Ashok Agrawala | University of Maryland, College Park (original) (raw)

Papers by Ashok Agrawala

Proceedings of the 3rd international conference on Mobile systems, applications, and services, 2005

Information Dynamics [Agrawala, 2000] is an information-centric framework that provides a suffici... more Information Dynamics [Agrawala, 2000] is an information-centric framework that provides a sufficient understanding of the characteristics of information used in systems for better system design and implementation. In this paper, we describe how to improve link-state routing based on this framework. Link-state routing protocols such as OSPF (Open Shortest Path First) [Moy, 1991] are currently used in many networks. In link-state routing, routes are determined based on link-delay estimates, which are periodically flooded throughout the network. This flooding of link-delay estimates is done without considering the relevance of these estimates to routing quality, i.e. without taking into account the usefulness of the link-delay information. We have developed a new approach that improves link-state routing by estimating future link delays and flooding these estimates only to the extent that they are relevant. This means that we consider the dynamics of the link-delay information and its usefulness. Simulation studies suggest that our approach can lead to significant reductions in routing traffic with noticeable improvements of routing quality in high-load conditions, demonstrating the effectiveness of the framework. We plan to further investigate the conditions where our information-dynamics approach is better than the standard approach.

A number of measurement studies have examined traffic characteristics in wireless networks. Most ... more A number of measurement studies have examined traffic characteristics in wireless networks. Most of these measurements [1], [6], [7] have been conducted from the wired portion of the network. In this paper we argue that such measurements are not sufficient to expose either the characteristics of the wireless medium or how such characteristics impact traffic patterns. While it is easier to make consistent measurements in the wired part of a network, such measurements can not observe the significant vagaries present in the wireless medium itself. As a consequence constructing an efficient and accurate measurement system from a wireless vantage point is important but usually quite difficult. In our work we have explored the various issues in implementing such a system to monitor traffic in an 802.11 based wireless network. We identify different challenges in making such measurements and provide detailed experimental evidence in their supports. Our work shows that the wireless measurement allows us to infer much richer information about the medium characteristics than is possible with a measurements made on the wired part of the network. We apply our measurement technique to study the end-to-end wireless network delay. We show that wireless monitoring can effectively identify the causes of end-to-end delays.

Acquisition, organization, management, retrieval, and distribution of information are fundamental... more Acquisition, organization, management, retrieval, and distribution of information are fundamental purposes of digital libraries and their supporting infrastructures. Interoperable digital libraries pose particularly difficult system design issues. Interoperability research has focused largely on syntactic and semantic interoperability. In this paper, a third form of interoperability, analytic interoperability is proposed, with a framework in which to consider it. Since information is the essential commodity of interest, a comprehensive interoperability design should take into account the fundamental properties of information, including representation, composition, relationships, and dynamics. Information Dynamics considers how the nature of information can be used to achieve analytic interoperability. 15. NUMBER OF PAGES 152

This work was a grant to enhance the Maruti operating system in several ways, in order to provide... more This work was a grant to enhance the Maruti operating system in several ways, in order to provide Mississippi State with a platform upon which their work on the Real-Time Message Passing Interface could be developed. Key technical achievements: (1) Developed predictable Myrinet communications for use in a real-time NOW; (2) Developed the MSU-Kernel to provide a POSIX OS for real-time NOWs; (3) Developed and implemented an algorithm for deploying a globally synchronized clock in a real-time NOW; (4) Developed an improved real-time scheduler for the Maruti hard real-time operating system at University of Maryland (UMD); and (5) Introduced a new parametric approach in Maruti for dynamic scheduling at UMD. Details of the results of the work are presented in papers, thesis and project reports.

• ,J kthat the number can be contained in a fixed amount of memory by storing it as the residue o... more • ,J kthat the number can be contained in a fixed amount of memory by storing it as the residue of a modulus. The number of messages required to implemetit the exclusion can be reduced by using sequential node-by-node processing, by using broadcast message techniques or by sending information through timing channels. The 4eaders and writers"'-problem is solved by a simple modification of the' algorithm. The modifications necessary to make the algorithm robust are described. UNCLASSIFIED WWI Abmtract An algorithm is proposed which creates mutual exclusion in a computer network whose nodes can communicate only by messages and do not share memory. The algorithm sends only 20(N-1) messages, where N is the number of nodes in the network, per critical section invocation. This number of messages is a minimum if parallel, distributed, symmetric control is used; hence, the algorithm is optimal in this respect. The time needed to achieve mutual exclusion is also minimal under some general assumptions. Like Lamport's "bakery algorithm," unbounded sequence numbers are used to provide first-come first-served priority into the critical section. It is shown that the number can be contained in a fixed amount of memory by storing it as the residue of a modulus. The number of messages required to implement the exclusion can be reduced by using sequential node-by-node processing, by using broadcast message techniques, or by sending information through timing channels. The "readers and writers" problem is solved by a simple modification of the algorithm. The modifications necessary to make the algorithm robust are described.

Proceedings Second International Workshop on Real-Time Computing Systems and Applications

Proceedings of the 12th EAI International Conference on Mobile and Ubiquitous Systems: Computing, Networking and Services, 2015

Location Based Services are providing one of the fastest growing market segments today. While the... more Location Based Services are providing one of the fastest growing market segments today. While the most common technique for location determination is GPS, several alternative approaches have been proposed for Wi-Fi environments, based on time of flight, signal strength, etc. Time based techniques not only require accurate timestamping mechanisms, but also precise and synchronized clocks, which is quite difficult and expensive in industry. On the other hand, signal strength based methods need a lot of ground truth data. These method also require time consuming work and efforts before the system comes into use. In considerations of costs and time consumption, we present in this paper an approach for determining the location of a general Wi-Fi device combining RTS/CTS and TDoA techniques. The proposed model is deployable in various environments and contains two different methods, with clock mapping functions and asynchronized clocks. We also explain limitations of current round trip time (RTT) based RTS/CTS systems. Extensive experiments have been conducted and demonstrated how an accuracy of about one foot can be obtained and also the assumption of RTT measurements have been verified.

Real-time applications are becoming increasingly popular in distributed environments. These real-... more Real-time applications are becoming increasingly popular in distributed environments. These real-time applications range from hard real-time applications with periodic or aperiodic tasks and intertask relative timing constraints to soft real-time applications with best effort timing requirements. This paper introduces a complete system model for scheduling and dispatching hard as well as soft real-time tasks with intertask temporal dependencies in distributed environments. The model uses a dynamic time based off-line scheduler to verify the feasibility of a distributed hard real-time task set, and a parametric runtime kernel that guarantees the temporally determinate dispatching of hard real-time task instances and best effort performance for soft real-time task instances. The use of the dynamic time based scheduling, provides off-line guarantees for all the timing requirements of the hard real-time tasks while the parametric dispatching mechanism maintains a flexible run-time envir...

Proceedings of the 12th IEEE International Conference on Network Protocols, 2004. ICNP 2004.

Physical layer capture (PLC) in 802.11b refers to the successful reception of the stronger (highe... more Physical layer capture (PLC) in 802.11b refers to the successful reception of the stronger (higher signal strength at receiver) frame in a collision. PLC causes significant imbalance in the throughputs of sources. Existing 802.11b simulators, including ns2 and Qualnet, assume that PLC occurs only if the stronger frame arrives first at the receiver. We show empirically that in reality PLC occurs even if the stronger frame arrives later (but within the physical layer preamble of the first frame). Consequently, throughput unfairness in reality can be significantly (up to 15%) higher than with the former PLC model. We have modified the ns2 simulator to account for this and Qualnet will be incorporating a fix in their next release. To identify which frames were involved in collisions, when their transmissions started, and which of them were retrieved, we have devised a novel technique using multiple sniffers and instrumented device drivers to reconstruct from the air interface all tx/rx events in a WLAN to within 4 × accuracy. This allows us to quantify the causal links from the PHY layer through the MAC layer to the observed application layer imbalance. It also shows that the arrival times of colliding frames routinely differ by as much as 20 × due to inherent uncertainties of 802.11b firmware clock synchronization and rx/tx turnaround delays, and that the frame to arrive first can be either the stronger or the weaker with equal likelihood.

