Performance of Wireless Mesh Networks: Experimental Studies of IEEE 802.11 and IEEE 802.16 solutions

Wireless Mesh Networks (WMNs) are emerging as novel networking paradigm capable of supporting application scenarios where quick installation and flexibility are a major requirement. The major advantage of WMNs, in fact, is the ease of deployment and reconfiguration of the network infrastructure, thus allowing the deployment of networks with a minimum planning phase, while maintaining compatibility with existing WLAN installations (i.e. WMN as WiFi hotspots wirelessy interconnected). From the economic perspective, WMNs are expected to lower the entrance barrier to Wireless Internet Service Provider (WISP). This in turn by allowing them to deploy a wireless back-haul in an incremental fashion. At the same time WMNs can provide dramatic benefits by enabling wireless connectivity where there is not an in-advance knowledge of where the network should be deployed (e.g., tactical and emergency/disaster situations). Moreover, WMNs have been receiving considerable attention by the research community only for the last couple of years, boosted by their commercial deployment. At the moment there is a large number of companies active in this field, which provide solutions mostly based on the IEEE 802.11 standard family [1]. At the same time, a large fraction of prominent universities and research centers are undertaking the deployment of experimental testbeds. Indeed, WMNs pose a series of challenges at various layers of the ISO/OSI protocol stack. The most evident is the way packets are routed, where open issues concern the protocol architecture (i.e., at which level packets should be routed on the mesh), the routing metric (from standard hop-count to more complex parameters based on link quality [2]) and the ability to provide quality of service guarantees to multimedia applications. In order to be successful, WMNs must cope with current trends in services. It is indeed widely acknowledged that the next-generation Internet will be characterized by an extreme variety of multimedia broadband services. Without the ability to successfully support the peculiarities of these services, WMNs run the risk to remain a niche market. Unlike “pure data” applications like FTP or HTTP, next-generation services are characterized by requirements in terms of network support, i.e., bandwidth, latency, packet delay jitter etc. On the other hand, these constraints fit badly the decentralized architecture of WMNs, where smart solutions are needed to provide such performance guarantees. It is therefore a primary need to perform performance measurements on real-world testbeds, in order to characterize the ability of WMNs to support multimedia flows and gain insight into the critical points of such systems, therefore providing smart guidelines for the design of innovative solutions able to boost WMNs deployment. The purpose of this work is to introduce the mesh architecture through wide experimental studies building different type of testbeds. In order to achieve this goal, in Chapter 1, after an introduction of the mesh networking paradigm, some performance measurements have been obtained on an experimental WiFi-based mesh testbed running at CREATE-NET premises. The tests aim at characterizing the suitability of current mesh networking solutions to support multimedia flows. Experimental results pointed out several critical issues causing an highly variable Quality of Service. For this reason I defined and implemented a scalable architecture able to perform well in the presence of a large number of neighboring nodes. In particular my major research idea was to exploit multiradio routing protocols. In this scenario I investigated and proposed innovative solutions, algorithms, mechanisms and protocols for the support of Trafc Differentiation and Cooperation/Fairness Enforcement in WMNs. The proposed solutions were implemented and validated through real-world experimentation carried over our testbed and the obtained results were a real break-through when compared to legacy protocols. Being shielded by the hazards of a live or production environment, testbeds provide rigorous, transparent and replicable testing conditions. Measurements run over testbeds can be exploited by the scientific community in order to evaluate the performance of newly developed protocols, providing important guidelines for the design of innovative solutions. Moreover, proposed solutions can be implemented and validated through real-world experiments conducted over testbeds. The Chapter 2 contains a survey on the most relevant hardware and software platforms that can be used to build a WMN testbed. It was not my intention to provide an exhaustive survey on all platforms that are suitable for a WMN deployment, instead I concentrated on open-source software and off-the-shelf hardware components. There are many different channels defined in the IEEE 802.11 standard. However, the performance of WiFi networks still greatly suffers from the interference between users, even if they are using different channels. In Chapter 3, I present a theoretical analysis of the interference between two channels, which is further verified by experiments. Specifically, I quantify the interference between two channels, which is determined by the level of RF energy that leaks from one channel to the other. As I have just explained, IEEE 802.11 operates on a frequency band occupying several non-overlapping channels. It is commonly considered that the transmissions on two non-overlapping channels do not interfere with each other and some multi-channel MAC protocols are designed based on this premise to improve the spatial reuse. Many papers on mesh networks also take advantage of such non-overlapping channels and develop channel assignment algorithms which can dynamically assign a channel to each link to reduce interference and to improve throughput. Recently, however, some researches ?nd that there does exist interference between non-overlapping channels. Therefore, the Chapter 4 attempts to answer to the following two questions when we use multi-channel in WMNs: (Q1.) Can we still use multiple channels effectively in wireless networks? (Q2.) If we can, what is the cost when we dynamically switch the channel? Can we afford changing channels frequently? While research on WMNs has been up to a large extent con?ned to the study of efficient routing protocols, there is a clear need to envision new network management tools, able to suf?ciently exploit the peculiarities of WMNs. In particular, a new generation of middleware tools for network monitoring and pro?ling must be introduced in order to speed up development and testing of novel protocol architectures. Currently, management functionalities are developed using conventional centralized approaches. The distributed and self-organizing nature of WMNs suggest a transition from network monitoring to network sensing. In Chapter 5, I propose JANUS, a novel framework for distributed monitoring of WMNs. I describe the JANUS architecture, present a possible implementation based on open-source software and report some experimental measurements carried out on a small-scale testbed. Currently deployed mesh networks use IEEE 802.11 wireless devices for wireless mesh connectivity. However, IEEE 802.11 medium access control is not so appropriate for commercial applications of mesh networks since the Distributed Coordination Function used to coordinate 802.11 transmissions cannot provide Quality of Service. Furthermore, the IEEE 802.11-based solutions are a technology with very limited transmission range that makes them unfeasible to cover large areas. This lack of technology to bridge the gap between local (i.e., WLAN) and global (i.e., Internet) wireless connectivity drove the development of the ?rst version of the IEEE 802.16 standard [3] that defines the physical layer and the medium access control sublayer targeted for wireless networking in Metropolitan Area Networks (MANs). At about the same time (i.e., in 2001), the WiMAX forum [4] was formed as a non-pro?t corporation to promote and certify the compatibility and interoperability of Broadband Wireless Access (BWA) products using the IEEE 802.16 specications. Among BWA technologies, the IEEE 802.16 standard is considered the leading technology for the provision of Internet-based broadband services in wide area networks. With its large range and high transmission rate, WiMAX can serve as a backbone for 802.11 hotspots for connecting to the Internet. Alternatively, users can connect mobile devices such as laptops and handsets directly to WiMAX base stations without using 802.11. IEEE 802.16 supports point-to-multipoint (PMP) single-hop transmission between a single base station and multiple subscriber stations as well as multihop mesh networking. Even though WiMAX-based mesh deployments may contribute to the success of such technology in a more mature stage [5], the current standard provides only optional support to such architecture. Hence, in this work, the focus is devoted to the PMP con?guration, as deployed in our testbed. In particular, the Chapter 6 initially provides an in-depth overview and discussion on the QoS support provided by WiMAX technology. Moreover, I reported the results of a set of measurements performed on the ?eld on a WiMAX Alvarion testbed, located in Turin, Italy. I fed the system with synthetic VoIP traffic, real-time guaranteed, competing with concurrent best effort traffic. I obtained E-model ?gures, thus characterizing the operation intervals of the system, depending on the codec source and on the number of calls. Finally, the Chapter 7 concludes the work showing the most important results