Cauligi S. Raghavendra

PEGASIS: Power-efficient gathering in sensor information systems

Sensor webs consisting of nodes with limited battery power and wireless communications are deployed to collect useful information from the field. Gathering sensed information in an energy efficient manner is critical to operate the sensor network for a long period of time. In W. Heinzelman et al. (Proc. Hawaii Conf. on System Sci., 2000), a data collection problem is defined where, in a round of communication, each sensor node has a packet to be sent to the distant base station. If each node transmits its sensed data directly to the base station then it will deplete its power quickly. The LEACH protocol presented by W. Heinzelman et al. is an elegant solution where clusters are formed to fuse data before transmitting to the base station. By randomizing the cluster heads chosen to transmit to the base station, LEACH achieves a factor of 8 improvement compared to direct transmissions, as measured in terms of when nodes die. In this paper, we propose PEGASIS (power-efficient gathering in sensor information systems), a near optimal chain-based protocol that is an improvement over LEACH. In PEGASIS, each node communicates only with a close neighbor and takes turns transmitting to the base station, thus reducing the amount of energy spent per round. Simulation results show that PEGASIS performs better than LEACH by about 100 to 300% when 1%, 20%, 50%, and 100% of nodes die for different network sizes and topologies.

PAMAS—power aware multi-access protocol with signalling for ad hoc networks

In this paper we develop a new multiaccess protocol for ad hoc radio networks. The protocol is based on the original MACA protocol with the adition of a separate signalling channel. The unique feature of our protocol is that it conserves battery power at nodes by intelligently powering off nodes that are not actively transmitting or receiving packets. The manner in which nodes power themselves off does not influence the delay or throughput characteristics of our protocol. We illustrate the power conserving behavior of PAMAS via extensive simulations performed over ad hoc networks containing 10-20 nodes. Our results indicate that power savings of between 10% and 70% are attainable in most systems. Finally, we discuss how the idea of power awareness can be built into other multiaccess protocols as well.

Efficient routing in intermittently connected mobile networks: the multiple-copy case

Intermittently connected mobile networks are wireless networks where most of the time there does not exist a complete path from the source to the destination. There are many real networks that follow this model, for example, wildlife tracking sensor networks, military networks, vehicular ad hoc networks (VANETs), etc. In this context, conventional routing schemes would fail, because they try to establish complete end-to-end paths, before any data is sent. To deal with such networks researchers have suggested to use flooding-based routing schemes. While flooding-based schemes have a high probability of delivery, they waste a lot of energy and suffer from severe contention which can significantly degrade their performance. With this in mind, we look into a number of ldquosingle-copyrdquo routing schemes that use only one copy per message, and hence significantly reduce the resource requirements of flooding-based algorithms. We perform a detailed exploration of the single-copy routing space in order to identify efficient single-copy solutions that (i) can be employed when low resource usage is critical, and (ii) can help improve the design of general routing schemes that use multiple copies. We also propose a theoretical framework that we use to analyze the performance of all single-copy schemes presented, and to derive upper and lower bounds on the delay of any scheme.

Data gathering algorithms in sensor networks using energy metrics

Gathering sensed information in an energy efficient manner is critical to operating the sensor network for a long period of time. The LEACH protocol presented by Heinzelman et al. (2000) is an elegant solution where clusters are formed to fuse data before transmitting to the base station. In this paper, we present an improved scheme, called PEGASIS (power-efficient gathering in sensor information systems), which is a near-optimal chain-based protocol that minimizes energy. In PEGASIS, each node communicates only with a close neighbor and takes turns transmitting to the base station, thus reducing the amount of energy spent per round. Simulation results show that PEGASIS performs better than LEACH. For many applications, in addition to minimizing energy, it is also important to consider the delay incurred in gathering sensed data. We capture this with the energy /spl times/ delay metric and present schemes that attempt to balance the energy and delay cost for data gathering from sensor networks. We present two new schemes to minimize energy /spl times/ delay using CDMA and non-CDMA sensor nodes. We compared the performance of direct, LEACH, and our schemes with respect to energy /spl times/ delay using extensive simulations for different network sizes. Results show that our schemes perform 80 or more times better than the direct scheme and also outperform the LEACH protocol.

An adaptive energy-efficient and low-latency MAC for data gathering in wireless sensor networks

Summary form only given. In many sensor network applications the major traffic pattern consists of data collected from several source nodes to a sink through a unidirectional tree. We propose DMAC, an energy efficient and low latency MAC that is designed and optimized for such data gathering trees in wireless sensor networks. We first show that previously proposed MAC protocols for sensor networks that utilize activation/sleep duty cycles suffer from a data forwarding interruption problem, whereby not all nodes on a multihop path to the sink are notified of data delivery in progress, resulting in significant sleep delay. DMAC is designed to solve the interruption problem and allow continuous packet forwarding by giving the sleep schedule of a node an offset that depends upon its depth on the tree. DMAC also adjusts the duty cycles adaptively according to the traffic load in the network. We further propose a data prediction mechanism and the use of more-to-send (MTS) packets in order to alleviate problems pertaining to channel contention and collisions. Our simulation results show that by exploiting the application-specific structure of data gathering trees in sensor networks, DMAC provides significant energy savings and latency reduction while ensuring high data reliability.

