An Optimal Medium Access Control with Partial Observations for Sensor Networks

We consider medium access control (MAC) in multihop sensor networks, where only partial information about the shared medium is available to the transmitter. We model our setting as a queuing problem in which the service rate of a queue is a function of a partially observed Markov chain representing the available bandwidth, and in which the arrivals are controlled based on the partial observations so as to keep the system in a desirable mildly unstable regime. The optimal controller for this problem satisfies a separation property: we first compute a probability measure on the state space of the chain, namely the information state, then use this measure as the new state on which the control decisions are based. We give a formal description of the system considered and of its dynamics, we formalize and solve an optimal control problem, and we show numerical simulations to illustrate with concrete examples properties of the optimal control law. We show how the ergodic behavior of our queuing model is characterized by an invariant measure over all possible information states, and we construct that measure. Our results can be specifically applied for designing efficient and stable algorithms for medium access control in multiple-accessed systems, in particular for sensor networks

FHJ: A Formal Model for Hierarchical Dispatching and Overriding

Multiple inheritance is a valuable feature for Object-Oriented Programming. However, it is also tricky to get right, as illustrated by the extensive literature on the topic. A key issue is the ambiguity arising from inheriting multiple parents, which can have conflicting methods. Numerous existing work provides solutions for conflicts which arise from diamond inheritance: i.e. conflicts that arise from implementations sharing a common ancestor. However, most mechanisms are inadequate to deal with unintentional method conflicts: conflicts which arise from two unrelated methods that happen to share the same name and signature. This paper presents a new model called Featherweight Hierarchical Java (FHJ) that deals with unintentional method conflicts. In our new model, which is partly inspired by C++, conflicting methods arising from unrelated methods can coexist in the same class, and hierarchical dispatching supports unambiguous lookups in the presence of such conflicting methods. To avoid ambiguity, hierarchical information is employed in method dispatching, which uses a combination of static and dynamic type information to choose the implementation of a method at run-time. Furthermore, unlike all existing inheritance models, our model supports hierarchical method overriding: that is, methods can be independently overridden along the multiple inheritance hierarchy. We give illustrative examples of our language and features and formalize FHJ as a minimal Featherweight-Java style calculus

Flow Control for Multiple-Access Queues

We study the problem of finding a characterization for the channel that results when a queue is operated under multiple access conditions. In such systems, the mechanism by which different sources gain access to the channel plays a fundamental role in defining what is the channel available to each source. In this paper therefore we study the structure and properties of these control devices in some detail. Under some (mild) technical conditions, and under modeling assumptions inspired by TCP/IP's flow control (the standard control algorithm in the current Internet) , we are able to characterize the optimal controller for this problem. We also present some numerical simulations, to help develop an intuition on what exactly this control box does

Storage of Images and Video in Database Systems

We consider the problem of constructing indices to image and video data analogous to those used in classical database systems, in order to speed up the retrieval of such data from physical storage (either local or remote). To achieve this goal, we propose new coding techniques which combine a wavelet representation, embedded coding of the wavelet coefficients, and segmentation of image-domain regions in the wavelet domain, to generate a bitstream in which each image region is encoded independently of other objects, and without explicitly storing shape boundary information. Our simulation results show that the new proposed algorithms achieve coding performance which compares favorably, both perceptually and objectively, to that achieved using state-of-the-art image/video coding techniques which do not support indexing. Our results establish the feasibility of physical database designs for image and video data in which the compression performance of state-of-the-art coders needs not be c..

Network scheduling for data archiving applications in sensor networks

Since data archiving in sensor networks is a communication intensive application, a careful power management of communication is of critical importance for such networks. An example is FPS, an adaptive power scheduling algorithm that combines slotted scheduling with a CSMA MAC [7]. In this paper, we first propose a new global power scheduling protocol called Multi-Flow Power Scheduling (MPS) that delivers more data and consumes less energy than existing power scheduling protocols. MPS sets up a transmission schedule through standard data aggregation and dissemination operations, however since it creates a global schedule it does not scale to large networks. We then present a new power scheduling protocol called Hybrid Power Scheduling (HPS) that retains the scalability of FPS while maintaining the energy efficiency and high data delivery rate of MPS. In a thorough simulation study, we compare HPS and MPS, and our results show the efficacy of HPS. 1

A Region-Based Representation of Images in MARS

We study the problem of representing images within a multimedia Database Management System (DBMS), in order to support fast retrieval operations without compromising storage efficiency. To achieve this goal, we propose new image coding techniques which combine a wavelet representation, embedded coding of the wavelet coefficients, and segmentation of image-domain regions in the wavelet domain. A bitstream is generated in which each image region is encoded independently of other regions, and without having to explicitly store information describing the regions. Simulation results show that our proposed algorithms achieve coding performance which compares favorably, both perceptually and objectively, to that achieved using state-of-the-art image/video coding techniques while additionally providing region-based support