Showing papers by "Nitin H. Vaidya published in 2007"

Rate-Adaptive Framing for Interfered Wireless Networks

Abstract: The majority of existing wireless rate controls are based on the implicit assumption that frames are corrupted due to the random, arbitrary environmental and thermal noises. They generally reduce the channel rate on frame losses, trading lower efficiency in frequency band utilization for more robust modulation so that the current noise level may be tolerable. In highly interfered wireless networks where frames are lost mainly due to interference from other wireless transceivers, simply reducing the channel rate prolongs the frame transmission time and therefore aggravates frame loss ratio. This positive feedback in the rate control loop quickly diverges the interfered transceivers into a suboptimal channel rate and drives the network into a state with high interference. In the worst case, interfered transceivers can be starved. In this paper we present RAF, the rate-adaptive framing that jointly controls the channel rate and frame size according to the observed interference patterns and noise level at the receiver. Based on the inputs from physical layer carrier sense, the receiver derives the optimal channel rate and frame size that maximize throughput, and informs the transmitter of such optimal configuration in a few bits in the per-frame acknowledgement. Through intensive simulations we show that RAF consistently outperforms ARF, RBAR, and OAR in all simulated scenarios.

Connectivity and Capacity of Multi-Channel Wireless Networks with Channel Switching Constraints

Abstract: This paper argues for the need to address the issue of multi-channel network performance under constraints on channel switching. We present examples from emergent directions in wireless networking to motivate the need for such a study, and introduce some models to capture channel switching constraints. For some of these models, we study connectivity and capacity of a wireless network comprising n randomly deployed nodes, equipped with a single interface each, when there are c=O(logn) channels of equal bandwidth W/c available. We consider an adjacent (c,f) channel assignment where a node may switch between f adjacent channels, but the adjacent channel block is randomly assigned. We show that the per-flow capacity for this channel assignment model is Theta(Wradic(f/cnlogn)). We then show the adjacent (c,2) assignment maps to the case of untuned radios. We also consider a random (c,f) assignment where each node may switch between a pre-assigned random subset of f channels. For this model, we prove that per-flow capacity is O(Wradic(prnd/nlogn)) (where prnd=1-(1-f/c)(1-f/(c-1))...(1-f/(c-f+1)) and Omega(Wradic(f/cnlogn))).

Capacity of multi-channel wireless networks with random (c, f) assignment

Abstract: With the availability of multiple unlicensed spectral bands, and potential cost-based limitations on the capabilities of individual nodes, it is increasingly relevant to study the performance of multi-channel wireless networks with channel switching constraints. To this effect, some constraint models have been recently proposed, and connectivity and capacity results have been formulated for networks of randomly deployed single-interface nodes subject to these constraints. One of these constraint models is termed random (c, f) assignment, wherein each node is pre-assigned a random subset of f channels out of c (each having bandwidth Wc), and may only switch on these. Previous results for this model established bounds on network capacity, and proved that when c=O(logn), the per-flow capacity is O(W√prndnlogn) and Ω(W√fcnlogn) (where prnd = 1 - (1-fc)(1-fc-1)...(1-fc-f+1) ≥ 1-e-f2c). In this paper we present a lower bound construction that matches the previous upper bound. This establishes the capacity as Θ(W√prndnlogn). The surprising implication of this result is that when f=Ω(√c), random (c, f) assignment yields capacity of the same order as attainable via unconstrained switching. The routing/scheduling procedure used by us to achieve capacity requires synchronized route-construction for all flows in the network, leading to the open question of whether it is possible to achieve capacity using asynchronous procedures.

Reliable Broadcast in Wireless Networks with Probabilistic Failures

Abstract: We consider the problem of reliable broadcast in a wireless network in which nodes are prone to failure. Each node can fail independently with probability p. Failures are permanent. The primary focus is on Byzantine failures, but we also handle crash-stop failures. We consider two network models: a regular grid, and a random network. Our necessary and sufficient conditions for the Byzantine failure model indicate that p should be less than frac12, and the critical node degree is Theta(dmin+(lnn/ln(1/2p))+ln(1/2(1-p))) (where dmin is the minimum node degree associated with a non-empty neighborhood, and is a small constant). For a random network we prove that, for failure probability less than frac12, the critical average degree for reliable broadcast is O(lnn/frac12-p+frac12ln(1/2(1-p))). We briefly discuss the issue of crash-stop failures for which we have results that improve upon previously existing results for this model, when p approaches 0. We also identify an interesting similarity in the structure of various known results in the literature pertaining to a set of related problems in the realm of connectivity and reliable broadcast.

