21,721 research outputs found
Performance improvements to the 802.11 wireless network medium access control sub-layer : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Computer Systems Engineering at Massey University
This thesis presents the outcome into the research and development of improvements to the 802.11 wireless networking medium access control (MAC) sublayer. The main products of the research are three types of improvement that increase the efficiency and throughput of the 802.11 protocol. Beginning with an overview of the original 802.11 physical layer and MAC sub-layer standard, the introductory chapters then cover the many supplements to the original standard (including a brief on the future 802.11n supplement). The current state of the 802.11 MAC sub-layer is presented along with an assessment of the realistic performance available from 802.11. Lastly, the motivations for improving the MAC sub-layer are explained along with a summary of existing research into this area. The main improvement presented within the thesis is that of packet aggregation. The operation of aggregation is explained in detail, along with the reasons for the significant available throughput increase to 802.11 from aggregation. Aggregation is then developed to produce even higher throughput, and to be a more robust mechanism. Additionally, aggregation is formally described in the form of an update to the existing 802.11 standard. Following this, two more improvements are shown that can be used either with or without the aggregation mechanism. Stored frame headers are designed to reduce repetition of control data, and combined acknowledgements are an expansion of the block acknowledgement system introduced in the 802.11e supplement. This is followed by a description of the simulation environment used to test the three improvements presented, such as the settings used and metrics created. The results of the simulations of the improvements are presented along with the discussion. The developments to the basic improvements are also simulated and discussed in the same way. Finally, conclusions about the improvements detailed and the results shown in the simulations are drawn. Also at the end of the thesis, the possible future direction of research into the improvements is given, as well as the aspects and issues of implementing aggregation on a personal computer based platform
When Backpressure Meets Predictive Scheduling
Motivated by the increasing popularity of learning and predicting human user
behavior in communication and computing systems, in this paper, we investigate
the fundamental benefit of predictive scheduling, i.e., predicting and
pre-serving arrivals, in controlled queueing systems. Based on a lookahead
window prediction model, we first establish a novel equivalence between the
predictive queueing system with a \emph{fully-efficient} scheduling scheme and
an equivalent queueing system without prediction. This connection allows us to
analytically demonstrate that predictive scheduling necessarily improves system
delay performance and can drive it to zero with increasing prediction power. We
then propose the \textsf{Predictive Backpressure (PBP)} algorithm for achieving
optimal utility performance in such predictive systems. \textsf{PBP}
efficiently incorporates prediction into stochastic system control and avoids
the great complication due to the exponential state space growth in the
prediction window size. We show that \textsf{PBP} can achieve a utility
performance that is within of the optimal, for any ,
while guaranteeing that the system delay distribution is a
\emph{shifted-to-the-left} version of that under the original Backpressure
algorithm. Hence, the average packet delay under \textsf{PBP} is strictly
better than that under Backpressure, and vanishes with increasing prediction
window size. This implies that the resulting utility-delay tradeoff with
predictive scheduling beats the known optimal tradeoff for systems without prediction
A queueing theory description of fat-tailed price returns in imperfect financial markets
In a financial market, for agents with long investment horizons or at times
of severe market stress, it is often changes in the asset price that act as the
trigger for transactions or shifts in investment position. This suggests the
use of price thresholds to simulate agent behavior over much longer timescales
than are currently used in models of order-books.
We show that many phenomena, routinely ignored in efficient market theory,
can be systematically introduced into an otherwise efficient market, resulting
in models that robustly replicate the most important stylized facts.
We then demonstrate a close link between such threshold models and queueing
theory, with large price changes corresponding to the busy periods of a
single-server queue. The distribution of the busy periods is known to have
excess kurtosis and non-exponential decay under various assumptions on the
queue parameters. Such an approach may prove useful in the development of
mathematical models for rapid deleveraging and panics in financial markets, and
the stress-testing of financial institutions
Kemahiran menggunakan peralatan dan perisian dalam menghasilkan produk ukur : satu tinjauan ke atas pelajar diploma ukur tanah di Politeknik Sultan Haji Ahmad Shah, Kuantan, Pahang
Projek ini adalah untuk melihat kemahiran yang diperlukan oleh pelajar Diploma Ukur Tanah dalam menggunakan peralatan ukur dan perisian berkaitan. Sampel kajian terdiri daripada 32 orang pelajar semester keenam yang sedang mengikuti kursus Diploma Ukur Tanah di Politeknik Sultan Haji Ahamd Shah, Kuantan Pahang. Perolehan data adalah melalui borang soal selidik. Pengkaji memberi tumpuan kepada persoalan kajian yang melihat kepada tiga aspek iaitu, jenis-jenis peralatan dan perisian ukur tanah di firma ukur tanah, aspek kemahiran-kemahiran yang dimiliki pelajar meliputi kemahiran menggunakan peralatan ukur, kemahiran menggunakan perisian ukur dan kemahiran-kemahiran asas meliputi teori yang diperlukan dalam keija-keija ukur dan dalam menghasilan produk uk ur. Dapatan kajian menunjukkan pelajar mahir menggunakan alat ukur manual dan kemahiran pelajar terhadap penggunaan perisian adalah tidak pelbagai. Hasil kajian juga menunjukkan bahawa pelajar mahir dalam mengaplikasikan teori-teori yang digunakan dalam keija ukur dan penghasilan produk ukur
Some aspects of queueing and storage processes : a thesis in partial fulfilment of the requirements for the degree of Master of Science in Statistics at Massey University
In this study the nature of systems consisting of a single queue are first considered. Attention is then drawn to an analogy between such systems and storage systems.
A development of the single queue viz queues with feedback is considered after first considering feedback processes in general. The behaviour of queues, some with feedback loops, combined into networks is then considered. Finally, the application of such networks to the analysis of interconnected reservoir systems is considered and the conclusion drawn that such analytic methods complement the more recently developed mathematical programming methods by providing analytic solutions for
sub systems behaviour and thus guiding the development of a system model
Concave Switching in Single and Multihop Networks
Switched queueing networks model wireless networks, input queued switches and
numerous other networked communications systems. For single-hop networks, we
consider a {()-switch policy} which combines the MaxWeight policies
with bandwidth sharing networks -- a further well studied model of Internet
congestion. We prove the maximum stability property for this class of
randomized policies. Thus these policies have the same first order behavior as
the MaxWeight policies. However, for multihop networks some of these
generalized polices address a number of critical weakness of the
MaxWeight/BackPressure policies.
For multihop networks with fixed routing, we consider the Proportional
Scheduler (or (1,log)-policy). In this setting, the BackPressure policy is
maximum stable, but must maintain a queue for every route-destination, which
typically grows rapidly with a network's size. However, this proportionally
fair policy only needs to maintain a queue for each outgoing link, which is
typically bounded in number. As is common with Internet routing, by maintaining
per-link queueing each node only needs to know the next hop for each packet and
not its entire route. Further, in contrast to BackPressure, the Proportional
Scheduler does not compare downstream queue lengths to determine weights, only
local link information is required. This leads to greater potential for
decomposed implementations of the policy. Through a reduction argument and an
entropy argument, we demonstrate that, whilst maintaining substantially less
queueing overhead, the Proportional Scheduler achieves maximum throughput
stability.Comment: 28 page
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