Nabeel Ahmed

CENTAUR: realizing the full potential of centralized wlans through a hybrid data path

Enterprise WLANs have made a dramatic shift towards centralized architectures in the recent past. The reasons for such a change have been ease of management and better design of various control and security functions. The data path of WLANs, however, continues to use the distributed, random-access model, as defined by the popular DCF mechanism of the 802.11 standard. While theoretical results indicate that a centrally scheduled data path can achieve higher efficiency than its distributed counterpart, the likely complexity of such a solution has inhibited practical consideration. In this paper, we take a fresh, implementation and deployment oriented, view in understanding data path choices in enterprise WLANs. We perform extensive measurements to characterize the impact of various design choices, like scheduling granularity on the performance of a centralized scheduler, and identify regions where such a centralized scheduler can provide the best gains. Our detailed evaluation with scheduling prototypes deployed on two different wireless testbeds indicates that DCF is quite robust in many scenarios, but centralization can play a unique role in 1) mitigating hidden terminals - scenarios which may occur infrequently, but become pain points when they do and 2) exploiting exposed terminals - scenarios which occur more frequently, and limit the potential of successful concurrent transmissions. Motivated by these results, we design and implement CENTAUR - a hybrid data path for enterprise WLANs, that combines the simplicity and ease of DCF with a limited amount of centralized scheduling from a unique vantage point. Our mechanisms do not require client cooperation and can support legacy 802.11 clients.

PixNet: interference-free wireless links using LCD-camera pairs

Given the abundance of cameras and LCDs in todays environment, there exists an untapped opportunity for using these devices for communication. Specifically, cameras can tune to nearby LCDs and use them for network access. The key feature of these LCD-camera links is that they are highly directional and hence enable a form of interference-free wireless communication. This makes them an attractive technology for dense, high contention scenarios. The main challenge however, to enable such LCD-camera links is to maximize coverage, that is to deliver multiple Mb/s over multi-meter distances, independent of the view angle. To do so, these links need to address unique types of channel distortions, such as perspective distortion and blur. This paper explores this novel communication medium and presents PixNet, a system for transmitting information over LCD-camera links. PixNet generalizes the popular OFDM transmission algorithms to address the unique characteristics of the LCD-camera link which include perspective distortion, blur, and sensitivity to ambient light. We have built a prototype of PixNet using off-the-shelf LCDs and cameras. An extensive evaluation shows that a single PixNet link delivers data rates of up to 12 Mb/s at a distance of 10 meters, and works with view angles as wide as 120 degree°.

SMARTA: a self-managing architecture for thin access points

Optimally choosing operating parameters for access points in an enterprise wireless LAN environment is a difficult and well-studied problem. Unlike past work, the SMARTA self-managing wireless LAN architecture dynamically adjusts both access point channel assignments and power levels in response to measured changes in the wireless environment to optimize arbitrary objective functions, while taking into account the irregular nature of RF propagation, and working with unmodified legacy clients. We evaluate the SMARTA architecture through simulation and show that our solution is not only feasible, but also provides significant improvements over existing approaches. For example, in a realistic scenario, SMARTA can provide 50% more throughput and 40% lower mean per-packet delay than a hand-optimized configuration. Moreover, SMARTA can automatically reconfigure channels and power levels in response to both small and large changes in the RF environment due to client movement.

CarSpeak: a content-centric network for autonomous driving

This paper introduces CarSpeak, a communication system for autonomous driving. CarSpeak enables a car to query and access sensory information captured by other cars in a manner similar to how it accesses information from its local sensors. CarSpeak adopts a content-centric approach where information objects -- i.e., regions along the road -- are first class citizens. It names and accesses road regions using a multi-resolution system, which allows it to scale the amount of transmitted data with the available bandwidth. CarSpeak also changes the MAC protocol so that, instead of having nodes contend for the medium, contention is between road regions, and the medium share assigned to any region depends on the number of cars interested in that region. CarSpeak is implemented in a state-of-the-art autonomous driving system and tested on indoor and outdoor hardware testbeds including an autonomous golf car and 10 iRobot Create robots. In comparison with a baseline that directly uses 802.11, CarSpeak reduces the time for navigating around obstacles by 2.4x, and reduces the probability of a collision due to limited visibility by 14x.

GUESS: gossiping updates for efficient spectrum sensing

Wireless radios of the future will likely be frequency-agile, that is, supporting opportunistic and adaptive use of the RF spectrum. Such radios must coordinate with each other to build an accurate and consistent map of spectral utilization in their surroundings. We focus on the problem of sharing RF spectrum data among a collection of wireless devices. The inherent requirements of such data and the time-granularity at which it must be collected makes this problem both interesting and technically challenging. We propose GUESS, a novel incremental gossiping approach to coordinated spectral sensing. It (1) reduces protocol overhead by limiting the amount of information exchanged between participating nodes, (2) is resilient to network alterations, due to node movement or node failures, and (3) allows exponentially-fast information convergence. We outline an initial solution incorporating these ideas and also show how our approach reduces network overhead by up to a factor of 2.4 and results in up to 2.7 times faster information convergence than alternative approaches.

