Elizabeth M. Belding

Interference-Aware Channel Assignment in Multi-Radio Wireless Mesh Networks

The capacity problem in wireless mesh networks can be alleviated by equipping the mesh routers with multiple radios tuned to non-overlapping channels. However, channel assignment presents a challenge because co-located wireless networks are likely to be tuned to the same channels. The resulting increase in interference can adversely affect performance. This paper presents an interference-aware channel assignment algorithm and protocol for multi-radio wireless mesh networks that address this interference problem. The proposed solution intelligently assigns channels to radios to minimize interference within the mesh network and between the mesh network and co-located wireless networks. It utilizes a novel interference estimation technique implemented at each mesh router. An extension to the conflict graph model, the multi-radio conflict graph, is used to model the interference between the routers. We demonstrate our solution’s practicality through the evaluation of a prototype implementation in a IEEE 802.11 testbed. We also report on an extensive evaluation via simulations. In a sample multi-radio scenario, our solution yields performance gains in excess of 40% compared to a static assignment of channels.

Blockage and directivity in 60 GHz wireless personal area networks: from cross-layer model to multihop MAC design

We present a cross-layer modeling and design approach for multigigabit indoor wireless personal area networks (WPANs) utilizing the unlicensed millimeter (mm) wave spectrum in the 60 GHz band. Our approach accounts for the following two characteristics that sharply distinguish mm wave networking from that at lower carrier frequencies. First, mm wave links are inherently directional: directivity is required to overcome the higher path loss at smaller wavelengths, and it is feasible with compact, low-cost circuit board antenna arrays. Second, indoor mm wave links are highly susceptible to blockage because of the limited ability to diffract around obstacles such as the human body and furniture. We develop a diffraction-based model to determine network link connectivity as a function of the locations of stationary and moving obstacles. For a centralized WPAN controlled by an access point, it is shown that multihop communication, with the introduction of a small number of relay nodes, is effective in maintaining network connectivity in scenarios where single-hop communication would suffer unacceptable outages. The proposed multihop MAC protocol accounts for the fact that every link in the WPAN is highly directional, and is shown, using packet level simulations, to maintain high network utilization with low overhead.

Green WLANs: On-Demand WLAN Infrastructures

Enterprise wireless local area networks (WLANs) that consist of a high-density of hundreds to thousands of access points (APs) are being deployed rapidly in corporate offices and university campuses. The primary purpose of these deployments is to satisfy user demands for high bandwidth, mobility, and reliability. However, our recent study of two such WLANs showed that these networks are rarely used at their peak capacity, and the majority of their resources are frequently idle. In this paper, we bring to attention that a large fraction of idle WLAN resources results in significant energy losses. Thousands of WLANs world-wide collectively compound this problem, while raising serious concerns about the energy losses that will occur in the future. In response to this compelling problem, we propose the adoption of resource on-demand (RoD) strategies for WLANs. RoD strategies power on or off WLAN APs dynamically, based on the volume and location of user demand. As a specific solution, we propose SEAR, a practical and elegant RoD strategy for high-density WLANs. We implement SEAR on two wireless networks to show that SEAR is easy to integrate in current WLANs, while it ensures no adverse impact on end-user connectivity and performance. In our experiments, SEAR reduces power consumption to 46%. Using our results we discuss several interesting problems that open future directions of research in RoD WLANs.

Cool-Tether: energy efficient on-the-fly wifi hot-spots using mobile phones

We consider the problem of providing ubiquitous yet affordable Internet connectivity to devices at home, at work, and on the move. In this context, we take advantage of two significant technology trends: the commoditization of WiFi WLAN technology and the rapid growth of cellular data services. We propose an architecture called Cool-Tether that harnesses the cellular radio links of one or more mobile smartphones in the vicinity, builds a WiFi hotspot on-the-fly, and provides energy-efficient, affordable connectivity. Prior approaches to supporting such a tethered mode operation have relied on the WiFi ad hoc mode, which impedes the key goal of conserving battery energy on mobile phones. To address the challenges of energy efficiency, Cool-Tether carefully optimizes the energy drain of the WAN (GPRS/ EDGE/ 3G) and WiFi radios on smartphones. In particular, Cool-Tether employs a cloud-based gatherer and an energy-aware striper that exploit the unique energy characteristics of the WAN radio. Cool-Tether also uses a novel reverse-infrastructure mode for WiFi, where the client host serves as a WiFi access point while the mobile phone gateway serves as a WiFi client. We prototype Cool-Tether on smartphones and, experimentally demonstrate savings in energy consumption between 38% - 71% compared to prior energy-agnostic solutions.

Routing stability in static wireless mesh networks

Considerable research has focused on the design of routing protocols for wireless mesh networks. Yet, little is understood about the stability of routes in such networks. This understanding is important in the design of wireless routing protocols, and in network planning and management. In this paper, we present results from our measurement-based characterization of routing stability in two network deployments, the UCSB MeshNet and the MIT Roofnet. To conduct these case studies, we use detailed link quality information collected over several days from each of these networks. Using this information, we investigate routing stability in terms of route-level characteristics, such as prevalence, persistence and flapping. Our key findings are the following: wireless routes are weakly dominated by a single route; dominant routes are extremely short-lived due to excessive route flapping; and simple stabilization techniques, such as hysteresis thresholds, can provide a significant improvement in route persistence.

