devices, channel assignment algorithm will have to consider external interference to avoid an signi cant impact on the network performance [37, 38]. Especially since many applica-tions of WMNs, such as providing Internet access to mobile clients, are envisioned mainly for urban scenarios. With the trend towards large-scale network deployments in regard to the number of nodes, the need for the algorithms to be adaptive towards node mobility, node failure, and changes in the channel conditions becomes more important. Finally, due to the huge eort to obtain measurement-based interference estimations, all algorithms rely on simple distance-based interference models. The recent development of more e -cient measurement-based approaches [50,73] combined with the results of studies showing the inaccuracy of the simple models [46, 49], will allow channel assignment algorithms to pro t of the more accurate interference estimations.
2.7 Related work: Frameworks for distributed channel assignment
The implementation process of channel assignment algorithms in testbed environments yields many challenges and pitfalls, since common operating systems are not designed to support channel assignment out of the box. Thus, the programmer has to deal with operat-ing system speci cs, drivers for the wireless interfaces and the capabilities and limitations of the particular hardware. This is also re ected in the results of the survey of the previ-ous section, in which only about half of the algorithms have been evaluated in a testbed environment.
A development framework for channel assignment algorithms can simplify the imple-mentation eort by introducing an abstraction layer for common operating system speci c tasks, such as channel switches and the con guration of the wireless interfaces. Further-more, the analysis of several diverse distributed algorithms has shown that despite their dierent strategies for channel assignment, a couple of common key services are required for almost all distributed algorithms. This includes a neighbor discovery service, node to node message exchange for channel switch synchronization, and interference models for the network-wide interference estimation and for creating con ict graphs. By providing a service library of such commonly used services, a framework for channel assignment can signi cantly speed up the development process. This is especially helpful, if several strate-gies are implemented to compare their performance. As next, we present frameworks that have been created to support researchers in the development process of channel assignment algorithms.
2.7.1 FreeMac
With the release of the MadWiFi driver and the OpenHAL [74], an open source imple-mentation of the binary hardware abstraction layer (HAL) distributed by Atheros, several frameworks emerged to support research on MAC protocols for commodity IEEE 802.11 hardware. FreeMAC [75], based on its predecessor MadMAC [76], is such a framework
24 Chapter 2. Channel assignment in wireless mesh networks
that provides an API to simplify multi-channel MAC protocol development.
The channel switching time is a crucial issue for the performance of dynamic channel assignment algorithms, and therefore, FreeMAC has altered the channel switching proce-dure to make it faster. Any neighbor discovery operations that usually take place after a channel switch have been removed. The achieved channel switching time, including a hardware reset of the wireless network interface and operating system speci c operations, was measured on a Linux-based test system with about 4 ms [75].
FreeMAC can function as the foundation of dynamic channel assignment algorithms that require fast channel switching. However, the framework does not provide any other services for channel assignment algorithms, such as neighbor discovery or communication among network nodes.
2.7.2 NET-X
The NET-X framework was created to implement the fast channel switching algorithm described in [66] for a wireless testbed environment based on the 2.4 Linux kernel [65, 77]. The algorithm divides the available wireless network interfaces on each node into
xed interfaces, which stay on a single channel for receiving, and switchable interfaces for sending, which can be dynamically switched to dierent channels depending on the receiver. The research focus of the NET-X framework is on algorithms for the channel assignment of the xed interfaces.
The implementation of the NET-X algorithm requires several changes to the Linux net-work stack and to the driver of the wireless netnet-work interfaces which is supported by the framework. Among them are per-channel queues, in order to store messages temporarily that are to be transmitted on a dierent channel than the switchable interface is cur-rently operating on and the reduction of the channel switching time. The architecture of NET-X comprises the channel abstraction layer (CAL), the kernel multi-channel rout-ing (KMCR), and the userspace daemon as shown in Figure 2.7.1. The CAL comprises broadcast and unicast components, queuing and scheduling mechanisms for the channel queues, and implements the con guration of the wireless network interfaces through the userspace daemon. The KMCR provides the necessary functionality to support reactive routing protocols, such as packet buering during the route discovery procedure. The userspace daemon comprises a multi-channel routing protocol and the interface handling.
An abstraction layer for interface handling is provided with a set of functions to control the interface capabilities. These comprise the radio parameters that aect the network performance, such as the utilized channel and transmission power.
Experiments with the NET-X framework have been performed to evaluate dierent strategies for the channel assignment of the xed interfaces [78] and for QoS-provisioning.
Extensions have been developed for queue management [79]. In summary, the services provided by NET-X are directly derived from the requirements of the implemented algo-rithm [66]. The framework is not intended to function as a foundation to develop dierent
2.7. Related work: Frameworks for distributed channel assignment 25
Channel Abstraction Layer
Broadcast Unicast Queuing and
Scheduling Wireless
Interface Con guration Userspace Daemon
Linux TCP/IP Stack
wlan0 wlan1
Multichannel Routing
Interface Management
Kernel Multi-Channel Routing Support Route
Handling
Figure 2.7.1.: Architecture of NET-X. Displayed are the three components of NET-X the channel abstraction layer (CAL), the kernel multi-channel routing (KMCR), and the userspace daemon and their interconnection.
distributed channel assignment algorithms, the focus is on algorithms to assign channels to the xed interfaces. Data structures for network and con ict graphs are not provided, also a topology monitoring functionality has not been in the scope of the framework.
CHAPTER 3
DES-Testbed: Wireless experimentation laboratory
The Distributed Embedded Systems Testbed (DES-Testbed) is a hybrid wireless network located on the campus of Freie Universität Berlin. It is hybrid in a way, that all DES-Nodes comprise a wireless mesh router equipped with multiple IEEE 802.11a/b/g radios and an MSB-A2 sensor node [80]. The deployment of the testbed started in 2008 and since then, the testbed has been used for experimentation in several European (OPNEX [81], WISEBED [82]) and national funded projects (G-Mesh-Lab [83]). All experiments de-scribed in this dissertation have been performed on the DES-Testbed.
The chapter starts with a description of the network architecture and the hard- and software components of the DES-Testbed. The experimentation methodology presents the work ow for testbed experiments and the corresponding tools to assist researchers in the studies. The chapter closes with experimental results concerning the network topology and the characteristics of the IEEE 802.11 radio channels.