Forward Loop Research Articles

Hop-by-hop routing algorithms for premium traffic

In Differentiated Service (DiffServ) networks, the routing algorithms used by the premium class traffic, due to the high priority afforded to that traffic, may have a significant impact not only on the premium class traffic itself, but on all other classes of traffic as well. The shortest hop-count routing scheme, used in current Internet, turns out to be no longer sufficient in DiffServ networks. This paper studies the problem of finding optimal routes for the premium-class traffic in a DiffServ domain, such that (1) no forwarding loop exists in the entire network in the context of hop-by-hop routing; and (2) the residual bandwidth on bottleneck links is maximized. This problem is called the Optimal Premium-class Routing (OPR) problem. We prove in this paper that the OPR problem is NP-hard.To handle the OPR problem, first, we analyze the strength and weaknesses of two existing algorithms (Widest-Shortest-Path algorithm and Bandwidth-inversion Shortest-Path algorithm). Second, we propose a novel heuristic algorithm, called the Enhanced Bandwidth-inversion Shortest-Path (EBSP) algorithm . We prove theoretically the correctness of the EBSP algorithm, i.e., we show that it is consistent and loop-free. Our extensive simulations in different network environments show clearly that the EBSP algorithm performs better when routing the premium traffic in complex, heterogeneous networks.

On the correctness of IBGP configuration

The Border Gateway Protocol (BGP) has two distinct modes of operation. External BGP (EBGP) exchanges reachability information between autonomous systems, while Internal BGP (IBGP) exchanges external reachability information within an autonomous system. We study several routing anomalies that are unique to IBGP because, unlike EBGP, forwarding paths and signaling paths are not always symmetric. In particular, we focus on anomalies that can cause the protocol to diverge, and those that can cause a router's chosen forwarding path to an egress point to be deflected by another router on that path. Deflections can greatly complicate the debugging of routing problems, and in the worst case multiple deflections can combine to form persistent forwarding loops. We define a correct IBGP configuration to be one that is anomaly free for every possible set of routes sent by neighboring autonomous systems. We show that determination of IBGP configuration correctness is NP-hard. However, we give simple sufficient conditions on network configurations that guarantee correctness.

Interactivating Feedback Loops within the Mammalian Clock: BMAL1 Is Negatively Autoregulated and Upregulated by CRY1, CRY2, and PER2

Transcriptional regulation appears to be fundamental to circadian oscillations of clock gene expression. These oscillations are believed to control output rhythms. The transcriptional feedback loop and a model of interlocked loops have been proposed as the basis for these oscillations. We characterized the genomic structure of the mouse Bmal1 gene (mBmal1) and defined the mBmal1 promoter region. Transcription of mBmal1 was activated by CRY1, CRY2, and PER2, and was repressed by BMAL1-CLOCK dimers. Therefore, CRY, PER2, and BMAL1-CLOCK play bidirectional roles in transcription when they are at high levels by late day and midnight, respectively. This underlies the opposite phase of BMAL1 compared to CRY and PER. We propose that a BMAL1 negative feedback loop interlocks with the CRY and PER2 negative feedback loop by inter-activation, forming a third positive forward loop. This transcriptional model suggests a molecular basis for the maintenance of stability, persistence, and period of circadian rhythms. The transcriptional potency of CRY is predominant within the mammalian clock, suggesting a clearance mechanism for CRY in period maintenance.

New IQC for quasi‐concave nonlinearities

AbstractA new set of integral quadratic constraints (IQC) is derived for a class of ‘rate limiters’, modelled as a series connections of saturation‐like memoryless nonlinearities followed by integrators. The result, when used within the standard IQC framework (in particular, with finite gain/passivity‐based argiments, Lyapunov theory, structured singular values, etc.), is expected to be widely useful in nonlinear system analysis. For example, it enables ‘discrimination’ between ‘saturation‐like’ and ‘deadzone‐like’ nonlinearities and can be used to prove stability of systems with saturation in cases when replacing the saturation block by another memoryless nonlinearity with equivalent slope restrictions makes the whole system unstable. In particular, it is shown that the L2 gain of a unity feedback system with a rate limiter in the forward loop cannot exceed \sqrt{2}.In addition, a new, more flexible version of the general IQC analysis framework is presented, which relaxes the homotopy and boundedness conditions, and is more aligned with the language of the emerging IQC software. Copyright © 2001 John Wiley & Sons, Ltd.

