**1. Introduction**

26 Will-be-set-by-IN-TECH

[23] Fu, J., Jiang, X., Ping, L. & Fan, R. [2009]. A novel rekeying protocol for 802.11s key management, *Proceedings of the 2009 International Conference on Information Management and Engineering*, ICIME '09, IEEE Computer Society, Washington, DC, USA, pp. 295–299.

[24] Gollmann, D. [1996]. What do we mean by entity authentication?, *SP '96: Proceedings of the 1996 IEEE Symposium on Security and Privacy*, IEEE Computer Society, Washington,

[25] Haasz, J. & Hampton, S. [2006]. Amendment: Mesh networking,

[26] He, C., Sundararajan, M., Datta, A., Derek, A. & Mitchell, J. C. [2005]. A modular correctness proof of IEEE 802.11i and TLS., *ACM Conference on Computer and*

[27] Krawczyk, H. [2003]. SIGMA: The 'SIGn-and-MAc' approach to authenticated

[28] Kuhlman, D., Moriarty, R., Braskich, T., Emeott, S. & Tripunitara, M. [2007]. A proof of security of a mesh security architecture, http://eprint.iacr.org/2007/364.pdf. [29] Meadows, C. & Pavlovic, D. [2004]. Deriving, attacking and defending the GDOI

[30] Rigney, C., Willens, S., Rubens, A. & Simpson, W. [June 2000]. Remote Authentication Dial In User Service (RADIUS) – RFC 2865, http://tools.ietf.org/html/rfc2865. [31] Roy, A., Datta, A., Derek, A. & Mitchell, J. C. [2007]. Inductive proof method for computational secrecy, *Proceedings of 12th European Symposium On Research In Computer*

[32] Roy, A., Datta, A., Derek, A., Mitchell, J. C. & Seifert, J.-P. [2006]. Secrecy analysis in protocol composition logic., *Proceedings of 11th Annual Asian Computing Science*

[33] Roy, A., Datta, A., Derek, A., Mitchell, J. C. & Seifert, J.-P. [2008]. Secrecy analysis in protocol composition logic., *Formal Logical Methods for System Security and Correctness*,

[34] Simon, D., Aboba, B. & Hurst, R. [2007]. The EAP TLS authentication protocol,

[35] Zhao, M., Walker, J. & Conner, W. S. [2007]. Overview of abbreviated handshake protocol,

http://www.ietf.org/internet-drafts/draft-simon-emu-rfc2716bis-11.txt.

https://mentor.ieee.org/802.11/documents doc 11-07/1998r01.

diffie-hellman and its use in the IKE-protocols., *CRYPTO*, pp. 400–425.

URL: *http://dx.doi.org/10.1109/ICIME.2009.14*

*Communications Security*, pp. 2–15.

protocol, *ESORICS*, pp. 53–72.

http://standards.ieee.org/board/nes/projects/802-11s.pdf.

DC, USA, p. 46.

*Security*.

*Conference*.

IOS Press.

*Wireless Mesh Network* (WMN) is an ad-hoc network with a fixed network infrastructure (see an example in figure 1). The physical structure of a WMN includes base stations, a backbone and mobile stations. The *base stations* (also known as mesh routers or mesh points) are static wireless nodes, forming the network infrastructure and providing wireless network access to the mobile stations. The *backbone* is a wireless ad-hoc network among the base stations. The fixed network infrastructure provides wireless network access to the mobile stations in a service area. *Service area* is a finite three-dimensional space. The *mobile stations* are wireless

**Figure 1.** Wireless mesh networks and radio coverage

©2012 Ivanov and Nett" 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.

#### 2 Will-be-set-by-IN-TECH 204 Wireless Mesh Networks – Effi cient Link Scheduling, Channel Assignment and Network Planning Strategies Achieving Fault-Tolerant Network Topology in Wireless Mesh Networks <sup>3</sup>

nodes which move within the service area and communicate to other stations via the WMN. The stations in a WMN use a *multi-hop routing protocol* for communication. This protocol automatically discovers the network topology and delivers the messages to the destination; if needed over multiple hops. We can think of a WMN as an infrastructure wireless network in which the backbone is replaced by a wireless one and the communication is done in a (multi-hop) ad-hoc way.

The *environmental dynamics* are unpredictable changes of the radio propagation and radio attenuation properties of the environment (e.g. new obstacles, movement of obstacles, increased humidity). They occur due to reconfiguration of the plant layout. Environmental dynamics occur, for instance, in Reconfigurable Manufacturing Systems (RMS) [11, 26]. An RMS is a production system with an adjustable structure, that is able to meet the market requirements with respect to capacity, functionality and cost. This adjustable structure at the system level includes changes in the plant layout, for instance "*adding, removing or modifying machine modules, machines, cells, material handling units and/or complete lines*" [11]. In RMS the environmental dynamics are unpredictable at design time, because the system layout adjustments are made on the fly to meet the actual production demand. The environmental dynamics negatively affect the radio coverage (radio signal strength between mobile stations and base stations) and the backbone network connectivity among base stations of an WMN. The first contribution of this chapter is a fault-tolerance method for guaranteeing radio coverage of wireless mesh networks in dynamic propagation environments. The basic idea of this approach is to automatically detect an *error state*, which is lack of redundancy in radio coverage and connectivity, and correct this error by reconfiguring the base stations before the radio coverage fails. The error detection is based on a radio propagation model: if an error is detected in the model, this is an indicator that an error in the real radio coverage exists. In order to be able to make this conclusion, this model is automatically calibrated to the real

