Thyaga Nandagopal

Characterizing the capacity region in multi-radio multi-channel wireless mesh networks

Next generation fixed wireless broadband networks are being increasingly deployed as mesh networks in order to provide and extend access to the internet. These networks are characterized by the use of multiple orthogonal channels and nodes with the ability to simultaneously communicate with many neighbors using multiple radios (interfaces) over orthogonal channels. Networks based on the IEEE 802.11a/b/g and 802.16 standards are examples of these systems. However, due to the limited number of available orthogonal channels, interference is still a factor in such networks. In this paper, we propose a network model that captures the key practical aspects of such systems and characterize the constraints binding their behavior. We provide necessary conditions to verify the feasibility of rate vectors in these networks, and use them to derive upper bounds on the capacity in terms of achievable throughput, using a fast primal-dual algorithm. We then develop two link channel assignment schemes, one static and the other dynamic, in order to derive lower bounds on the achievable throughput. We demonstrate through simulations that the dynamic link channel assignment scheme performs close to optimal on the average, while the static link channel assignment algorithm also performs very well. The methods proposed in this paper can be a valuable tool for network designers in planning network deployment and for optimizing different performance objectives.

Fast and reliable estimation schemes in RFID systems

RFID tags are being used in many diverse applications in increasingly large numbers. These capabilities of these tags span from very dumb passive tags to smart active tags, with the cost of these tags correspondingly ranging from a few pennies to many dollars. One of the common problems that arise in any RFID deployment is the problem of quick estimation of the number of tags in the field up to a desired level of accuracy. Prior work in this area has focused on the identification of tags, which needs more time, and is unsuitable for many situations, especially where the tag set is dense. We take a different, more practical approach, and provide very fast and reliable estimation mechanisms. In particular, we analyze our estimation schemes and show that the time needed to estimate the number of tags in the system for a given accuracy is much better than schemes presented in related work. We show that one can estimate the cardinality of tag-sets of any size in near-constant time, for a given accuracy of estimation.

Characterizing achievable rates in multi-hop wireless mesh networks with orthogonal channels

This paper considers the problem of determining the achievable rates in multi-hop wireless mesh networks with orthogonal channels. We classify wireless networks with orthogonal channels into two types, half duplex and full duplex, and consider the problem of jointly routing the flows and scheduling transmissions to achieve a given rate vector. We develop tight necessary and sufficient conditions for the achievability of the rate vector. We develop efficient and easy to implement Fully Polynomial Time Approximation Schemes for solving the routing problem. The scheduling problem is a solved as a graph edge-coloring problem. We show that this approach guarantees that the solution obtained is within 50% of the optimal solution in the worst case (within 67% of the optimal solution in a common special case) and, in practice, is close to 90% of the optimal solution on the average. The approach that we use is quite flexible and can be extended to handle more sophisticated interference conditions, and routing with diversity requirements.

Anonymous Tracking Using RFID Tags

The increasing use of RFID tags in many applications have brought forth valid concerns of privacy and anonymity among users. One of the primary concerns with RFID tags is their ability to track an individually tagged entity. While this capability is currently thought to be necessary for supporting some features of RFID systems, such practice can lead to potential privacy violations. In this paper, we propose a privacy-preserving scheme that enables anonymous estimation of the cardinality of a dynamic set of RFID tags, while allowing the set membership to vary in both the spatial and temporal domains. In addition, the proposed scheme can identify the dynamics of the changes in the tag set population. The main idea of the scheme is to avoid explicit identification of tags. We demonstrate that the proposed scheme is highly adaptive and can accurately estimate tag populations across many orders of magnitude, ranging from a few tens to millions of tags. The associated probing latency is also substantially lower (les 10%) than that of the schemes which require explicit tag identification. We also show that our proposed scheme performs well even in highly dynamic environments, where the tag set keeps changing rapidly.

The effect of interference on the capacity of multihop wireless networks

The effect of interference on the achievable rate region in multihop wireless networks under three increasingly constraining interference models network throughput characterization namely primary conflict avoidance, receiver conflict avoidance and transmitter-receiver conflict avoidance is studied in this paper. The necessary conditions for achievability in the three models as linear constraints, and the structure of these constraints to develop efficient fully polynomial time approximation algorithms that route end-to-end flows between multiple source destination pairs are exploited. The techniques developed in this paper are applicable to a wide range of problems, including capacity problems, arising in multihop networks with interference constraints.

