. For example, in an amusement park where Ferris wheel occasionally blocks the signal from a wireless access point. If there are dozens or hundreds of other nodes around, the mesh network will adjust to find a clear signal.

Mesh networks are "self healing," since the network automatically finds the fastest and most reliable paths to send data, even if nodes are blocked or lose their signal.

However, the feature of dynamically self-organized and self-configured of WMN (Gkelias and Leung, 2008; Akyildiz et al. 2005; Gungor et al. 2008; Huang et al. 2008; Hou et al. 2008;

Zhang et al. 2008; Agg’elou, 2009) whereby the nodes in the mesh network automatically

establish and maintain network connectivity brings many advantages to the end-users. Accordingly, with the use of advanced radio technologies such as multiple radio interfaces and smart antennas increased the network capacity significantly. Also, the gateway and bridge functionalities in mesh routers enable the integration of wireless mesh networks with various other existing wireless networks such as wireless sensor networks, wireless-fidelity (Wi-Fi), and Wi-Max respectively. Zhang et al. (2008) have noted that there are still several

challenges and issues preventing WMNs to be widely deployed in large scales. The first major issue is that the performance (throughput, delay, or packet loss rate) of WMNs drops sharply with increasing number of wireless hops the packets traverse through. The multi- radio, multi-channel technique is being researched to overcome this problem. The second major issue is the lack of an integrated cross-layer solution to provide security in WMNs, which has received meagre attention in the literature. Clearly, without a well designed security solution, WMNs are vulnerable to various types of internal and external attacks that may cause significant inconvenience to the users and operators. Hence, to ensure security in any application, the following general goals are desired:

Confidentiality or Privacy Integrity Availability Authentication Authorization Accounting

According to Baiamonte et al. (2008), MWN have two types of nodes: mesh routers and mesh clients. Both types of nodes operate not only as hosts but also as routers, forwarding packets on behalf of other nodes that may not be within direct wireless transmission range of their destinations. Also, a mesh router may have gateway/bridge functionalities. The following are the benefits and characteristics of WMN (Gungor et al., 2008; Akyildiz et al. 2005):

Increased Reliability Low Installation Costs Large Coverage Area

Automatic Network Connectivity

6.2.2.5.2 WMN Core Components

WMN have four core components; Mesh access points, Prime infrastructure, Wireless LAN controller and Mesh software architecture (Cisco, 2012):

Mesh access points

The proposed Cisco Aironet 1550 Series Outdoor Mesh Access Point is a modularized wireless outdoor 802.11n access point design that supports point-to-multipoint mesh wireless connectivity, wireless client access simultaneously and also operate as a relay node for other access points that are not directly connected to a wired network. Through intelligent wireless routing provided by the Adaptive Wireless Path Protocol (AWPP), the access point can identify its neighbours and intelligently choose the optimal path to the wired network by calculating the cost of each path in terms of signal strength and the number of hops required to get to a controller.

Prime infrastructure

The Prime Infrastructure is used to design, control, and monitor WMN from a central location. However, the Prime Infrastructure provides a graphical platform for wireless mesh planning, configuration, and management. The Prime Infrastructure therefore runs on a server platform with an embedded database, which provides scalability that allows hundreds of controllers and thousands of Cisco mesh access points to be managed.

Wireless LAN controller (referred to as controller)

The wireless mesh solution is supported on Cisco 2500, 5500, and 8500 Series Wireless LAN Controllers. According to Cisco (2011), the controller works in conjunction with Cisco lightweight access points and the Cisco Wireless Control System (WCS) to provide system- wide wireless LAN functions. It also provides real-time communication between wireless access points and other devices to deliver centralized security policies, guest access, Wireless Intrusion Prevention System (WIPS), context-aware (location), RF management, quality of services for mobility services such as voice and video, and office extended access

point (OEAP) support for the teleworker solution. Figure 6.5 showsa 2504 controller network

topology and network connections with the required medium dependent interface (MDI) Ethernet cables.

Mesh Software Architecture

The mesh network architecture such as Cisco Wireless Control System (WCS) is the industry's most comprehensive management platform for lifecycle management of 802.11n and 802.11a/b/g, enterprise-class wireless networks. It delivers cost-effective management solution that enables successful planning, deployment, monitoring, troubleshooting and reporting of indoor and outdoor wireless networks. Though, it runs on a server platform with an embedded database but it provides the scalability necessary to manage hundreds of wireless LAN controllers which in turn can manage thousands of Cisco Aironet lightweight access points. As shown in Figure 6.5 above, Cisco wireless LAN controllers can be located on the same LAN as Cisco WCS, on separate routed subnets, or across a wide-area connection (Cisco, 2007; Cisco, 2012).