Achieving Fault-Tolerant Network Topology in Wireless Mesh Networks 205

The second contribution of this chapter is an automatic base station planning algorithm for the reconfiguration phase of the fault-tolerance approach. In this phase base stations are added to the network in order to correct errors in the radio coverage and connectivity. The question is: what is the minimum number of base stations to be added and at which positions in order to restore the original state of radio coverage and connectivity. Our approach is an optimization algorithm, which uses knowledge from the calibrated radio propagation model and answers

In section 2 we will discuss related work. In section 3 we will present our fault-tolerance approach for ensuring the availability of radio coverage and connectivity of wireless mesh networks. In section 4 we will present our approach for automatic base station planning in wireless mesh networks, which is used in the reconfiguration phase on the fault-tolerance

Firstly, we will present related work aiming at availability of the radio coverage and connectivity. Then we will discuss related work to the automatic base station planning

The availability of the service *radio coverage* is a necessary condition for reliable communication in wireless networks. The issue of reliable communication via wireless medium has been extensively investigated during the design of every wireless communication

approach. Section 6 provides a conclusion and a summary of future work.

enviromnent by using radio signal strength measurements.

this question in a sufficient time frame.

**2.1. Availability of the radio coverage**

**1.4. Structure of the chapter**

**2. Related work**

algorithm.

We consider a wireless mesh network which supports a business process and is under the administration of an organization. This is not a MANET (Mobile Ad-hoc Network) consisting of self-dependent mobile nodes, like it is often in the literature. The organization has control over the network infrastructure and aims at providing radio coverage and connectivity in a clearly defined service area. The *management appliance* is a central instance for basic configuration and diagnosis of the WMN, including topology monitoring, protocol settings, traffic management, etc.

*Radio coverage* and *connectivity* are basic *services* of a wireless mesh network which are required for communication. Radio coverage ensures that the mobile stations can access the network infrastructure (backbone) while they are located or moving in the service area. Connectivity ensures that the topology of the backbone is connected.

#### **1.1. Radio coverage**

The service *radio coverage* is *correct*, if the service area is *covered* by the base stations. The service area is covered, if the unification of radio cells of all base stations contains the whole service area. The radio cell of a base station is a part of the space around it, in which a mobile station observes the base station with a radio signal strength sufficient for communication. The sufficient radio signal strength in the service area is a basic requirement for the mobile stations to be able to access the WMN. The radio coverage service ensures this sufficient signal strength in the service area. Service location is a point of the service area, specified by its coordinates. A service location is covered, if the unification of radio cells of all base stations contains the service location.

### **1.2. Connectivity**

The service *connectivity* is correct, if the backbone graph is connected. The *backbone graph* is a graph with the base stations as vertices and the routing layer links among them as edges. A *link* exists if two wireless devices can communicate through the wireless medium obeying some qualitative parameters (see section 4.3 for more information). The backbone graph represents the network topology at the routing layer. This graph is connected, if a *path* (a sequence of edges) exists between every two vertices. A connected backbone graph means a connected routing layer topology which is a basic requirement for communication through the WMN. The connectivity service ensures that the backbone graph is connected.

At the example WMN in figure 1 the radio coverage and the connectivity are correct. The unification of radio cells contains the service area and the backbone graph is connected.

### **1.3. Problem exposition and contributions**

In this chapter, we address the problem of guaranteeing radio coverage of Wireless Mesh Networks, which are exposed to environmental dynamics.

The *environmental dynamics* are unpredictable changes of the radio propagation and radio attenuation properties of the environment (e.g. new obstacles, movement of obstacles, increased humidity). They occur due to reconfiguration of the plant layout. Environmental dynamics occur, for instance, in Reconfigurable Manufacturing Systems (RMS) [11, 26]. An RMS is a production system with an adjustable structure, that is able to meet the market requirements with respect to capacity, functionality and cost. This adjustable structure at the system level includes changes in the plant layout, for instance "*adding, removing or modifying machine modules, machines, cells, material handling units and/or complete lines*" [11]. In RMS the environmental dynamics are unpredictable at design time, because the system layout adjustments are made on the fly to meet the actual production demand. The environmental dynamics negatively affect the radio coverage (radio signal strength between mobile stations and base stations) and the backbone network connectivity among base stations of an WMN.

The first contribution of this chapter is a fault-tolerance method for guaranteeing radio coverage of wireless mesh networks in dynamic propagation environments. The basic idea of this approach is to automatically detect an *error state*, which is lack of redundancy in radio coverage and connectivity, and correct this error by reconfiguring the base stations before the radio coverage fails. The error detection is based on a radio propagation model: if an error is detected in the model, this is an indicator that an error in the real radio coverage exists. In order to be able to make this conclusion, this model is automatically calibrated to the real enviromnent by using radio signal strength measurements.

The second contribution of this chapter is an automatic base station planning algorithm for the reconfiguration phase of the fault-tolerance approach. In this phase base stations are added to the network in order to correct errors in the radio coverage and connectivity. The question is: what is the minimum number of base stations to be added and at which positions in order to restore the original state of radio coverage and connectivity. Our approach is an optimization algorithm, which uses knowledge from the calibrated radio propagation model and answers this question in a sufficient time frame.

#### **1.4. Structure of the chapter**

In section 2 we will discuss related work. In section 3 we will present our fault-tolerance approach for ensuring the availability of radio coverage and connectivity of wireless mesh networks. In section 4 we will present our approach for automatic base station planning in wireless mesh networks, which is used in the reconfiguration phase on the fault-tolerance approach. Section 6 provides a conclusion and a summary of future work.
