The Hong Kong University of Science and Technology
Department of Computer Science


PhD Thesis Defence


"On Improving Internet Quality of Service Using a Control Theoretic
Approach"

By

Miss Xiaolin Chang


Abstract

Rapid development in telecommunication technologies and ever-growing
network-user demands have made network congestion a critical problem in
today's Internet. Congestion not only leads to significant network-centric
performance degradation such as low link utilization, but also damages the
user-centric Quality-of-Service (QoS). In addition, the increasingly
heterogeneous network environments and varying and unpredictable traffic
conditions mean that any mechanism without adaptability hardly be able to
deliver QoS requirements.

This thesis considers the issue of congestion control and two QoS issues
brought up by congestion in various network infrastructures including
wired networks and wireless LANs. The two QoS issues are: (i) bandwidth
assurance in the Assured Forwarding (AF) service in Differentiated
Services (DiffServ) wired networks; and (ii) application level fairness
between downlink and uplink flows in an infrastructure WLAN using the DCF
mode. This thesis explores using a control theoretic approach to analyze
and design mechanisms for these problems based only on the local
knowledge.

First we describe a generic controller structure and discuss its features.
Then, we use it to design a stable queue-based adaptive AQM mechanism for
the current Internet. We aim to alleviate two tradeoffs: (i) between fast
transient response and stability, and (ii) between fast transient response
and small steady state error. Stability analysis of the closed-loop
nonlinear system is conducted, providing guidelines for designing the
algorithm and selecting control parameters.

The current Internet is a best-effort network and most AQM schemes focus
on providing QoS assurance from a network-centric standpoint. The DiffServ
approach has been proposed as a scalable mechanism in order to provide
QoS. However, the AF service in this approach fails providing bandwidth
assurance under certain conditions. We instantiate the generic controller
structure in the AF service framework and analyze the features of the
existing ingress-based mechanisms for improving bandwidth assurance among
AF flows. Based on the analysis, we propose a simple but robust controller
for this problem. We carry out extensive simulations to validate the
analysis and demonstrate that the proposed mechanism outperforms the
mechanisms proposed in the literature over a wide range of network
dynamics.

Congestion control and fairness issues are not only prominent for the
Internet infrastructure when wired backbones and centralized control
entities are available, but are also essential for various applications
that use wireless technologies for flexible data communication. Therefore,
this dissertation extends the research methodologies and technologies for
Internet congestion control and QoS problems to wireless LANs. We explore
the feasibility of combining AQM schemes with the underlying MAC protocol
design in WLANs to provide fairness for traffic flows traversing between
wired and wireless domains.


Date:			Monday, 30 May 2005

Time:			10:30a.m.-12:30p.m.

Venue:			Room 2302
			Lifts 17-18

Chairman:		Prof. Kar-Yan Tam (ISMT)

Committee Members:	Prof. Jogesh Muppala (Supervisor)
                      	Prof. Brahim Bensaou
                        Prof. Mounir Hamdi
                        Prof. Li Qiu (ELEC)
                        Prof. John C.S. Lui (Comp. Sci. & Engg., CUHK)


**** ALL are Welcome ****