2003 IEEE Wireless Communications and Networking, 2003. WCNC 2003.

We define energy-efficient broadcast and multicast schemes for reliable communication in multi-ho... more We define energy-efficient broadcast and multicast schemes for reliable communication in multi-hop wireless networks. Unlike previous techniques, the choice of neighbors in the broadcast and multicast trees in these schemes, are based not only on the link distance, but also on the error rates associated with the link. Our work is based on some existing techniques that create such trees which have a constant approximation ratio to optimal solutions. However, these existing techniques are applicable only to the ideal scenario where wireless links are error-free. Through simulations we show that our scheme achieves upto 45% improvement over previous schemes on realistic 100-node network topologies.

PCCC 2005. 24th IEEE International Performance, Computing, and Communications Conference, 2005.

There has been a growing interest in the applications of wireless sensor networks in unattended e... more There has been a growing interest in the applications of wireless sensor networks in unattended environments. In such applications, sensor nodes are usually deployed randomly in an area of interest. Knowledge of accurate node location is essential in such network setups in order to correlate the gathered data to the origin of the sensed phenomena and assure the relevance of the reported information. In addition, awareness of the nodes' positions can enable employing efficient management strategies such as geographic routing and conducting important analyses such as node coverage properties. In this paper, we present an efficient anchor-free protocol for localization in wireless sensor networks. Each node discovers its neighbors that are within its transmission range and estimates their ranges. Our algorithm fuses local range measurements in order to form a network wide unified coordinate systems while minimizing the overhead incurred at the deployed sensors. Scalability is achieved through grouping sensors into clusters. Simulation results show that the proposed protocol achieves precise localization of sensors and maintains consistent error margins. In addition, we capture the effect of error accumulation of the node's range estimates and network's size and connectivity on the overall accuracy of the unified coordinate system.

Proceedings IEEE 24th Annual Joint Conference of the IEEE Computer and Communications Societies.

In this paper, we propose an enhancement to the existing IEEE 802.11 Distributed Coordination Fun... more In this paper, we propose an enhancement to the existing IEEE 802.11 Distributed Coordination Function (DCF) MAC to improve channel spatial reuse efficiency, and thus improve overall network data throughput. Our modification, named the Location Enhanced DCF (LED) for IEEE 802.11, incorporates location information in DCF frame exchange sequences so that stations sharing the communication channel are able to make better interference predictions and blocking assessments. Utilizing an underlying physical layer design that supports frame capture, the LED enhanced interference estimation can increase overall network data throughput by permitting more concurrent transmissions. In this paper we also analytically study the potential performance enhancement of the LED over the original IEEE 802.11 DCF. The results are verified using the ns-2 simulator, which shows that up to 35% of DCF blocking decisions are unnecessary and our LED method can achieve up to 22% more throughput than the original DCF.

Proceedings of the 2004 ACM workshop on Wireless security - WiSe '04, 2004

Many studies on measurement and characterization of wireless LANs (WLANs) have been performed rec... more Many studies on measurement and characterization of wireless LANs (WLANs) have been performed recently. Most of these measurements have been conducted from the wired portion of the network based on wired monitoring (e.g. sniffer at some wired point) or SNMP statistics. More recently, wireless monitoring, the traffic measurement from a wireless vantage point, is also widely adopted in both wireless research and commercial WLAN management product development. Wireless monitoring technique can provide detailed PHY/MAC information on wireless medium. For the network diagnosis purpose (e.g. anomaly detection and security monitoring) such detailed wireless information is more useful than the information provided by SNMP or wired monitoring. In this paper we have explored various issues in implementing the wireless monitoring system for an IEEE 802.11 based wireless network. We identify the pitfalls that such system needs to be aware of, and then provide feasible solutions to avoid those pitfalls. We implement an actual wireless monitoring system and demonstrate its effectiveness by characterizing a typical computer science department WLAN traffic. Our characterization reveals rich information about the PHY/MAC layers of the IEEE 802.11 protocol such as the typical traffic mix of different frame types, their temporal characteristics and correlation with the user activities. Moreover, we identify various anomalies in protocol and security of the IEEE 802.11 MAC. Regarding the security, we identify malicious usages of WLAN, such as email worm and network scanning. Our results also show excessive retransmissions of some management frame types reducing the useful throughput of the wireless network.

