11,933 research outputs found
TCP over CDMA2000 Networks: A Cross-Layer Measurement Study
Modern cellular channels in 3G networks incorporate sophisticated power control and dynamic rate adaptation which can have significant impact on adaptive transport layer protocols, such as TCP. Though there exists studies that have evaluated the performance of TCP over such networks, they are based solely on observations at the transport layer and hence have no visibility into the impact of lower layer dynamics, which are a key characteristic of these networks. In this work, we present a detailed characterization of TCP behavior based on cross-layer measurement of transport layer, as well as RF and MAC layer parameters. In particular, through a series of active TCP/UDP experiments and measurement of the relevant variables at all three layers, we characterize both, the wireless scheduler and the radio link protocol in a commercial CDMA2000 network and assess their impact on TCP dynamics. Somewhat surprisingly, our findings indicate that the wireless scheduler is mostly insensitive to channel quality and sector load over short timescales and is mainly affected by the transport layer data rate. Furthermore, with the help of a robust correlation measure, Normalized Mutual Information, we were able to quantify the impact of the wireless scheduler and the radio link protocol on various TCP parameters such as the round trip time, throughput and packet loss rate
Queue Dynamics With Window Flow Control
This paper develops a new model that describes the queueing process of a communication network when data sources use window flow control. The model takes into account the burstiness in sub-round-trip time (RTT) timescales and the instantaneous rate differences of a flow at different links. It is generic and independent of actual source flow control algorithms. Basic properties of the model and its relation to existing work are discussed. In particular, for a general network with multiple links, it is demonstrated that spatial interaction of oscillations allows queue instability to occur even when all flows have the same RTTs and maintain constant windows. The model is used to study the dynamics of delay-based congestion control algorithms. It is found that the ratios of RTTs are critical to the stability of such systems, and previously unknown modes of instability are identified. Packet-level simulations and testbed measurements are provided to verify the model and its predictions
Effect of Wideband Wireless Access Systems Interference Robustness on the Quality of Video Streaming
The transmission of audio and video streaming
services over different conduits (wireless access systems,
Internet, etc.) is becoming ever more popular. This widespread
increase is accompanied by the attendant new and difficult task
of maintaining the quality of service of streaming video. The use of very accurate coding techniques for transmissions over wireless networks alone cannot guarantee a complete eradication of distortions characteristic of the video signal. A software-hardware composite system has been developed for investigating the effect of single bit error and bit packet errors in wideband wireless access systems on the quality of H.264/AVC standard bursty video streams. Numerical results of the modeling and analysis of the effect of interference robustness on quality of video streaming are presented and discussed
Design and analysis for TCP-friendly window-based congestion control
The current congestion control mechanisms for the Internet date back to the early 1980’s and were
primarily designed to stop congestion collapse with the typical traffic of that era. In recent years the
amount of traffic generated by real-time multimedia applications has substantially increased, and the
existing congestion control often does not opt to those types of applications. By this reason, the Internet
can be fall into a uncontrolled system such that the overall throughput oscillates too much by a single
flow which in turn can lead a poor application performance. Apart from the network level concerns,
those types of applications greatly care of end-to-end delay and smoother throughput in which the
conventional congestion control schemes do not suit. In this research, we will investigate improving the
state of congestion control for real-time and interactive multimedia applications. The focus of this work
is to provide fairness among applications using different types of congestion control mechanisms to get
a better link utilization, and to achieve smoother and predictable throughput with suitable end-to-end
packet delay
Performance Modelling and Optimisation of Multi-hop Networks
A major challenge in the design of large-scale networks is to predict and optimise the
total time and energy consumption required to deliver a packet from a source node to a
destination node. Examples of such complex networks include wireless ad hoc and sensor
networks which need to deal with the effects of node mobility, routing inaccuracies, higher
packet loss rates, limited or time-varying effective bandwidth, energy constraints, and the
computational limitations of the nodes. They also include more reliable communication
environments, such as wired networks, that are susceptible to random failures, security
threats and malicious behaviours which compromise their quality of service (QoS) guarantees.
In such networks, packets traverse a number of hops that cannot be determined
in advance and encounter non-homogeneous network conditions that have been largely
ignored in the literature. This thesis examines analytical properties of packet travel in
large networks and investigates the implications of some packet coding techniques on both
QoS and resource utilisation.
Specifically, we use a mixed jump and diffusion model to represent packet traversal
through large networks. The model accounts for network non-homogeneity regarding
routing and the loss rate that a packet experiences as it passes successive segments of a
source to destination route. A mixed analytical-numerical method is developed to compute
the average packet travel time and the energy it consumes. The model is able to capture
the effects of increased loss rate in areas remote from the source and destination, variable
rate of advancement towards destination over the route, as well as of defending against
malicious packets within a certain distance from the destination. We then consider sending
multiple coded packets that follow independent paths to the destination node so as to
mitigate the effects of losses and routing inaccuracies. We study a homogeneous medium
and obtain the time-dependent properties of the packet’s travel process, allowing us to
compare the merits and limitations of coding, both in terms of delivery times and energy
efficiency. Finally, we propose models that can assist in the analysis and optimisation
of the performance of inter-flow network coding (NC). We analyse two queueing models
for a router that carries out NC, in addition to its standard packet routing function. The
approach is extended to the study of multiple hops, which leads to an optimisation problem
that characterises the optimal time that packets should be held back in a router, waiting
for coding opportunities to arise, so that the total packet end-to-end delay is minimised
The effect of light assisted collisions on matter wave coherence in superradiant Bose-Einstein condensates
We investigate experimentally the effects of light assisted collisions on the
coherence between momentum states in Bose-Einstein condensates. The onset of
superradiant Rayleigh scattering serves as a sensitive monitor for matter wave
coherence. A subtle interplay of binary and collective effects leads to a
profound asymmetry between the two sides of the atomic resonance and provides
far bigger coherence loss rates for a condensate bathed in blue detuned light
than previously estimated. We present a simplified quantitative model
containing the essential physics to explain our experimental data and point at
a new experimental route to study strongly coupled light matter systems.Comment: 10 pages, 4 figure
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