Son K. Dao

Modeling and querying moving objects

We propose a data model for representing moving objects in database systems. It is called the Moving Objects Spatio-Temporal (MOST) data model. We also propose Future Temporal Logic (FTL) as the query language for the MOST model, and devise an algorithm for processing FTL queries in MOST.

Power management for throughput enhancement in wireless ad-hoc networks

We introduce the notion of power management within the context of wireless ad-hoc networks. More specifically, we investigate the effects of using different transmit powers on the average power consumption and end-to-end network throughput in a wireless ad-hoc environment. This power management approach would help in reducing the system power consumption and hence prolonging the battery life of mobile nodes. Furthermore, it improves the end-to-end network throughput as compared to other ad-hoc networks in which all mobile nodes use the same transmit power. The improvement is due to the achievement of a tradeoff between minimizing interference ranges, reduction in the average number of hops to reach a destination, reducing the probability of having isolated clusters, and reducing the average number of transmissions (including retransmissions due to collisions). The protocols would first dynamically determine an optimal connectivity range wherein they adapt their transmit powers so as to only reach a subset of the nodes in the network. The connectivity range would then be dynamically changed in a distributed manner so as to achieve the near optimal throughput. Minimal power routing is used to further enhance performance. Simulation studies are carried out in order to investigate these design approaches. It is seen a network with such a power managed scheme would achieve a better end-to-end throughput performance (about 10% improvement with a slotted aloha MAC protocol) and lower transmit power (about an 80% Improvement) than a network without such a scheme.

Distributed power control in ad-hoc wireless networks

Mobile ad-hoc networking involves peer-to-peer communication in a network with a dynamically changing topology. Achieving energy efficient communication in such a network is more challenging than in cellular networks since there is no centralized arbiter such as a base station that can administer power management. We propose and evaluate a power control loop, similar to those commonly found in cellular CDMA networks, for ad-hoc wireless networks. We use a comprehensive simulation infrastructure consisting of group mobility, group communication and terrain blockage models. A major focus of research in ad-hoc wireless networking is to reduce energy consumption because the wireless devices are envisioned to have small batteries and be incapable of energy scavenging. We show that this power control loop reduces energy consumption per transmitted byte by 10-20%. Furthermore, we show that it increases overall throughput by 15%.

Mermaid—A front-end to distributed heterogeneous databases

Mermaid is a system that allows the user of multiple databases stored under various relational DBMSs running on different machines to manipulate the data using a common language, either ARIEL or SQL. It makes the complexity of this distributed, heterogeneous data processing transparent to the user. In this paper, we describe the architecture, system control, user interface, language and schema translation, query optimization, and network operation of the Mermaid system. Future research issues are also addressed.

Querying the uncertain position of moving objects

In this paper we propose a data model for representing moving objects with uncertain positions in database systems. It is called the Moving Objects Spatio-Temporal (MOST) data model. We also propose Future Temporal Logic (FTL) as the query language for the MOST model, and devise an algorithm for processing FTL queries in MOST.

Scalable unidirectional routing with zone routing protocol (ZRP) extensions for mobile ad-hoc networks

Ad-hoc networks consist of peer-to-peer communicating nodes that are highly mobile. As such, an ad-hoc network lacks infrastructure and the topology of the network changes dynamically. The task of routing data from a source to a destination in such a network is challenging. Several routing protocols have been proposed for wireless ad-hoc networks. Most of these protocols, however, pre-suppose the presence of bi-directional links between the nodes in the network. In reality the ad-hoc network may consist of heterogeneous nodes with different power capabilities and hence, different transmission ranges. When this is the case, a given node might be able to receive the transmission of another given node but might not be able to successfully transmit to the latter. Thus, unidirectional links are formed. Most of the current routing protocols are unsuitable for deployment when such unidirectional links are present. We consider a routing protocol called the zone routing protocol (ZRP) that has been proposed for wireless ad-hoc networks with bi-directional links. The zone routing protocol employs a hybrid proactive (table driven) and reactive (on-demand) methodology to provide scalable routing in the ad-hoc network. However, in the presence of unidirectional links some routes remain undiscovered if ZRP is used. We propose extensions to ZRP to support its deployment when unidirectional links are present. In particular, we propose a query enhancement mechanism that recursively builds partial routes to a destination. Simulation results show that even at a high mobility of 20 m/s, the queries resulting due to the enhancement mechanism result in the computation of valid routes more than 80% of the time. These results are valid even when a large number (40% of nodes have half the transmission range as that of the remaining nodes) of unidirectional links are present in the network.

