Recent studies have shown that mobile applications are increasingly becoming "chatty", resulting in excessive signaling load on cellular networks, potentially causing major network outages. One of the main causes of this signaling overload is the static dormancy timer that 3GPP-based networks employ to control the release of radio resources allocated to a device. To systematically manage the ever-increasing signaling load, we design a novel network-aware system named REACT for dynamically tuning the dormancy timers. In doing so, we highlight a key trade-off between signaling overhead on the network and energy consumed on mobile devices. Contrary to the prior work, our solution models and predicts traffic of mobile apps at the network side with high degree of accuracy, and then dynamically tunes the dormancy timer at a per-device, per-base-station or per-RNC granularity, the approach requires no modifications on mobile devices. Our evaluation shows that REACT reduces signaling overhead by more than a factor of 2 compared to current static setting of dormancy timers. Furthermore, network-aware dynamic tuning of dormancy timers enables a flexible system that can aggressively save energy on devices when the overall network load is low, and manages signaling overhead effectively during high network load.

We conduct an extensive measurement study in an operational cellular network in dense urban settings and identify two key sources of inefficiencies in standard 3GPP handoff technique: wrong timing of handoff and sub-optimal base station association at a point. Based on our study, we propose a novel handoff technique that leverages (1) historical geo-tagged signal strength data from visible base stations at typical mobile user locations and (2) knowledge of the user trajectory. First, we formulate the handoff problem as a bi-criteria optimization problem and present a dynamic programming formulation that provides pareto-optimal solutions that maximize overall signal strength along user trajectory for a range of handoffs. Using an efficient utility based selection technique, we select a solution that balances the tradeoff between reducing number of handoffs and increasing the overall signal strength experienced by the user. Next, we present a much faster greedy heuristic that augments the solution obtained from the dynamic programming formulation when optimizing user performance in the presence of dynamic network load at each base station. A key contribution of our work is to show that striking a tradeoff between the two objectives is much harder in dense macro cellular base station deployments and an effective handoff technique should balance both these objectives well. We show that our technique takes a well balanced approach and achieves more than 25% reduction in the number of handoffs over the standard 3GPP handoff technique in an operational cellular network without incurring reduction in average signal strength, and even improving it by 1dB/sec in some cases. Finally, we propose a novel 3GPP standards compliant trajectory-aware location based handoff protocol and discuss the practical implementation aspects in cellular networks.

We conduct the first detailed measurement analysis of network resource usage and subscriber behavior using a large-scale data set collected inside a nationwide 3G cellular data network. The data set tracks close to a million subscribers over thousands of base stations. We analyze individual subscriber behaviors and observe a significant variation in network usage among subscribers. We characterize subscriber mobility and temporal activity patterns and identify their relation to traffic volume. We then investigate how efficiently radio resources are used by different subscribers as well as by different applications. We also analyze the network traffic from the point of view of the base stations and find significant temporal and spatial variations in different parts of the network, while the aggregated behavior appears predictable. Broadly, our observations deliver important insights into network-wide resource usage. We describe implications in pricing, protocol design and resource and spectrum management.

We study the localization problem in sensor networks by using local angle measurement. Localization using local angle information was recently proposed as an effective localization technique, which can be used for geographical routing with guaranteed delivery. However, the existing approach is based on linear programming (LP) and can not be implemented distributedly. We propose, design, and evaluate DAL: a purely distributed localization protocol in sensor networks using local angle measurement. Localization with local angle poses unique challenge in sensor networks due to information uncertainties identified in this paper. DAL specifically addresses these challenges. Via extensive simulations using ns2 and our own simulator, we show that the performance of DAL is comparable with that of the centralized LP approach in most cases. Our preliminary results with noisy angle measurement show that DAL keeps the global geometry of the sensor network fairly well.

We introduce a measurement-based optimization framework for topology control in dense 802.11 networks using sectorized antennas. We first formulate a topology control optimization problem, where nodes activate their sectorized antenna patterns to minimize network interference and maximize network capacity. In contrast to previous approaches, our formulation is based on a physical interference model that relies on received signal strength (RSS) measurements on multiple antenna patterns of each link. We then introduce a distributed measurement protocol to measure RSS of these antenna patterns and a greedy distributed topology control protocol that uses this information to achieve topologies of minimal interference. The protocols are experimentally evaluated on a dense 802.11 wireless testbed. Extensive measurements show that the protocols operate very close to optimal and yield significant increase in network throughput compared to omni-directional antennas. These results hold for various traffic scenarios in both wireless LAN and mesh network configurations.