Congestion-Aware Rate Adaptation in Wireless Networks: A Measurement-Driven Approach

Traditional rate adaptation solutions for IEEE 802.11 wireless networks perform poorly in congested networks. Measurement studies show that congestion in a wireless network leads to the use of lower transmission data rates and thus reduces overall network throughput and capacity. The lack of techniques to reliably identify and characterize congestion in wireless networks has prevented development of rate adaptation solutions that incorporate congestion information in their decision framework. To this end, our main contributions in this paper are two-fold. First, we present a technique that identifies and measures congestion in an 802.11 network in real time. Second, we design Wireless congestion Optimized Fallback (WOOF), a measurement-driven rate adaptation scheme for 802.11 devices that uses the congestion measurement to identify congestion related packet losses. Through experimental evaluation, we show that WOOF achieves up to 300% higher throughput in congested networks, compared to other well-known adaptation algorithms.

FreeMAC: framework for multi-channel mac development on 802.11 hardware

Exponential growth in the number of wireless devices that operate in the limited unlicensed frequency spectrum necessitates the next generation of radio devices to be reconfigurable and sensitive to changes in network conditions and spectrum availability. Most modern wireless devices offer increased software programmability and control over radio communication parameters. Since a large portion of the MAC protocol is implemented in software, with the firmware providing a set of functional primitives, it is possible to design and implement alternate MAC protocols in real testbeds equipped with commodity 802.11 devices. This paper describes FreeMAC, a reconfigurable MAC protocol development framework that enables the design and implementation of a general class of multi-channel MAC protocols on a typical Linux system. FreeMAC provides support for frequent channel switching and fine control over the timing of packet transmissions. We also propose a mechanism to reduce the latency in the scheduling of periodic operations of a software MAC protocol that have strict timing requirements. Results from our six node testbed indicate that using our approach, the scheduling latency of slot transitions in a TDMA-style MAC can be improved by up to an order of magnitude, with minimal overhead. FreeMAC also exports a number of radio configuration parameters as API functions to enable cross layer interactions among wireless networking protocols. As a proof of concept, we implement a simple multi-channel TDMA MAC on our testbed to demonstrate the utility of FreeMAC as a development framework.

The impact of channel bonding on 802.11n network management

The IEEE 802.11n standard allows wireless devices to operate on 40MHz-width channels by doubling their channel width from standard 20MHz channels, a concept called channel bonding. Increasing channel width should increase bandwidth, but it comes at the cost of decreased transmission range and greater susceptibility to interference. However, with the incorporation of MIMO (Multiple-Input Multiple-Output) technology in 802.11n, devices can now exploit the increased transmission rates from wider channels at a reduced sacrifice to signal quality and range. The goal of our work is to understand the characteristics of channel bonding in 802.11n networks and the factors that influence that behavior to ultimately be able to predict behavior so that network performance is maximized. We discuss the impact of channel bonding choices as well as the effects of both co-channel and adjacent channel interference on network performance. We discover that intelligent channel bonding decisions rely not only on a links signal quality, but also on the strength of neighboring links and their physical rates.

IQU: practical queue-based user association management for WLANs

Flash crowds and high concentrations of users in wireless LANs (WLANs) cause significant interference problems and unsustainable load at access points. This leads to poor connectivity for users, severe performance degradation, and possible WLAN collapse. To validate this claim, we present two case studies of large, heavily loaded operational WLANs. These studies provide significant insight into the degraded performance and collapse of a WLAN during heavy use. To address these problems, we propose IQU, a practical queue-based user association management system for heavily loaded WLANs. IQU grants users fair opportunities to access the WLAN while maintaining high overall throughput, even when the WLAN is heavily loaded. The basic premise of IQU is to control user associations with the WLAN through request queues and work period allocations. We implement a prototype of IQU and evaluate it on a wireless testbed. Our evaluation demonstrates that IQU significantly improves network throughput under heavy load; the tradeoff is that users have to wait for network access. We explore the impact of IQU parameters on system performance, and validate the robustness of IQU under heavy load conditions. Through IQU, WLANs can be utilized efficiently and network collapse prevented.

Internet usage and performance analysis of a rural wireless network in Macha, Zambia

There have been a number of rural wireless networks providing Internet access over the last decade but little is known about how the Internet is being used, how these networks perform and whether they follow similar trends when compared with Internet usage patterns in developed regions. We analyse a set of network traces from the Linknet wireless network in Zambia, which provides Internet access to approximately 300 residents of a rural village using a satellite link and a combination of point-to-point links, hotspots and wireless mesh networks. Our analysis reveals largely web-based traffic as opposed to the peer-to-peer traffic dominance that one finds in urban areas. Social networking sites receive the most hits, and large file downloads from operating system repositories contribute the most to the bandwidth consumption. A number of network pathologies in the gateway as well as the wireless mesh network are also analysed and a set of recommendations conclude the work.