Showing papers by "Samir R. Das published in 2005"

A topology control approach for utilizing multiple channels in multi-radio wireless mesh networks

Abstract: We consider the channel assignment problem in a multi-radio wireless mesh network that involves assigning channels to radio interfaces for achieving efficient channel utilization. We propose the notion of a traffic-independent base channel assignment to ease coordination and enable dynamic, efficient and flexible channel assignment. We present a novel formulation of the base channel assignment as a topology control problem, and show that the resulting optimization problem is NP-complete. We then develop a new greedy heuristic channel assignment algorithm (termed CLICA) for finding connected, low interference topologies by utilizing multiple channels. Our extensive simulation studies show that the proposed CLICA algorithm can provide large reduction in interference (even with a small number of radios per node), which in turn leads to significant gains in both link layer and multihop performance in 802.11-based multi-radio mesh networks.

Exploiting path diversity in the link layer in wireless ad hoc networks

Abstract: We develop an anycast mechanism at the link layer for wireless ad hoc networks. The goal is to exploit path diversity in the link layer by choosing the best next hop to forward packets when multiple next hop choices are available. Such choices can come from a multipath routing protocol, for example. This technique can reduce transmission retries and packet drop probabilities in the face of channel fading. We develop an anycast extension of the IEEE 802.11 MAC layer based on this idea. We implement the protocol in an experimental proof-of-concept testbed using the Berkeley motes platform and S-MAC protocol stack. We also implement it in the popular ns-2 simulator and experiment with the AOMDV multipath routing protocol and Rician fading channels. We show that anycast performs significantly better than 802.11 in terms of packet delivery, particularly when the path length is large or fading is substantial.

Design and evaluation of iMesh: an infrastructure-mode wireless mesh network

Abstract: We have designed and evaluated iMesh, an infrastructure-mode 802.11-based mesh network. IEEE 802.11 access points double as routers making the network architecture completely transparent to mobile clients, who view the network as a conventional wireless LAN. Layer-2 handoffs between access points trigger routing activities inside the network, which can be thought of as layer-3 handoffs. We describe the design rationale and a testbed implementation of iMesh. We present results related to the handoff performance. The results demonstrate excellent handoff performance, the overall latency varying between 50-100 ms, depending on different layer-2 techniques, even when a five-hop long route update is needed. Various performance measurements also demonstrate the clear superiority of a flat routing scheme relative to a more traditional, Mobile IP-like scheme to handle layer-3 handoff.

Efficient gathering of correlated data in sensor networks

Abstract: In this paper, we design techniques that exploit data correlations in sensor data to minimize communication costs (and hence, energy costs) incurred during data gathering in a sensor network. Our proposed approach is to select a small subset of sensor nodes that may be sufficient to reconstruct data for the entire sensor network. Then, during data gathering only the selected sensors need to be involved in communication. The selected set of sensors must also be connected, since they need to relay data to the data-gathering node. We define the problem of selecting such a set of sensors as the connected correlation-dominating set problem, and formulate it in terms of an appropriately defined correlation structure that captures general data correlations in a sensor network.We develop a set of energy-efficient distributed algorithms and competitive centralized heuristics to select a connected correlation-dominating set of small size. The designed distributed algorithms can be implemented in an asynchronous communication model, and can tolerate message losses. We also design an exponential (but non-exhaustive) centralized approximation algorithm that returns a solution within O(log n) of the optimal size. Based on the approximation algorithm, we design a class of efficient centralized heuristics that are empirically shown to return near-optimal solutions. Simulation results over randomly generated sensor networks with both artificially and naturally generated data sets demonstrate the efficiency of the designed algorithms and the viability of our technique -- even in dynamic conditions.

Fault tolerant connected sensor cover with variable sensing and transmission ranges

Abstract: Sensor networks are often deployed in a redundant fashion. In order to prolong the network lifetime, it is desired to choose only a subset of sensors to keep active and put the rest to sleep. In order to provide fault tolerance, this small subset of active sensors should also provide some degree of redundancy. In this paper, we consider the problem of choosing a minimum subset of sensors such that they maintain a required degree of coverage and also form a connected network with a required degree of fault tolerance. In addition, we consider a more general, variable radii sensor model, wherein every sensor can adjust both its sensing and transmission ranges to minimize overall energy consumption in the network. We call this the variable radiik1-Connected, k2-Cover problem. To address this problem, we propose a distributed and localized Voronoi- based algorithm. The approach extends the relative neighborhood graph (RNG) structure to preserve k-connectivity in a graph, and design a distributed technique to inactivate desirable nodes while preserving k-connectivity of the remaining active nodes. We show through extensive simulations that our proposed techniques result in overall energy savings in random sensor networks over a wide range of experimental parameters.

Routing in Mobile Ad Hoc Networks

Abstract: Efficient, dynamic routing is one of the key challenges in mobile ad hoc networks. In the recent past, this problem was addressed by many research efforts, resulting in a large body of literature. We survey various proposed approaches for routing in mobile ad hoc networks such as flooding, proactive, on-demand and geographic routing, and review representative protocols from each of these categories. We further conduct qualitative comparisons across various approaches. We also point out future research issues in the context of individual routing approaches as well as from the overall system perspective.

Cache placement in sensor networks under update cost constraint

Abstract: In this paper, we address an optimization problem that arises in context of cache placement in sensor networks. In particular, we consider the cache placement problem where the goal is to determine a set of nodes in the network to cache/store the given data item, such that the overall communication cost incurred in accessing the item is minimized, under the constraint that the total communication cost in updating the selected caches is less than a given constant. In our network model, there is a single server (containing the original copy of the data item) and multiple client nodes (that wish to access the data item). For various settings of the problem, we design optimal, near-optimal, heuristic-based, and distributed algorithms, and evaluate their performance through simulations on randomly generated sensor networks.

Cache placement in sensor networks under update cost constraint

Abstract: In this paper, we address an optimization problem that arises in context of cache placement in sensor networks. In particular, we consider the cache placement problem where the goal is to determine a set of nodes in the network to cache/store the given data item, such that the overall communication cost incurred in accessing the item is minimized, under the constraint that the total communication cost in updating the selected caches is less than a given constant. In our network model, there is a single server (containing the original copy of the data item) and multiple client nodes (that wish to access the data item). For various settings of the problem, we design optimal, near-optimal, heuristic-based, and distributed algorithms, and evaluate their performance through simulations on randomly generated sensor networks.