**1. Introduction**

78 Wireless Mesh Networks – Efficient Link Scheduling, Channel Assignment and Network Planning Strategies

2001.

[34] H. Y. Hsieh and R. Sivakumar, "Improving Fairness and Throughput in Multi-Hop Wireless Networks," *Proceedings of IEEE International Conference on Networks*, pp.569-578,

[35] S. Lakshmanan, R. Sivakumar, and K. Sundaresan, "Multi-Gateway Association in Wireless Mesh Networks," *Ad Hoc Networks*, Vol. 7(3), pp. 622–637, May 2009.

> In recent years, wireless mesh networks (WMNs) were deployed as a type of next generation wireless broadband networks. WMNs provide wireless broadband accessibility to extend the Internet connectivity to the last mile and improve the network coverage. WMN consists of a set of mesh routers and mesh clients (Fig. 1). *Mesh routers* are usually stationary and form multi-hop wireless backbone network (i.e. mesh routers are interconnected with each other via wireless medium). Some or all of the mesh routers also serve as access points for mobile users (*mesh clients*) under their coverage. Usually one or more mesh routers have direct connections to wired network and serve as Internet gateways for the rest of the network. These nodes are called *mesh gateways*. Compared to traditional wireless LANs, the main feature of WMNs is their multi-hop wireless backbone capability (Conti et al., 2007).

> Traditionally, wireless networks are equipped with only one IEEE 802.11 radio interface. However, a single-interface inherently restricts the whole network by using only one single channel (Fig. 3a). In order to communicate successfully, two neighboring routers have to build a logical link which operates on a common channel. Due to that, all wireless nodes have to use only one radio interface, all logical links in network must use the same channel. If two neighboring links operate on the same channel and transfer data simultaneously, then they definitely interfere with each other. The network capacity and the performance may degrade significantly because of the interference (Gupta & Kumar, 2000). The key factor for reducing the effect of interference is the using of non-overlapping channels (standard IEEE 802.11b/g provides 3 and standard IEEE 802.11a up to 12 non-overlapping channels) (Ramachandran et al., 2006). In practice, IEEE 802.11b/g defines 11 communication channels (number of communication channels varies due to regulations of different countries) but only 3 of them are non-overlapping (Fig.2).

© 2012 Pollak and Wieser, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Channel Assignment Schemes Optimization for Multi-Interface Wireless Mesh Networks Based on Link Load 81

One of the most promising approaches lies in using multiple radio interfaces and multiple non-overlapping channels (Fig. 3c). This solution is better than previous one, because of providing the effective usage of given frequency spectrum (Conti et al., 2007). This architecture overcomes deficiencies of single interface solution. It allows using of multiple interfaces per node to allow the simultaneous transmission and reception on different radio interfaces tuned to different channels, which can essentially improve network capacity. However, the number of radio interfaces is always much higher than the number of effective channels, which causes an existence of many different links between mesh routers operating on the same channel. For this reason, the suitable channel assignment method is needed to maintain the connectivity between mesh nodes and to minimize the effect of interference

The channel assignment (CA) in a multi-interface WMN consists of a task to assign channels to the radio interfaces by such a way to achieve efficient channel utilization and to minimize the interference. The problem of optimally assigning channels in an arbitrary mesh topology has been proved to be NP-hard (non-deterministic polynomial-time hard) based on its mapping to a graph-coloring problem. Therefore, channel assignment schemes predominantly employ heuristic techniques to assign channels to radio interfaces belonging

The channel assignment algorithms can be divided into three main categories: fixed, dynamic and hybrid, depending on the frequency with which it is modified by the channel assignment scheme. In a fixed scheme, the CA is almost constant, while in a dynamic one it is continuously updated to improve performance. A hybrid scheme applies a fixed scheme

The main objective of this chapter is to give to reader the compact information about problems connected with optimal using of radio interfaces and radio channels in wireless mesh networks. The optimal using is computed from several different points of view, e.g. network topology, number of data flows, number of nodes by comparison of selected QoS

for some radio interfaces and dynamic one for the others (Yulong Chen et al., 2010).

(Raniwala et al., 2004).

to WMN nodes.

**Figure 3.** Different types of WMNs

**Figure 1.** WMN architecture

**Figure 2.** Channel spectrum occupation in IEEE 802.11b/g

Using multiple non-overlapping channels in single interface network disconnects the subset of nodes using one channel from other nodes that are not using the same channel (Fig. 3b). For this reason this approach generally requires MAC layer modification and per packet channel switching capability for radio interfaces (Marina & Das, 2005). Before every data transmission a channel selection mechanism evaluates the available channels and selects a channel to transmit. There are also some problems introduced with channel switching mechanism. These problems include multi-channel hidden terminal problem, broadcast problem, deafness problem and channel deadlock problem (Raniwala et al., 2004).