14th IEEE Proceedings on Personal, Indoor and Mobile Radio Communications, 2003. PIMRC 2003.

Service discovery technologies are exploited to enable services to advertise their existence in a... more Service discovery technologies are exploited to enable services to advertise their existence in a dynamic way, and can be discovered, configured and used by other devices with a minimum of manual efforts. Automatic service discovery will play essential role in future network scenarios. Especially, the development Mobile Ad Hoc Networks (MANETs) to support the proliferation of mobile devices and emergence of pervasive computing gives rise to the challenges of the service discovery techniques, because MANET allows these devices to communicate dynamically without fixed infrastructure and centralized administration. In this paper, we present a dynamic service discovery infrastructure that uses XML to describe services and match using the semantic content of service descriptions for MANET. We believe that the architecture we have designed is a necessary component of any discovery of non-infrastructure services effectively and correctly. We further exploit the secure and performance issues of this infrastructure.

IEEE Workshop on Experimental Distributed Systems

The M A R U T I operating system is designed to support hard realtime applications on distributed... more The M A R U T I operating system is designed to support hard realtime applications on distributed computer systems while providing a fault tolerant operation. It is an object oriented design and the communication mechanism allows transparent use of the resources of a distributed system. Fault tolerance is provided through a consistent set of mechanisms that supports a number of policies. Most importantly, M A R U T I supports guaranteed-service scheduling, in which jobs that are accepted by the system are guaranteed to meet the time constraints of the computation requests with a specified degree of fault tolerance. As a consequence, MARUTI applications can be executed in a predictable fashion. The development of current hard real-time applications requires that the analyst estimate the resource requirements for all parts of the computation, and then make sure that the resources are available to meet the time constraints. It tends to be a cumbersome process. As a part of the MARUTI system, a set of tools has been developed which supports the hard real-time applications during various phases of their life cycle. The present version of MARUTI has been implemented as a prototype running on a UNIX platform. Experiences with the development of this prototype are also presented. 1 MARUTI Overview The main focus of the MARUTI project at the University of Maryland is to examine the constructs of future distributed, hard real-time, fault tolerant, secure operating systems[4, 8, 91. This operating system contains a number of new approaches for solving many of the problems in real-time systems of tomorrow. The development is a reserch effort aimed ~~ 'This work is supported in part by contract DSAG60-87-C-0066 from the U. S. Army Strategic Defense Command to the Department of Computer Science, University of Maryland. The views, opinions, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of Defense position, policy, or decision, unless so designated by other official documentation. Computer facilities where in part provided by NSF grant CCR-8811954 'Reactive systems are those that accept new jobs while executing already-accepted guaranteed jobs[5]

IEEE Internatonal Conference on Mobile Adhoc and Sensor Systems, 2004

We define techniques to compute energy-efficient paths, using the IEEE 802.11 fragmentation mecha... more We define techniques to compute energy-efficient paths, using the IEEE 802.11 fragmentation mechanism, within the framework of on-demand routing protocols. We focus on one specific on-demand routing protocol, namely the ad-hoc on-demand vector routing protocol (AODV), and show how it should be adapted to compute energy-efficient paths. The choice of energy-efficient paths depends on link error rates on different wireless

IEEE Globecom 2006, 2006

Clustering is a standard approach for achieving efficient and scalable performance in wireless se... more Clustering is a standard approach for achieving efficient and scalable performance in wireless sensor networks. Most of the published clustering algorithms strive to generate the minimum number of disjoint clusters. However, we argue that guaranteeing some degree of overlap among clusters can facilitate many applications, like inter-cluster routing, topology discovery and node localization, recovery from cluster head failure, etc. We formulate the overlapping multi-hop clustering problem as an extension to the k-dominating set problem. Then we propose MOCA; a randomized distributed multi-hop clustering algorithm for organizing the sensors into overlapping clusters. We validate MOCA in a simulated environment and analyze the effect of different parameters, e.g. node density and network connectivity, on its performance. The simulation results demonstrate that MOCA is scalable, introduces low overhead and produces approximately equal-sized clusters.