Section 6.6. Wireless Mesh Networks (WMNs)

6.6. Wireless Mesh Networks (WMNs)

A wireless mesh network (WMN) is a dynamically self-organized wireless network that maintains mesh connectivity. WMNs help users stay online anywhere , anytime , for an unlimited time. One key component that makes this happen is the type of wireless router used in mesh infrastructures . We look at applications of WMNs, and WiMax networks, the conductivities of P2P networks with backbone wireless mesh networks.

6.6.1. WiMAX Technology and IEEE 802.16

The worldwide interoperability for microwave access (WiMAX) technology is a certification mark for the IEEE 802.16 standard. This standard is implemented for point-tomultipoint broadband wireless access. WiMAX is a wireless WAN technology that can connect IEEE 802.11 WiFi hotspots with one another and to other parts of the Internet. WiMAX also provides a wireless alternative to cable and DSL for broadband access. WiMAX devices are capable of forming wireless connections to allow Internet packets to be carried across a network. Conceptually, WiMAX is similar to WiFi technology but has been improved for use over much greater distances.

The IEEE 802.16 standard offers a significant improvement for communications, as it defines a MAC layer that supports multiple physical-layer specifications, potentially making WiMAX a great framework for wireless broadband communications. The 802.16 MAC is a scheduling MAC whereby the user device competes once for initial entry into the network. After being in the network, the base station allocates a time slot to the user . This time slot can enlarge or constrict, and no other users can use it. Unlike 802.11, the 802.16 scheduling algorithm exhibits stability given overload and offers better bandwidth efficiency. Another advantage is that 802.16 lets the base station offer QoS by balancing the assignments of users.

The IEEE 802.16 standard has determined the frequency range of 10 GHz to 66 GHz. WiMAX improves on the WiFi standard by providing increased bandwidth and stronger encryption. WiMAX makes excellent use of multipath signals. IEEE 802.16 dictates up to 50 km of connectivity services between users without a direct line of sight. This does not mean that a user 50 km away with no line of sight has connectivity, and practically, this distance is 5 km to 8 km. The data rate with WiMAX is up to 70 Mb/s, which is sufficient to simultaneously support more than 60 businesses with T-1-type connectivity. The line of sight is about 1,000 homes at 1 Mb/s DSL-level connectivity.

WiMAX antennas can share a cell tower without impacting the normal operations of the cellular network. A WiMAX antenna can even be connected to an Internet backbone via optical fibers or directional microwave link. WiMAX may be considered for cities or countries willing to skip a wired infrastructure, establishing a wireless infrastructure in an inexpensive, decentralized, deployment-friendly, and effective manner.

6.6.2. Applications of Mesh Networks

In WMNs, two types of nodes perform the routing: mesh routers and mesh users . Figure 6.19 shows detailed connectivity of a backbone mesh network to WiFi, WiMAX, and wireless cellular networks. In this figure, a WiFi network, a cellular network, and a WiMAX network are connected through mesh routers with gateway bridges . A router with gateway bridge capability enables the integration of WMNs with other type networks, although traditional routers with regular network interface cards (NICs) can connect to mesh networks.

Figure 6.19. Overview of a backbone mesh network and connections to WiFi, WiMAX, and wireless cellular networks

Mesh users can also operate as routers for mesh networking, making the connectivity much simpler and faster than conventional wireless networks with base stations . Figure 6.20 shows another scenario, in which a wireless mesh network backbone is connected to wireless mesh users. Users are communicating in an ad hoc fashion, with each individual user acting as a router and connected to a mesh router gateway. In this network, wired users, as shown by a LAN in the figure, can also be connected to WMN, using a mesh router gateway. User meshing provides peer-to-peer networks among users (discussed in Chapter 16).

Figure 6.20. Connectivity between a backbone wireless mesh network to wireless users and other networking devices

The inclusion of multiple wireless interfaces in mesh routers significantly enhances the flexibility of mesh networks. WMNs offer advantages of low cost, easy network maintenance, and remarkably more reliable service coverage than conventional ad hoc networks. Mesh networks are being designed for metropolitan and enterprize networking, and most of standards, such as IEEE 802.11, IEEE 802.15, and IEEE 802.16, are accepted in WMN infrastructures. A widely accepted radio technology is the series of IEEE 802.11 standards.

The benefits of a mesh network are as follows :

• Scalability. The WMN infrastructure is designed to be scalable as the need for network access increases .

• Ad hoc networking support. WMNs have the capability to self-organize and be connected to certain points of ad hoc networks for a short period of time.

• Mobility support of end nodes. End nodes are supported through the wireless infrastructure.

• Connectivity to wired infrastructure. Gateway mesh routers may integrate heterogeneous networks in both wired and wireless fashions .

To achieve scalability in WMNs, all protocolsfrom the MAC layer to the application layermust be scalable. "Topology-" and "routing-aware" MAC can substantially improve the performance of WMNs.

The QoS provisioning in wireless mesh networks is different from that of classical ad hoc networks. Several applications are broadband services with heterogeneous QoS requirements. Consequently, additional performance metrics, such as delay jitter and aggregate and per node throughput, must be considered in establishing a route. Application-specific security protocols must also be designed for WMNs. Security protocols for ad hoc networks cannot provide any reliability, as the traffic in such networks can resemble the one flowing in the wired Internet.

6.6.3. Physical and MAC Layers of WMNs

The physical layer of wireless mesh networks benefits from existing modulation and coding rates. Orthogonal frequency multiple access (OFDM) and ultrawide band (UWB) techniques are used to support high-speed wireless communications.

Physical Layer

WMN communication quality and system capacity have been improved through the use of multiantenna systems, such as antenna diversity, smart antenna, and MIMO ( multiple-input multiple-output ) systems. MIMO algorithms send information over two or more antennas. The radio signals reflect objects, making multiple paths that in conventional radios cause interference and fading. A MIMO system uses these paths to carry more information. Another improvement in WMNs includes the use of cognitive radios , which dynamically capture unoccupied spectrum. One of the unique features of this technology is that all components of its radio, including RF bands, channel-access modes, and even channel modulations, are programmable.

MAC Layer

The WMN MAC layer is different from classical wireless networks. In WMNs, the MAC layer

1. Is designed to face more than one-hop communication

2. Is distributed to support multipoint-to-multipoint communications

3. Has self-organization features

4. Has moderately lower mobility than in classical wireless networks

Wireless mesh MAC protocols can be designed for both a single channel or multiple channels or to even operate simultaneously. A multiple-channel MAC setup improves network performance significantly by increasing network capacity. Because of the poor scalability of CSMA/CA schemes, these techniques are not efficient solutions for single-channel MAC protocols. The best solution for WMNs is the enhanced versions of TDMA or CDMA, owing to low complexity and cost.

Multichannel MACs can be deployed in various ways. With multichannel single-transceiver MAC, only one channel can be active at a time in each network node, as only one transceiver is available. With multichannel multitransceiver MAC, several channels can be active simultaneously and only one MAC-layer module is assigned to coordinate all channels. With multiradio MAC, each node has multiple radios, and each radio has its own MAC layer and physical layer.