A Spatial Backoff Algorithm Using the Joint Control of Carrier Sense Threshold and Transmission Rate

Abstract: Traditional medium access control (MAC) protocols utilize temporal mechanisms such as access probability or backoff interval adaptation for contention resolution. Temporal contention resolution aims to separate transmissions from different nodes in time to achieve successful transmissions. We explore an alternative approach for wireless networks - named "spatial backoff - that adapts the "space" occupied by the transmissions. By adapting the space occupied by transmissions, the set of "locally" competing nodes, and thus, the channel contention level, can be adjusted to reach a suitable level. There are different ways to realize spatial backoff. In this paper, we propose a dynamic spatial backoff algorithm using the joint control of carrier sense threshold and transmission rate. Our results suggest that spatial backoff is promising to improve the channel utilization.

Net-X: a multichannel multi-interface wireless mesh implementation

Abstract: The use of multiple wireless channels has been advocated as one approach for enhancing network capacity. In many scenarios, hosts will be equipped with fewer radio interfaces than available channels. Under these scenarios, several protocols, which require interfaces to switch frequently, have been proposed. However, implementing protocols which require frequent interface switching in existing operating systems is non-trivial. In this paper, we identify the features needed in the operating system kernel for supporting frequent interface switching. We present a new channel abstraction module to support frequent interface switching. We identify modifications to interface device drivers to reduce switching delay. The channel abstraction module and an example multichannel protocol that uses the module have been implemented in a multichannel multi-interface testbed. Our implementation efforts are part of the Net-X project which is aimed at developing operating system support for exploiting various forms of diversity available in a wireless network in the form of multiple channels, interfaces, transmission rates, transmission power-levels, etc.

MAC-Layer Capture: A Problem in Wireless Mesh Networks using Beamforming Antennas

Abstract: Beamforming antennas have been shown to improve spatial reuse in wireless networks. Protocols that aim to exploit beamforming antennas have leveraged benefits from directional transmission as well as directional reception. We argue that exploiting antenna characteristics for transmission and reception alone, is not sufficient Controlling the antenna during the idle state of a node is necessary to attain further improvements. A node that does not control its antenna during the idle state will waste time in receiving packets not intended for it. These packets will eventually be dropped at the MAC layer, reducing channel utilization. With suitable beamforming, a node might be able to utilize this time for concurrent communication. We call this the problem of MAC-layer Capture in view of a node getting engaged in receiving unproductive packets. We present MAC and routing protocols to address MAC-layer capture. Simulation results show that improvements from avoiding MAC-layer capture can be substantial in static mesh networks.

Reliable Local Broadcast in a Wireless Network Prone to Byzantine Failures.

Abstract: Reliable broadcast can be a very useful primitive for many distributed applications, especially in the context of sensoractuator networks. Recently, the issue of reliable broadcast has been addressed in the context of the radio network model that is characterized by a shared channel, and where a transmission is heard by all nodes within the sender’s neighborhood. This basic defining feature of the radio network model can be termed as the reliable local broadcast assumption. However, in actuality, wireless networks do not exhibit such perfect and predictable behavior. Thus any attempt at distributed protocol design for multi-hop wireless networks based on the idealized radio network model requires the availability of a reliable local broadcast primitive that can provide guarantees of such idealized behavior. We present a simple proof-of-concept approach toward the implementation of a reliable local broadcast primitive with probabilistic guarantees, with the intent to highlight the potential for lightweight scalable solutions to achieve probabilistic reliable local broadcast in a wireless network.

DSCR: A More Stable MAC Protocol for Wireless Networks

Abstract: Channel capacity is a scarce resource in wireless networks. However, the way in which IEEE 802.11 DCF uses random backoff time to resolve the channel contention leads to inefficient utilization of this scarce resource. In particular, in a highly loaded network, the portion of channel bandwidth wasted due to collisions is significantly high. Prior work proposes various ways to improve the performance of 802.11. In this paper, we present a different mechanism which uses two “virtual” stages of contention resolution to achieve better and more stable performance than IEEE 802.11 DCF. The proposed mechanism is compatible with 802.11, and can be incorporated in IEEE 802.11 without major changes. Simulation results, as well as some analysis, are presented to demonstrate the effectiveness of this mechanism.

Heterogeneous Multi-Channel Wireless Networks: Scheduling and Routing Issues

Abstract: We consider multi-channel networks where nodes may be equipped with heterogeneous radios, each potentially capable of operation on a limited portion of the total availa ble spectrum. Moreover even the channels may not all be identical; they may possibly have different propagation ch aracteristics, and may support different sets of transmiss ion rates. Much prior research on multi-channel networks has assumed identical channels and radio capabilities. However heterogeneity of channels and radios introduces a host of new issues that must be handled. In recent theoretical work we considered asymptotic transport capacity of multi-channel networks subject to switching constraints. This constitutes a class of instances involving heterogeneous r adios, albeit identical channels. We leverage some of the insights obtained from our theoretical results, and now con sider a more general model involving heterogeneity of radios and channels for networks of realistic scale. We id entify the key issues that differentiate heterogeneous multi-channel networks, and describe a design framework for routing and channel/interface assignment.