Sectorized antennas provide an attractive solution to increase wireless network capacity through higher spatial reuse. Despite their increasing popularity, the real-world performance characteristics of such antennas in dense wireless mesh networks are not well understood. In this demo, we demonstrate our multi-sector antenna prototypes and their performance through video streaming over an indoor wireless network in the presence of interfering nodes. We use our graphical tool to vary the sender, receiver, and interferer antenna configurations and the resulting performance is directly visible in the video quality displayed at the receiver.

Sectorized antennas can increase wireless network capacity through greater spatial reuse. Despite their increasing popularity, their real-world performance characteristics in dense wireless mesh networks are not well understood. This paper conducts a systematic experimental study on a mesh network testbed using commodity 802.11 hardware and multi-sector antennas.

We use a steerable beam directional antenna mounted on a moving vehicle to localize roadside WiFi access points (APs), located outdoors or inside buildings. Localizing APs is an important step towards understanding the topologies and network characteristics of large scale WiFi networks that are deployed in a chaotic fashion in urban areas. The idea is to estimate the angle of arrival of frames transmitted from the AP using signal strength information on different directional beams of the antenna - as the beam continuously rotates while the vehicle is moving. This information together with the GPS locations of the vehicle are used in a triangulation approach to localize the APs. We show how this method must be extended using a clustering approach to account for multi-path reflections in cluttered environments. Our technique is completely passive requiring minimum effort beyond driving the vehicle around in the neighborhood where the APs need to be localized, and is able to improve the localization accuracy by an order of magnitude compared with trilateration approaches using omnidirectional antennas, and by a factor of two relative to other known techniques using directional antennas.

In this paper, we consider multihop wireless mesh networks, where each router node is equipped with multiple radio interfaces, and multiple channels are available for communication. We address the problem of assigning channels to communication links in the network with the objective of minimizing the overall network interference. Since the number of radios on any node can be less than the number of available channels, the channel assignment must obey the constraint that the number of different channels assigned to the links incident on any node is at most the number of radio interfaces on that node. The above optimization problem is known to be NP-hard. We design centralized and distributed algorithms for the above channel assignment problem. To evaluate the quality of the solutions obtained by our algorithms, we develop a semidefinite program and a linear program formulation of our optimization problem to obtain lower bounds on overall network interference. Empirical evaluations on randomly generated network graphs show that our algorithms perform close to the above established lower bounds, with the difference diminishing rapidly with increase in number of radios. Also, ns-2 simulations, as well as experimental studies on testbed, demonstrate the performance potential of our channel assignment algorithms in 802.11-based multiradio mesh networks.

In this work, we investigate the use of directional antennas and beam steering techniques to improve performance of 802.11 links in the context of communication between amoving vehicle and roadside APs. To this end, we develop a framework called MobiSteer that provides practical approaches to perform beam steering. MobiSteer can operate in two modes - cached mode - where it uses prior radiosurvey data collected during "idle" drives, and online mode, where it uses probing. The goal is to select the best AP and beam combination at each point along the drive given the available information, so that the throughput can be maximized. For the cached mode, an optimal algorithm for AP and beam selection is developed that factors in all overheads.

This paper presents first results of experiments in vehicular-to-roadside communication using directional antennas. With directional antennas on one side, the duration of connection to a fixed access point or a road side communication unit can be extended and on the other side the interference caused to others can be reduced. In this work results of experiments with electronical steerable directional antennas mounted on a car communicating with stationary access points are presented. The measurements show the benefit of using directional antennas in different environments typical for vehicular communications. The duration of potential 802.11b connections have been compared using directional and omnidirectional antenna patterns when driving through suburban environment. This comparison is based on passive scanning for access points in order to validate the approach in realistic scenarios. The results clearly prove a substantial potential improvement when using directional antennas.