Algebraic Estimators of the Stability Margins of Linear Control Systems

The article is concerned with the problem of estimating the stability margins for a system with a single negative feedback loop from the distribution of real roots of polynomials characterizing the frequency hodographs of the forward loop.

Unidirectional circular layout for overhead material handling systems

The reentrant flow type of the semiconductor wafer manufacturing process creates a large amount of overhead material flows in the interbay system. This paper proposes unidirectional circular layout for overhead multi-carrier systems. The layout configuration can potentially reduce the control complexity of heavy traffic flow, streamline the empty carrier supply, and meet the delivery requirements. The layout contains a central loop to reach every stocker at high speed and supply empty carriers, provide prompt delivery service to each loop addition in a dynamic and responsive manner. It is preferred to other track architectures, as minimal traffic control is required for the large amount of wafer flows in semiconductor fabs. The proposed construction procedure proceeds in two stages. In the first stage, four quantitative measures are provided to select good candidates of main loop layouts in order to minimize the total flow times distances, construction costs and service response time. Once the main loop is chosen, minimum carrier flow requirement and the critical segments can be identified. The required carrier flows depend on the minimum flow requirement on the critical segments. In the second stage, forward and backtrack reducible loops are found as loop additions to connect to exterior stockers (stockers outside the main loop) as well as to eliminate the flows on critical segments. A dynamic programming procedure is presented to minimize the total construction and operating costs in the hybrid loop addition process. This two-stage procedure identifies a number of good and feasible layouts in which multiple layouts are maintained throughout the execution process.

Magnetization reversal processes in exchange-biased systems

The magnetization reversal mechanism in sputtered bilayer films of NiFe coupled to a range of antiferromagnets has been studied using Lorentz microscopy and magnetic measurements. The reversal mechanism on the forward and recoil loops appears different and the results have been interpreted in terms of a recently published seven-point model. Reversal is controlled by the magnetic domain structure in the antiferromagnet. Time-dependent studies show that the reversal field for the NiFe layer decreases for both the forward and recoil loops, as the time for which the film is held above the saturation field of the NiFe layer increases. This can be explained by viscous rotation of the magnetization in thermally activated domains in the antiferromagnetic layer.

Robust stability of the systems with mixed uncertainties under the IQC descriptions

This paper studies the problem of robust stability checking for a single-input single-output uncertain feedback system which consists of a linear uncertain plant in the forward loop and a non-linear dynamic uncertain unit in the feedback loop. It is supposed that the linear part of the system is of parametric uncertainties described by a polytopic perturbation mode, and that the non-linear part of the system is of dynamic uncertainties characterized by an integral quadratic constraint (IQC). The problem of stability checking is discussed for different structures of the IQC multiplier based on the concepts of biconvex and convex-concave functions and their properties. For different uncertainty structures of the system, a finite vertex checking result and an edge checking result are presented. As an application of the above results, the problem of robust H performance checking is discussed for the case that the uncertain plant of a feedback system is parameterized by a polytopic perturbation mode, and an edge checking result is given under a fairly mild assumption. Also, the absolute stability of an interval system is investigated, and the vertex results of circle criterion and Popov criterion are obtained. Finally, a demonstrating example is included.