An ensemble of replication and erasure codes for cloud file systems

Geographically distributed storage is an important method of ensuring high data availability in cloud computing and storage systems. With the increasing demand for moving file systems to the cloud, current methods of providing such enterprise-grade resiliency are very inefficient. For example, replication based methods incur large storage cost though they provide low access latencies. While erasure coded schemes reduce storage cost, they are associated with large access latencies and high bandwidth cost. In this paper, we propose a novel scheme named CAROM, an ensemble of replication and erasure codes, to provide resiliency in cloud file systems with high efficiency. While maintaining the same consistency semantics seen in todays cloud file systems, CAROM provides the benefit of low bandwidth cost, low storage cost, and low access latencies. We perform a large-scale evaluation using real-world file system traces and demonstrate that CAROM outperforms replication based schemes in storage cost by up to 60% and erasure coded schemes in bandwidth cost by up to 43%, while maintaining low access latencies close to those in replication based schemes.

Frugal storage for cloud file systems

Enterprises are moving their IT infrastructure to cloud service providers with the goal of saving costs and simplifying management overhead. One of the critical services for any enterprise is its file system, where users require real-time access to files. Cloud service providers provide several building blocks such as Amazon EBS, or Azure Cache, each with very different pricing structures that differ on the basis of storage, access and bandwidth costs. Moving an entire file system to the cloud using such services is not cost-optimal if we rely on only one of these services. In this paper, we propose FCFS, a storage solution that drastically reduces the cost of operating a file system in the cloud. Our solution integrates multiple storage services and dynamically adapts the storage volume sizes of each service to provide a cost-efficient solution with provable performance bounds. Using real-world large scale data sets spanning a variety of work loads from an enterprise data center, we show that FCFS can reduce file storage and access costs in current cloud services by a factor of two or more, while allowing users to utilize the benefits of the various cloud storage services.

Scalable WiFi multicast services for very large groups

IEEE 802.11-based wireless local area networks, referred to as WiFi, have been globally deployed and the vast majority of mobile devices are currently WiFi-enabled. While WiFi has been proposed for multimedia content distribution, its lack of adequate support for multicast services hinders its ability to provide multimedia content distribution to a large number of devices. We propose AMuSe, a scalable and adaptive interference mitigation solution for WiFi multicast services which is based on accurate receiver feedback and that incurs a small control overhead. Specifically, we develop a scheme for dynamic selection of a subset of the multicast receivers as feedback nodes, which periodically send information, such as channel quality or received packet statistics, to the multicast sender. This feedback information is used by the multicast sender to optimize the multicast service quality, e.g., by dynamically adjusting the transmission bit-rate. Our proposed solution does not require any changes to the standards or any modifications to the WiFi devices. We have implemented the proposed solution in the ORBIT testbed and evaluated its performance in large groups with approximately 250 receivers, both with and without interference sources. Our online experiments demonstrate that our system provides practical multicast services that can accommodate hundreds of receivers.

Lowering Inter-datacenter Bandwidth Costs via Bulk Data Scheduling

Cloud service providers (CSP) of today operate multiple data centers, over which they provide resilient infrastructure, data storage and compute services. The links between data centers have very high capacity, and are typically purchased by the CSPs using established billing practices, such as 95-thpercentile billing or average-usage billing. These links are used to serve both client traffic as well as CSP-specific bulk data traffic, such as backup jobs, etc. Past studies have shown a diurnal pattern of traffic over such links. However, CSPs pay for the peak bandwidth, which implies that they are under-utilizing the capacity for which they have paid for. We propose a scheduling framework that considers various classes of jobs that are encountered over such links, and propose GRESE, an algorithm that attempts to minimize overall bandwidth costs to the CSP, by leveraging the flexible nature of the deadlines of these bulk data jobs. We demonstrate the problem is not a simple extension of any well-known scheduling problems, and show how the GRESE algorithm is effective in curtailing CSP bandwidth costs.

Identifying RFID tag categories in linear time

Given a large set of RFID tags, we are interested in determining the categories of tags that are present in the shortest time possible. Since there can be more than one tag present in a particular category, pure randomized strategies that rely on resolving individual tags are very inefficient. Instead, we rely on a pseudo-random strategy that utilizes a uniform hash function to accurately identify all t categories present among a given set of ψ tags with high probability. We propose two algorithms: (a) a single frame algorithm that determines the optimal frame size, and (b) a probabilistic version where the frame size is fixed, and we select the probability to minimize the number of frames needed for identification. Both of these algorithms run in time linear to the number of categories present, t. We show that our approach significantly outperforms existing algorithms for category identification. The performance of our algorithms is within a constant factor of the lower bound.