42 research outputs found
Markov modulated periodic arrival process offered to an ATM multiplexer
When a superposition of on/off sources is offered to a deterministic server, a particular queueing system arises whose analysis has a significant role in ATM based networks. Periodic cell generation during active times is a major feature of these sources. In this paper a new analytical method is provided to solve for this queueing system via an approximation to the transient behavior of the nD/D/1 queue. The solution to the queue length distribution is given in terms of a solution to a linear differential equation with variable coefficients. The technique proposed here has close similarities with the fluid flow approximations and is amenable to extension for more complicated queueing systems with such correlated arrival processes. A numerical example for a packetized voice multiplexer is finally given to demonstrate our results
Estimation of buffer overflow probabilities and economies of scale in ATM multiplexers by analysis of a model of packetized voice traffic
We obtain upper bounds on the probability of buffer overflow for an ATM multiplexer of L identical packetized voice sources. The multiplexer is modelled by a FCFS single server queue. The arrivals at the multiplexer are a homogenous superposition of the arrivals from L independent identical sources, with each source modelled by a copy of a discrete time Markov Chain which we call the Cell Level Model. Throughout, appropriate parameters are scaled with L, to maintain a constant load over all superposition sizes.
The probability that, the queue-length (q^) of the queue in a finite buffer exceeds the buffer size b, is bounded above by the probability that the queue-length (qL) of the queue m an infinite buffer exceeds length b In order to bound the former above, we find upper bounds or approximations for the latter by using the theory of,
• Large Deviations, to determine its asymptotics for large b,
• Martingales, to obtain upper bounds, valid for all positive b,
• Large Deviations, to determine its asymptotics for large L for time rescaled
(proportional to L) arrival processes.
These demonstrate the multiplexing gam and economies of scale obtainable from large and small buffers and large multiplexers, respectively
On the large deviations of a class of modulated additive processes
We prove that the large deviation
principle holds for a class of
processes inspired by semi-Markov
additive processes. For the processes
we consider, the sojourn times in the
phase process need not be independent
and identically distributed. Moreover
the state selection process need not
be independent of the sojourn times.
We assume that the phase process takes values in a finite set and that the
order in which elements in the set,
called states, are visited is selected stochastically. The sojourn times
determine how long the phase process
spends in a state once it has been
selected. The main tool is a
representation formula for the sample
paths of the empirical laws of the
phase process.
Then, based on assumed joint large
deviation behavior of the state
selection and sojourn processes, we
prove that the empirical laws of the
phase process satisfy a sample path
large deviation principle. From this
large deviation principle, the large deviations behavior of a class of
modulated additive processes is deduced.
As an illustration of the utility of the general results, we provide an alternate proof of results for modulated L´evy processes. As a practical application of
the results, we calculate the large
deviation rate function for a processes
that arises as the International Telecommunications Union’s
standardized stochastic model of two-way conversational speech
Quality aspects of Internet telephony
Internet telephony has had a tremendous impact on how people communicate.
Many now maintain contact using some form of Internet telephony.
Therefore the motivation for this work has been to address the quality aspects
of real-world Internet telephony for both fixed and wireless telecommunication.
The focus has been on the quality aspects of voice communication,
since poor quality leads often to user dissatisfaction. The scope of the work
has been broad in order to address the main factors within IP-based voice
communication.
The first four chapters of this dissertation constitute the background
material. The first chapter outlines where Internet telephony is deployed
today. It also motivates the topics and techniques used in this research.
The second chapter provides the background on Internet telephony including
signalling, speech coding and voice Internetworking. The third chapter
focuses solely on quality measures for packetised voice systems and finally
the fourth chapter is devoted to the history of voice research.
The appendix of this dissertation constitutes the research contributions.
It includes an examination of the access network, focusing on how calls are
multiplexed in wired and wireless systems. Subsequently in the wireless
case, we consider how to handover calls from 802.11 networks to the cellular
infrastructure. We then consider the Internet backbone where most of our
work is devoted to measurements specifically for Internet telephony. The
applications of these measurements have been estimating telephony arrival
processes, measuring call quality, and quantifying the trend in Internet telephony
quality over several years. We also consider the end systems, since
they are responsible for reconstructing a voice stream given loss and delay
constraints. Finally we estimate voice quality using the ITU proposal PESQ
and the packet loss process.
The main contribution of this work is a systematic examination of Internet
telephony. We describe several methods to enable adaptable solutions
for maintaining consistent voice quality. We have also found that relatively
small technical changes can lead to substantial user quality improvements.
A second contribution of this work is a suite of software tools designed to
ascertain voice quality in IP networks. Some of these tools are in use within
commercial systems today