Medium access control in a network of ad hoc mobile nodes with heterogeneous power capabilities

MAC layer protocols for wireless ad hoc networks typically assume that the network is homogeneous with respect to the transmit power capability of individual nodes in the network. The IEEE 802.11 MAC protocol has been popular for use in ad hoc networks. We investigate the performance of this protocol when it is used in a network with nodes that transmit at various power levels. We show that overall throughput is lower than the throughput of a network in which all nodes transmit at identical power levels. In addition, low power nodes have a disadvantage in accessing the medium due to higher levels of interference from the high power nodes. We consider propagating the control messages generated by a node wishing to initiate communication to distant nodes so that they may forbear transmissions for some time, thereby allowing clear access to the initiating node. We find that the overhead incurred due to the additional message transmissions outweighs the potential gain achieved by propagating these messages. This indicates that the signalling mechanism used in the IEEE 802.11 standard or the variants thereof are not sufficient to alleviate the loss in throughput and the lack of fairness engendered by networks that are heterogeneous with regard to the transmit power capabilities of individual nodes.

Heterogeneous networking: a new survivability paradigm

We believe that a network, to be survivable, must be heterogeneous. Just like a species that draws on a small gene pool can succumb to a single environmental threat, so a homogeneous network is vulnerable to a malicious attack that exploits a single weakness common to all of its components. In contrast, in a network in which each critical functionality is provided by a diverse set of protocols and implementations, attacks that focus on a weakness of one such protocol or implementation will not be able to bring down the entire network, even though all elements are not be bulletproof and even if some of components are compromised.Following this survivability through heterogeneity philosophy, we propose a new survivability paradigm, called heterogeneous networking, for improving a networks defense capabilities. Rather than following the current trend of converging towards single solutions to provide the desired functionality at every element of the network architecture, this methodology calls for systematically increasing the networks heterogeneity without sacrificing its interoperability.

Mermaid — Experiences with network operation

AIDA is a distributed data management (DDM) front-end system which runs on top of existing databases stored in different data management systems (DBMSs). It appears to the user to be a distributed DBMS, although many of the DBMS functions are actually provided by the underlying DBMS. It currently runs above three relational DBMSs, the IDM database machine connected to a VAX, Ingres on two Suns, and Mistress on a third Sun. One of the key components of the AIDA system is Mermaid which provides query optimization, networking, and system control. Mermaid required research into query optimization where the costs of operations on the different processors and DBMSs is variable, the network cost is variable, and capabilities of the different DBMSs is variable. We extended existing distributed query optimization algorithms to support replicated and fragmented relations. In this paper, we discuss our experiences with the development and testing of the Mermaid system and with the integration of Mermaid with the AIDA translator. We discuss the types of commands, the system controller, and our experiences with distributed debugging. Then we give the results of our initial performance tests.

View maintenance in mobile computing

Mobile users of portable computers will soon have onfine access to a large number of databases via wireless networks. For example, while on the road, passengers will access airfine and other carriers schedules, and weather information; investors will access prices of financial instruments, sales people will access inventory and other corporate data, commuters will access traffic information. However, access to an onfine database may be unavailable when the user is out-of-range of the wireless network. Furthermore, sending access-requests from the mobile computer to the onfine database may be expensive due to limited uplink bandwidth, and also due to the fact that sending messages is a significant drain on the portable battery. These two problems can be alleviated by maintaining a materialized view at the mobile computer. This view will be updated using wireless data messages, as the onfine database changes. This will also localize access, thus improving access time to the view. Therefore, to better deal with the problem of disconnection and to improve response time, the view should be materialized at the mobile computer. As explained above, sometimes communication cost can also be reduced by materialized views; but not always. For example, if the materialized