Interference mitigation in enterprise WLANs through speculative scheduling

Wireless LANs are commonplace installations in enterprise environments. Their ease of use and deployment, however, are accompanied by a difficulty in their management and security. Proposed solutions to these problems are based on centralization; in the control plane through centralized authentication and allocation of channels and power levels, and in the data plane through time slotted medium access using centralized scheduling for interference mitigation. While centralization of some control plane tasks has been shown to be feasible, centralization on the data plane is significantly harder to realize. This is because it needs to take into account the inherent variability of the wireless medium while offering bounds on delay and jitter on the control paths. In this work, we present a study of the various problems that arise in centralization of the data plane in an enterprise WLAN. We believe that a pragmatic solution for data plane centralization is the key approachto provisioning an enterprise WLAN consisting of a dense deployment of APs.

A successive refinement approach to wireless infrastructure network deployment

There has been a recent proliferation in wireless infrastructure network deployments. In a typical deployment, an installer uses either a one-time site survey or rules of thumb to place wireless access points and allocate them with channels and power levels. Because the access point location problem is inherently complex and one that requires tradeoffs among competing requirements, these approaches can result in either dead spots or significant unintended interference among wireless access points. This degrades network performance for end clients, with throughput reduction factors of 4x found in field measurements. In this paper, we take a first step towards improving client performance by coordinating choices of channels and power levels at wireless access points using a successive refinement approach. Our contributions are two-fold: first, we develop a mathematical model that crisply defines the solution space and identifies the characteristics of an optimal channel and power-level configuration. Second, we present heuristics that, under some simplifying assumptions, yield near-optimal configurations. We use Monte Carlo simulations to evaluate the performance of our heuristics. We find that the choice of heuristics for transmit power control impacts performance more than the channel allocation strategy, especially at high densities. Also, surprisingly, randomly assigning channels to access points appears to be an effective strategy at higher deployment densities. Taken together, we believe that this study paves the way to designing rapidly deployable real-world infrastructure networks that also have good performance

OmniVoice: a mobile voice solution for small-scale enterprises

We consider the problem of providing mobility support for Voice-over-IP (VoIP) traffic in small-scale enterprises. There is considerable interest in providing on-the-go support for VoIP through the use of WiFi-enabled smart phones. However, existing solutions either do not support client mobility or require client modifications, making them difficult to deploy in practice. In this paper, we present OmniVoice, an 802.11 compliant solution that supports mobility for VoIP traffic without any client modifications. To effectively support such traffic, OmniVoice eliminates client handoff delays and manages interference from non-VoIP background traffic. It achieves this by using (a) a single-channel WLAN design and (b) a lightweight central controller for scheduling non-interfering AP-to-client transmissions and dynamically associating clients. The controller minimizes interference with the help of an interference map that models potential exposed and hidden terminal conflicts in the WLAN, while allowing for the serialization of transmissions that would otherwise compete for medium access. We have implemented and extensively evaluated OmniVoice on a 40 node wireless testbed. OmniVoice meets the QoS requirements for VoIP in all operating scenarios and is unaffected by interference from non-VoIP traffic. In particular, OmniVoice provides 100% greater throughput and 130% greater uninterrupted connectivity time to mobile VoIP users as compared to an off-the-shelf 802.11 solution.

SAFIRE: A Self-Organizing Architecture for Information Exchange between First Responders

Disaster response requires quick and timely mobilization of relief efforts to save lives and property. Fundamental to these efforts is a reliable communications infrastructure that allows the disaster response teams to coordinate and exchange information in an efficient manner. Existing solutions for disaster response are inadequate as they suffer from interoperability problems and lack the appropriate amount of flexibility. In this paper, we propose SAFIRE, a novel multi-hop architecture for facilitating fast and reliable information exchange between first responders. The salient features of SAFIRE are (1) A decentralized cognitive radio-based approach for supporting direct communication between first responders, (2) A publish-subscribe mechanism for exchanging information among first responders, and (3) A flexible multi-layered policy framework for optimally configuring the system. We present the challenges in designing SAFIRE, and outline its basic components. We believe our exploration of such an architecture opens up a set of unique challenges related to the integration of different systems to realize SAFIRE, giving rise to new avenues for research In communication systems for disaster response.

Poster abstract: measuring multi-parameter conflict graphs for 802.11 networks

Dropping prices and increasing demand for a mobile workforce have caused a proliferation of wireless LANs in modern enterprises. In an attempt to increase wireless capacity and provide complete coverage, enterprises are turning to dense deployments of access points (APs). Unfortunately, APs on the same channel that ‘share the air’ are interference-limited, due to hidden and exposed terminals. Thus, increasing AP density can, paradoxically, reduce aggregate throughput. Proposals aiming to mitigate RF interference in 802.11 networks, such as dynamic frequency selection, power control, and traffic scheduling [1] assume the existence of a conflict graph (CG) as input. A conflict graph is a data structure that encodes interference information between links in the network. Specifically, for each link, it lists the set of other links which could cause interference either at the transmitter or receiver of the link. Note that the existence of a conflict edge between two links in a CG is dependent on factors such as the power level of the interferer and the data rate used on the link. Thus, what is needed is to determine for each link, the conditions under which all other potentially interfering transmitters could cause interference on that link. Measuring such configurations using today’s state of the art approaches [4] turns out to be highly unscalable. Our goal, therefore, is to find efficient ways of measuring conflict that incorporates all relevant effects (i.e. build a multi-parameter CG), while still being computationally efficient. We conduct a measurement study to gain a deeper understanding of how data rate, transmit power and temporal changes affect the conflict that exists between two links in a network. The insight we obtain allows us to construct a multi-parameter CG without measuring the entire state space of possible configurations. Contributions: The following is a summary of some of our findings: