Updated Research Statement: PDF
 

Research Summary
      My research interests focus in pervasive computing, peer-to-peer systems, networking, and wireless networks. I am particularly interested in energy efficient protocols, modeling and analyzing routing and medium access control (MAC) protocols, cross layer design mechanisms, directional antenna protocols, and pervasive applications for ad hoc networks.
      
       Various properties of wireless networks, such as mobility, frequent disconnections and
varying channel conditions, make designing efficient protocols for such networks a challenging task. Therefore, enhancing the performance of wireless networks requires alleviating the effect of the physical layer characteristics (e.g., channel noise) and developing cross layer mechanisms to exploit the those characteristics in favor of enhancing network performance. In my dissertation work, I focused on the impact of cannel noise, physical layer capture effect, and the use of the directional antenna on the design of reliable and efficient routing and MAC protocols taking into account cross layer interaction between both layers as well as the physical layer.

Through my dissertation work, I got involved in the following research projects:

IEEE 802.11 Capacity Enhancement
The wireless IEEE 802.11 MAC protocol, just like most other contention based MAC protocols, are based on Carrier Sense Multiple Access (CSMA) mechanism. In CSMA, a station may transmit if and only if the medium is sensed to be idle. The purpose is to prevent any station from causing interference to an ongoing transmission occupying the medium. I proposed 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. The 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. Frame capture uses the well known ”physical layer capture” phenomena in radio channels that allows the receiver to capture a frame if the frame’s detected power sufficiently exceeds the joint interfering power of interfering contenders by a minimum certain threshold factor. In paper "Location Enhancement to IEEE 802.11 DCF", I studied analytically 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.


Wireless Networks in Noisy Environments
Wireless communication suffers from transmission errors due to the channel noise. In order to increase the transmission reliability, IEEE 802.11 standard implements retransmission mechanism in which a packet is retransmitted over a link if no MAC layer acknowledgment is received. One of the goals in wireless networks is to minimize energy consumption during communication. Therefore, to construct reliable and energy efficient routes in ad hoc networks, routing protocols should take into account the channel noise in the vicinity of the nodes and evaluate the candidate routes based on the potential retransmissions over links as shown in our paper "Energy-Efficient Reliable Paths for On-Demand Routing Protocols". In addition, IEEE 802.11 adopts a fragmentation mechanism in which large packets are partitioned into smaller fragment to increase their transmission reliability. This fragmentation mechanism should be considered by the routing protocols in evaluating the reliable and energy efficient routes. In this project, I developed mechanisms to compute energy-efficient paths, using the IEEE 802.11 fragmentation mechanism, within the framework of on-demand routing protocols. I showed in paper "IEEE 802.11 Fragmentation-Aware Energy-Efficient Ad-Hoc Routing Protocols" how our scheme accounts for channel characteristics in computing such paths and how it exploits the IEEE 802.11 fragmentation mechanism to generate optimum energy-efficient paths. Results showed that the proposed variants of on-demand routing protocols can achieve orders of magnitude improvement in energy-efficiency of reliable data paths.

Also, I extended the study of the noisy environments to the performance of the IEEE 802.11 DCF for infrastructure networks. I showed that using the standard binary exponential backoff (BEB) mechanism in noisy environments results in a poor throughput performance due to its inability of differentiating between the causes of unsuccessful packet transmissions. I proposed in paper "IEEE 802.11 DCF Enhancements for Noisy Environments" an enhanced BEB mechanism that enhances the IEEE 802.11 with a capability of differentiating between different types of unsuccessful transmissions and showed that the new mechanism enhances the network performance significantly with respect to the network error rates (noise level). In paper "Performance of IEEE 802.11 based Wireless Sensor Networks in Noisy Environments", I studied the performance of BEB mechanism in the frame work of the sensor networks. 


Directional MAC layer
In contrast to omni-direction transmissions in which the transmitted signal propagates in all direction, a node equipped with directional antenna is capable of transmitting a signal that propagates only with a beam of width as narrow as 5^o to 10^o in a certain direction. The main goal of the directional antenna is the spatial reuse of the medium in order to increase the network capacity. Spatial reuse could be defined as the number of nodes that can transmit simultaneously to the total number of transmitting nodes. In addition to the use of directional NAV, omni/directional reception, omni/directional RTS/CTS, omni/directional carrier sense, I extend the traditional queue model in the IEEE 802.11 standard to multi-directional queue model to exploit the directional antenna. In such model each queue would be corresponding to a certain range of directions and received packets are inserted in the queue corresponding to its transmission direction. Consequently, if a certain direction is blocked, instead of blocking the transmission of the rest of the packets all the packets as the traditional queue mechanism, only packets belong to the queue corresponding to that transmission direction will be blocked and the MAC will try to transmit another packet in a non blocked direction.