Data gathering in sensor networks using the energy*delay metric

In this paper we consider the problem of data collection from a sensor web consisting of N nodes, where nodes have packets of data in each round of communication that need to be gathered and fused with other nodes’ packets into one packet and transmitted to a distant base station. Nodes have power control in their wireless communications and can transmit directly to any node in the network or to the base station. With unit delay cost for each packet transmission, if all nodes transmit data directly to the base station, then both high energy and high delay per round will occur. In our prior work [6], we developed an algorithm to minimize the energy cost per round, where a linear chain of all the nodes are formed to gather data, and nodes took turns to transmit to the base station. If the goal is to minimize the delay cost, then a binary combining scheme can be used to accomplish this task in about log N units of delay with parallel communications and incurring a slight increase in energy cost. The goal is to find data gathering schemes that balance the energy and delay cost, as measured by energy*delay. We conducted extensive simulation experiments with a number of schemes for this problem with 100 nodes in playing fields of 50m x 50m and 100m x 100m and the base station located at least 100 meters and 200 meters, respectively, from any node. With CDMA capable sensor nodes, a chain-based binary scheme performs best in terms of energy*delay. If the sensor nodes are not CDMA capable, then parallel communications are possible only among spatially separated nodes, and a chain-based 3 level hierarchy scheme performs well. These schemes perform 60 to 100 times better than direct scheme and also outperform a cluster based scheme, called LEACH [3].

Performance evaluation of the IEEE 802.15.4 MAC for low-rate low-power wireless networks

IEEE 802.15.4 is a new standard to address the need for low-rate low-power low-cost wireless networking. We provide in this paper one of the first simulation-based performance evaluations of the new medium access protocol in IEEE 802.15.4, focusing on its beacon-enabled mode for a star-topology network. We describe its key features such as the superframe structure, which allows devices to access channels in a contention access period (CAP) or a collision free period (CFP) and the beacon-based synchronization mechanism. Our performance evaluation study reveals some of the key throughput-energy-delay tradeoffs inherent in this MAC protocol. We provide an analysis comparing the energy costs of beacon tracking and non-tracking modes for synchronization, showing that the optimum choice depends upon the combination of duty cycles and data rates.

Performance analysis of mobility-assisted routing

Traditionally, ad hoc networks have been viewed as a connected graph over which end-to-end routing paths had to be established.Mobility was considered a necessary evil that invalidates paths and needs to be overcome in an intelligent way to allow for seamless ommunication between nodes.However, it has recently been recognized that mobility an be turned into a useful ally, by making nodes carry data around the network instead of transmitting them. This model of routing departs from the traditional paradigm and requires new theoretical tools to model its performance. A mobility-assisted protocol forwards data only when appropriate relays encounter each other, and thus the time between such encounters, called hitting or meeting time, is of high importance.In this paper, we derive accurate closed form expressions for the expected encounter time between different nodes, under ommonly used mobility models. We also propose a mobility model that can successfully capture some important real-world mobility haracteristics, often ignored in popular mobility models, and alculate hitting times for this model as well. Finally, we integrate this results with a general theoretical framework that can be used to analyze the performance of mobility-assisted routing schemes. We demonstrate that derivative results oncerning the delay of various routing s hemes are very accurate, under all the mobility models examined. Hence, this work helps in better under-standing the performance of various approaches in different settings, and an facilitate the design of new, improved protocols.

A dynamic load-balancing policy with a central job dispatcher (LBC)

A dynamic load-balancing policy is proposed with a central job dispatcher called the LBC policy for distributed systems. The design of this policy is motivated by the operation of a single-queue multiserver queueing system, and the average job response time is the same as that of a single-queue multiserver system, which is the best achievable performance when the communication delay is reduced to zero. Hence, near-minimum average job response time is expected for distributed systems with high-speed communication subnets. The performance is studied for systems with nonnegligible job transfer delays in the following three aspects: average job response time, overhead due to information exchanges, and sensitivity to heterogeneous load. >

An adaptive energy-efficient and low-latency MAC for tree-based data gathering in sensor networks

Summary A specific characteristic of sensor network applications is that the major traffic consists of data collection from various sensor source nodes to a sink via a unidirectional tree. In this paper, we propose DMAC, an energy efficient and low latency MAC that is designed and optimized for such data gathering trees in wireless sensor networks. We first show that previously proposed MAC protocols for sensor networks that utilize activation/sleep duty cycles suffer from a data forwarding interruption problem, whereby not all nodes on a multihop path to the sink can be notified of data delivery in progress, resulting in significant sleep delay. DMAC is designed to solve the interruption problem, by giving the active/sleep schedule of a node an offset that depends upon its depth on the tree. This scheme allows continuous packet forwarding because all nodes on the multihop path can be notified of the data delivery in progress. DMAC also adjusts node duty cycles adaptively according to the traffic load in the network by varying the number of active slots in an schedule interval. We further propose a data prediction mechanism and the use of more to send (MTS) packets in order to alleviate problems pertaining to channel contention and collisions. Our simulation results as well as experimental results with the Mote platform show that by exploiting the applicationspecific structure of data gathering trees in sensor networks, DMAC provides significant energy savings and latency reduction while ensuring high data reliability. Copyright