Kevin C. Almeroth

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.

Towards realistic mobility models for mobile ad hoc networks

One of the most important methods for evaluating the characteristics of ad hoc networking protocols is through the use of simulation. Simulation provides researchers with a number of significant benefits, including repeatable scenarios, isolation of parameters, and exploration of a variety of metrics. The topology and movement of the nodes in the simulation are key factors in the performance of the network protocol under study. Once the nodes have been initially distributed, the mobility model dictates the movement of the nodes within the network. Because the mobility of the nodes directly impacts the performance of the protocols, simulation results obtained with unrealistic movement models may not correctly reflect the true performance of the protocols. The majority of existing mobility models for ad hoc networks do not provide realistic movement scenarios; they are limited to random walk models without any obstacles. In this paper, we propose to create more realistic movement models through the incorporation of obstacles. These obstacles are utilized to both restrict node movement as well as wireless transmissions. In addition to the inclusion of obstacles, we construct movement paths using the Voronoi diagram of obstacle vertices. Nodes can then be randomly distributed across the paths, and can use shortest path route computations to destinations at randomly chosen obstacles. Simulation results show that the use of obstacles and pathways has a significant impact on the performance of ad hoc network protocols.

The use of multicast delivery to provide a scalable and interactive video-on-demand service

In typical proposals for video-on-demand (VoD) systems, customers are serviced individually by allocating and dedicating a transmission channel and a set of server resources to each customer. This approach leads to an expensive to operate, nonscalable system. We consider a VoD system that uses multicast delivery to service multiple customers with a single set of resources. The use of multicast communication requires that part of the on-demand nature of the system be sacrificed to achieve scalability and cost-effectiveness. One drawback to using multicast communication is that it complicates the provision or interactive VCR-style functions. Interactivity can be provided by either increasing the complexity of the customer set-top box (STB) or by modifying the semantics of the interactive functions to make them easier to provide. We describe a framework and mechanisms by which such interactive functions can be incorporated into a multicast delivery VoD system. Through the use of simulation, we evaluate and compare the performance of a unicast VoD system and multicast VoD systems offering various levels of interactivity.

The evolution of multicast: from the MBone to interdomain multicast to Internet2 deployment

Multicast communication-the one-to-many or many-to-many delivery of data-is a hot topic. It is of interest in the research community, among standards groups, and to network service providers. For all the attention multicast has received, there are still issues that have not been completely resolved. One result is that protocols are still evolving, and some standards are not yet finished. From a deployment perspective, the lack of standards has slowed progress, but efforts to deploy multicast as an experimental service are in fact gaining momentum. The question now is how long it will be before multicast becomes a true Internet service. The goal of this article is to describe the past, present, and future of multicast. Starting with the Multicast Backbone (MBone), we describe how the emphasis has been on developing and refining intradomain multicast routing protocols. Starting in the middle to late 1990s, particular emphasis has been placed on developing interdomain multicast routing protocols. We provide a functional overview of the currently deployed solution. The future of multicast may hinge on several research efforts that are working to make the provision of multicast less complex by fundamentally changing the multicast model. We survey these efforts. Finally, attempts are being made to deploy native multicast routing in both Internet2 networks and the commodity Internet. We examine how multicast is being deployed in these networks.

Understanding congestion in IEEE 802.11b wireless networks

The growing popularity of wireless networks has led to cases of heavy utilization and congestion. In heavily utilized wireless networks, the wireless portion of the network is a major performance bottleneck. Understanding the behavior of the wireless portion of such networks is critical to ensure their robust operation. This understanding can also help optimize network performance. In this paper, we use link layer information collected from an operational, large-scale, and heavily utilized IEEE 802.11b wireless network deployed at the 62nd Internet Engineering Task Force (IETF) meeting to study congestion in wireless networks. We motivate the use of channel busy-time as a direct measure of channel utilization and show how channel utilization along with network throughput and goodput can be used to define highly congested, moderately congested, and uncongested network states. Our study correlates network congestion and its effect on link-layer performance. Based on these correlations we find that (1) current rate adaptation implementations make scarce use of the 2 Mbps and 5.5 Mbps data rates, (2) the use of Request-to-Send/Clear-to-Send (RTS-CTS) prevents nodes from gaining fair access to a heavily congested channel, and (3) the use of rate adaptation, as a response to congestion, is detrimental to network performance.

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.

Multicast group behavior in the Internet's multicast backbone (MBone)

The multicast backbone (MBone) is a network overlaying the global Internet and designed to support multipoint applications. The authors first give an overview of the development and architecture of the current MBone. One important characteristic of the MBone is its reliance on IP multicast which allows receivers to join and leave groups asynchronously. The authors describe the Mlisten data collection tool that was created to provide a mechanism for capturing information about when members join and leave a multicast group. Using data collected with Mlisten, the authors present statistics about some of the MBone sessions they monitored. Results are provided for key parameters including multicast participant interarrival times and participant durations, multicast tree size and characteristics, and intersession relationships. Collecting data about MBone usage can improve our understanding of how multicast communication and real-time protocols are being used today.

Real-world environment models for mobile network evaluation

Simulation environments are an important tool for the evaluation of new concepts in networking. The study of mobile ad hoc networks depends on understanding protocols from simulations, before these protocols are implemented in a real-world setting. To produce a real-world environment within which an ad hoc network can be formed among a set of nodes, there is a need for the development of realistic, generic and comprehensive mobility, and signal propagation models. In this paper, we propose the design of a mobility and signal propagation model that can be used in simulations to produce realistic network scenarios. Our model allows the placement of obstacles that restrict movement and signal propagation. Movement paths are constructed as Voronoi tessellations with the corner points of these obstacles as Voronoi sites. Our mobility model also introduces a signal propagation model that emulates properties of fading in the presence of obstacles. As a result, we have developed a complete environment in which network protocols can be studied on the basis of numerous performance metrics. Through simulation, we show that the proposed mobility model has a significant impact on network performance, especially when compared with other mobility models. In addition, we also observe that the performance of ad hoc network protocols is effected when different mobility scenarios are utilized.

Delay tolerant mobile networks (DTMNs): controlled flooding in sparse mobile networks

The incredible growth in the capabilities and functionality of mobile devices has enabled new applications to emerge. Due to the potential for node mobility, along with significant node heterogeneity, characteristics such as very large delays, intermittent links and high link error rates pose a new set of challenges. Along with these challenges, end-to-end paths are assumed not to exist and message relay approaches are often adopted. While message flooding happens to be a simple and robust solution for such cases, its cost in terms of network resource consumption is unaffordable. In this paper, we focus on the evaluation of different controlled message flooding schemes over large-scale, sparse mobile networks. We study the effect of these schemes on message delay and network resource consumption. Our simulations show that our schemes can save substantial network resources while incurring a negligible increase in the message delivery delay.

Do social networks improve e-commerce?: a study on social marketplaces

Social networks have made a significant impact on how Internet users communicate, search for and share data today. Numerous proposals have been made to improve existing distributed systems by leveraging the inherent trust built into social links. For example, many believe that by augmenting online marketplaces with social networking, we can improve trust between transaction partners and increase user satisfaction. In this paper, we perform a detailed study of Overstock Auctions, an auction site that has recently integrated social links into user profiles. Using data on connections between roughly 400,000 Overstock users, we evaluate the impact of social connections on business transactions. Our results show that while the majority of users do not engage in social networking, those who transact with friends from their social network generally obtain significant benefits in the form of higher user satisfaction.