     In addition to my dissertation work, I got involved in the following research projects related to outdoor applications for ad hoc networks, peer-to-peer systems, and pervasive computing systems:


TrafficView: A Scalable Traffic Monitoring System
Vehicles are part of people's life in modern society, into which more and more high-tech devices are integrated. Most of the current research focuses on the functionalities of individual vehicles, and less attention has been paid to the cooperation among vehicles and road facilities, which forms the whole transportation system. Moreover, a common platform for inter-vehicle communication is necessary to realize an intelligent transportation system supporting safe driving, dynamic route schedule, emergency message dissemination, traffic condition monitoring, etc. TrafficView, which is a part of the e-Road project, defines a framework to disseminate and gather information about the vehicles on the road. Using such a system will provide a vehicle driver with road traffic information, which helps driving in situations as foggy weather, or finding an optimal route in a trip several miles long. The design, implementation, and simulation of TrafficView are given
in papers "TrafficView: Traffic Data Dissemination using Car-to-Car Communication" and "TrafficView: A Scalable Traffic Monitoring System". Algorithms to handle GPS readings accuracy in addition to presenting experimental results for TrafficView prototype are presented in "TrafficView: A Driver Assistant Device for Traffic Monitoring based on Car-to-Car Communication". We performed a demo about TrafficView at MobiCom 2005. For more details about TrafficView and list of its publications, visit TrafficView Project Web Site.

EZCab: A Cab Booking Application Using Short-Range Wireless Communication
We envision that the use of embedded devices in cars will soon become a reality. We have developed EZCab, a real-life ubiquitous computing application built over MANET, that allows people to book nearby cabs in densely populated urban areas using their cell phones or PDAs equipped with short-range wireless network interfaces. Current existing cab booking systems rely on centralized scheme for cab dispatching such as making phone calls to a taxi company or “gesturing”, or using short message service (SMS). Although centralized/traditional cab dispatching is guaranteed, but it suffers limited, and suffer from the lack of scalability due to: 1) all requests have to go through one or multiple cab dispatchers, which introduces waiting time or delays for the clients, especially during periods of high rate cab requests, and 2) in order to dispatch the nearest cab to the client, all cabs in the city have to be monitored to find the closest one to the client’s location. The EZCab dispatching system, on the other hand, is simpler, faster, and scalable since it works in a completely decentralized fashion, and there is no need to track the cab’s locations. EZCab uses a peer-to-peer approach whose key benefits are scalability and practicality. The design, implementation, and simulation of EZCab are given in "EZCab: A Cab Booking Application Using Short-Range Wireless Communication". For more details and publications, visit EZCab Project Web Site.

IBN: Instance-Based Network
In this work we consider the design principles of the Instance-Based Network (IBN), an extended version of a generic Content-Based Network (CBN). IBN acts as an overlay communication platform over which end-point entities, called contents, communicate independently from their physical locations while providing the flexibility of having different instances of the same content. The semantics of different instances are assigned by the application using the IBN. Routing in the IBN is instance based; the IBN can route a message to a specific content instance or to the closest instance, if no exact match is found for the destination content instance. Moreover, the IBN replicates the stored contents in order to provide fault tolerance. Possible applications for the IBN applications include:

ATP: Autonomous Transport Protocol
The basic service provided by the Autonomous Transport Protocol (ATP) is a reliable transport connection between two endpoints, identified by content identifiers, independent of their physical location. Autonomy allows dynamic endpoints relocation on different end hosts without disrupting the transport connection between them. ATP depends on the existence of an underlying Instance Based Network (IBN) to achieve its goals. ATP layers at
the intermediate nodes can actively participate in the connection. Data is transferred by a combination of active and passive operations, where the ATP layer of a node can decide whether to actively push the data to the destination or to passively wait for the destination endpoint to pull the data. The decision to whether to use the active or passive modes can be taken by a local policy on the node running the ATP protocol. The design, implementation, and
experimental and simulation results of the systems are given in "ATP: Autonomous Transport Protocol" and "ATP Technical Report". For more details, visit ATP Project Web Site.

Rover: Location-Aware Mobile Computing
Rover enables location, time and context-aware applications for wireless devices that scale to very large user populations. In our current systems, we have implemented the Rover clients on Compaq IPAQ handheld PDAs running Windows CE and PocketLinux, with the location service being provided to these devices using GPS in the outdoor environment, and using the Horus system in the indoor environment. In this project, I have been involved in detailed design of the Rover system and have participated in implementing different system modules. For more details refer to Rover website and to our papers: "Rover: Scalable Location-Aware Computing" and "Implementation of a Scalable Context-Aware Computing System".