One of the most promising approaches lies in using multiple radio interfaces and multiple non-overlapping channels (Fig. 3c). This solution is better than previous one, because of providing the effective usage of given frequency spectrum (Conti et al., 2007). This architecture overcomes deficiencies of single interface solution. It allows using of multiple interfaces per node to allow the simultaneous transmission and reception on different radio interfaces tuned to different channels, which can essentially improve network capacity. However, the number of radio interfaces is always much higher than the number of effective channels, which causes an existence of many different links between mesh routers operating on the same channel. For this reason, the suitable channel assignment method is needed to maintain the connectivity between mesh nodes and to minimize the effect of interference (Raniwala et al., 2004).

The channel assignment (CA) in a multi-interface WMN consists of a task to assign channels to the radio interfaces by such a way to achieve efficient channel utilization and to minimize the interference. The problem of optimally assigning channels in an arbitrary mesh topology has been proved to be NP-hard (non-deterministic polynomial-time hard) based on its mapping to a graph-coloring problem. Therefore, channel assignment schemes predominantly employ heuristic techniques to assign channels to radio interfaces belonging to WMN nodes.

The channel assignment algorithms can be divided into three main categories: fixed, dynamic and hybrid, depending on the frequency with which it is modified by the channel assignment scheme. In a fixed scheme, the CA is almost constant, while in a dynamic one it is continuously updated to improve performance. A hybrid scheme applies a fixed scheme for some radio interfaces and dynamic one for the others (Yulong Chen et al., 2010).

**Figure 3.** Different types of WMNs

80 Wireless Mesh Networks – Efficient Link Scheduling, Channel Assignment and Network Planning Strategies

Non-overlapping channels

Using multiple non-overlapping channels in single interface network disconnects the subset of nodes using one channel from other nodes that are not using the same channel (Fig. 3b). For this reason this approach generally requires MAC layer modification and per packet channel switching capability for radio interfaces (Marina & Das, 2005). Before every data transmission a channel selection mechanism evaluates the available channels and selects a channel to transmit. There are also some problems introduced with channel switching mechanism. These problems include multi-channel hidden terminal problem, broadcast

problem, deafness problem and channel deadlock problem (Raniwala et al., 2004).

**Figure 1.** WMN architecture

**Figure 2.** Channel spectrum occupation in IEEE 802.11b/g

The main objective of this chapter is to give to reader the compact information about problems connected with optimal using of radio interfaces and radio channels in wireless mesh networks. The optimal using is computed from several different points of view, e.g. network topology, number of data flows, number of nodes by comparison of selected QoS

parameters. In the second part of the chapter, the new proposed centralized channel assignment concept called First Random Channel Assignment algorithm (FRCA) is compared with two other channel assignment techniques (CCA, LACA) by the same QoS parameters.

Channel Assignment Schemes Optimization for Multi-Interface Wireless Mesh Networks Based on Link Load 83

We have included in our simulations several QoS parameters, data flows, number of nodes and network topologies to find the optimum number of radio interfaces for services which

A simulation WMN model was developed in NS-2 network simulator, with additional function to support multi-channel and multi-interface solution (Calvo & Campo, 2007). Each mesh node used the number of interfaces between 1 to 8 and the same number of channels. Two different network topologies were created. The first one was grid topology, which consisted of 25 static wireless mesh nodes placed in an area of 1000 x 1000 meters. Transmission range for each node was set to 200 meters (Fig.4a). The second topology consists of 25 nodes, which were randomly placed in an area of 1000 x 1000 meters (Fig.4b). For simulation evaluations, ten random topologies and computed average values of chosen QoS parameters were studied. We have used the WMN with 25 nodes, because of the typical number of mesh nodes in WMN (25 to 30) (Skalli et al., 2006). For traffic generation, 5 CBR (Constant Bit Rate) flows were used and the packet size was set to 512 bytes. The same radio default parameters as in (ns-2, 2008) were used, except that we set the channel data rate to 11 Mbit/s.

> **Parameter Value** Test Area 1000x1000 m Mac protocol IEEE 802.11 Propagation model Two ray ground Routing protocol AODV

Antenna type Omni-directional Traffic type CBR Packet size 512 bytes Simulation time 100 seconds

**Figure 4.** Grid (a) and random (b) topology of static WMN created in NS-2 simulator

required the real time transmission (e.g. video conference).

Simulation parameters are summarized in Table 1.

**3.1. Simulation environment** 

**Table 1.** Simulation parameters

The rest of this chapter is organized as follows. In section 2, the related work is summarized. In section 3, the methods and simulation results to find the optimal number of radio interfaces per node are introduced. In section 4 the mathematical background and graph based mathematical model is described and in the next section different types of channel assignment methods based on links load are analyzed. Section 6 concludes the chapter.
