Robust flow control in data-communication networks with multiple time-delays

Robust controller design for a flow control problem where uncertain multiple time-varying time-delays exist is considered. Although primarily data-communication networks are considered, the presented approach can also be applied to other flow control problems and can even be extended to other control problems where uncertain multiple time-varying time-delays exist. Besides robustness, tracking and fairness requirements are also considered. To solve this problem, an H∞ optimization problem is set up and solved. Unlike previous approaches, where only a suboptimal solution could be found, the present approach allows to design an optimal controller. Simulation studies are carried out in order to illustrate the time-domain performance of the designed controllers. The obtained results are also compared to the results of a suboptimal controller obtained by an earlier approach. Copyright © 2009 John Wiley & Sons, Ltd

Comparison of PI controllers designed for the delay model of TCP/AQM networks

One of the major problems of communication networks is congestion. In order to address this problem in TCP/IP networks, Active Queue Management (AQM) scheme is recommended. AQM aims to minimize the congestion by regulating the average queue size at the routers. To improve upon AQM, recently, several feedback control approaches were proposed. Among these approaches, PI controllers are gaining attention because of their simplicity and ease of implementation. In this paper, by utilizing the fluid-flow model of TCP networks, we study the PI controllers designed for TCP/AQM. We compare these controllers by first analyzing their robustness and fragility. Then, we implement these controllers in ns-2 platform and conduct simulation experiments to compare their performances in terms of queue length. Taken together, our results provide a guideline for choosing a PI controller for AQM given specific performance requirements. © 2013 Elsevier B.V. All rights reserved

Characterization and Computation of Partial Synchronization Manifolds for Diffusive Delay-Coupled Systems

Sometimes a network of dynamical systems shows a form of incomplete synchronization, characterized by synchronization of some but not all of its component systems. This type of incomplete synchronization is called partial synchronization or cluster synchronization. Partial synchronization is associated with the existence of partial synchronization manifolds, which are linear invariant subspaces of $\mathcal{C}$, the state space of the network of systems. We focus on partial synchronization manifolds in networks of identical systems, characterized by linear diffusive coupling described by a weighted graph, and allowing for time-delay in the coupling. We present equivalent existence criteria for partial synchronization manifolds in terms of invariant spaces, the block-structure of a reordered adjacency matrix, and the solvability of a Sylvester equation. We emphasize decomposable networks, according to the rational dependency structure of the coupling weights, and according to the delay values, respectively. It is obvious that if the existence conditions for partial synchronization manifolds are satisfied for all subnetworks simultaneously, they hold for the original network, yet the converse result is not always true, as we shall illustrate with an example. Furthermore, as our main results, we show that if the decomposition is according to the weights and the basis weights are rationally independent numbers, or if the decomposition is according to different delay values, then finding a partial synchronization manifold for the original network is equivalent to finding common partial synchronization manifolds for the subnetworks, i.e., restricting to the analysis of the subnetworks does not impose any conservatism, which simplifies the analysis significantly. For the case of decomposable networks according to the weights, with rationally independent basis weights, we provide a fourth existence criterion for partial synchronization manifolds in terms of a balanced coloring of an associated multigraph. In addition, we briefly describe publicly available software for detecting partial synchronization manifolds. Our equivalent existence criteria, which depend on the network and delay structure but not on the dynamics of the systems at the nodes, are sufficient for the presence of a partial synchronization manifold. We show that under a mild assumption on the systems at the nodes, namely, left-invertibility, these conditions are necessary as well. In all criteria it turns out that the distinction between noninvasive and invasive delayed coupling is important, i.e., whether or not a coupling term between two systems vanishes whenever the latter are synchronizedRead More: http://epubs.siam.org/doi/10.1137/15M101775

Comparing PI controllers for delay models of TCP/AQM networks

In this paper, ns-2 simulations and related comparisons of four different PI controllers designed for TCP/AQM networks will be presented. The simulations are performed for various scenarios. © 2010 IFAC

Association between obsessive compulsive disorder and tumor necrosis factor-á gene -308 (G>A) and -850 (C>T) polymorphisms in Turkish children

Obsessive compulsive disorder (OCD) is a neurobio-logical disease characterized with obsessions and compulsions. Obsessive compulsive disorder occurs with an autoimmune mechanism after Group A â hemolytic streptococcus (GABHS) infection. Tumor necrosis factor (TNF) is an important cytokine, as well as having an important role in the apoptosis mechanism of autoimmune diseases. It is expressed by the TNF-á gene. The aim of this study was to examine the relationship between the TNF-á gene promoter region -308 (G>A) and -850 (C>T) polymor-phisms and OCD. In this study, ages of the OCD patients and the control group ranged between 4 and 12 years. We studied two patient groups, one included childhood onset OCD patients (n = 49) and the control group was composed of healthy children (n = 58). Patients were diagnosed according to the Diagnostic and Statistical Manual of Mental Disorder (DSM-IV) criteria and with Kiddie Schedule for Affective Disorders and Schizophrenia-Present and Lifetime (KSAD-S-PL) version. For identifying the poly-morphisms, polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP) and polyacryl-amide gel electrophoresis (PAGE) methods were used. For the -308 polymorphism, 45 of 49 OCD patients' results were completed, and for the -850 polymorphism, 47 of 49 OCD patients' results were completed. According to our statistical results, there is a positive relationship between OCD and the -308 polymorphism (p 0.001) but no association between OCD and the -850 polymorphism (p = 0.053). There is no positive relationship between antistreptolysin O (ASO) titers and the -308 polymorphism (p = 0.953) but there is an important significance between the -850 polymorphism and ASO (p = 0.010). There is no positive relationship between gender of patients and OCD (p = 0.180) and no positive association between ASO and gender (p = 0.467). According to our results, we hypothesize that we can propose the mutant AA genotype for the -308 polymorphism, and that the mutant CT genotype for the -850 polymorphism may be used as molecular indicators for OCD