Router plugins: a software architecture for next-generation routers

Present-day Internet protocol routers typically employ monolithic operating systems that are not easily upgradable and extensible. With the rapid rate of protocol development it is becoming increasingly important to dynamically upgrade router software in an incremental fashion. We have designed and implemented a high-performance, modular, extended services router software architecture in the Net BSD operating system kernel. This architecture allows code modules, called plugins, to be dynamically added and configured at run time. One of the novel features of our design is the ability to bind different plugins to individual flows; this allows for distinct plugin implementations to seamlessly coexist in the same runtime environment. We achieve high performance through a carefully designed modular architecture, an innovative packet classification algorithm that is highly efficient, and by caching that exploits the flow-like characteristics of Internet traffic. Compared to a monolithic best effort kernel, our implementation requires an average increase in packet processing overhead of only 8%, or 600 cycles per packet when running on an Intel Pentium Pro at 233 MHz. By shortcutting the forward loop based on the per-flow state we establish, we can forward packets up to three times faster than the best effort kernel.

Web-based training, performance and controlling

On-line media and self-directed learning environments are among the most effective training solutions in terms of cost, time and logistics. In the last few years, the percentage of employees participating in training courses has increased. At the same time, there has been a decline of training budgets. Therefore, careful educational controlling must guard direct and indirect costs listed by amount, mode and source. Additionally, the success of on-line courses must be measured in terms of training and functional sectors. Most educational departments generally use controlling processes which consist of consecutive phases. Modern economic controlling, however, focuses on the permanent improvement of process outcomes. Recent examples of Web-based training courses and on-line tools illustrate how continuous feed forward and feed backward loops may be implemented in on-line learning. Controlling the transition from learning to working and checking the sustainability of learning outcomes improves the quality and success of training and makes it possible to calculate the success of training applications. Thus, modern educational controlling approaches should be introduced which comprise exact calculation of financial investments and gains, optimal planning of organisational processes and goal-oriented definitions of strategical and operational learning objectives. Web-based training will then allow the educational and IT staff to take over a strategical role to establish an innovative learning and working culture.

Predictive analogue generator excitation controller

A generator excitation control scheme is introduced which is capable of providing good and consistent voltage control performance, despite wide variations in the system operating conditions. The control scheme is organised so that it comprises a main forward loop, based on a standard voltage error signal, as in a conventional AVR, supplemented by an auxiliary loop incorporating direct axis voltage and current signals. The former loop serves to provide the voltage regulation requirements while the latter serves to compensate for the influence of generator load current on dynamic behaviour.

An original O(N) formulation for simulation of robotic systems

In the last decade a significant number of papers have been published on the efficient formulation of the inverse and forward dynamic equations of motion for robotic manipulators and in general for multibody systems, in view of real-time control or real-time simulation of the system. Until recently these dynamic formulations have been confined to serial computers. The breakthrough of parallel computers however causes to reconsider dynamical formulations from this new perspective. This paper presents an original order N formulation for simulation of the dynamics of a robotic manipulator without inversion of the massmatrix. The proposed algorithm differs from the well-known recursive O( N ) methods. Instead of a backward and two forward recursions, this method requires only one forward recursion and the solution of a small linear system of algebraic equations to generate the equations of motion for open-loop systems. The computations within this forward recursion loop can easily be arranged for parallel processing resulting in a very low computational load. The proposed method can be used for rigid multibody systems in tree structure with arbitrary types of joints. This algorithm also proves to be very promising in case of mechanical systems with closed loops.

Intracellular characterization of neurons in the locust brain signaling impending collision.

1. In response to a rapidly approaching object, intracellular recordings show that excitation in the locust lobula giant movement detecting (LGMD) neuron builds up exponentially, particularly during the final stages of object approach. After the cessation of object motion, inhibitory potentials in the LGMD then help to terminate this excitation. Excitation in the LGMD follows object recession with a short, constant latency but is cut back rapidly by hyperpolarizing potentials. The timing of these hyperpolarizing potentials in the LGMD is variable, and their latency following object recession is shortest with the highest velocities of motion simulated. The hyperpolarizing potentials last from 50-300 ms and are often followed by re-excitation. The observed hyperpolarizations of the LGMD can occur without any preceding excitation and are accompanied by a measurable conductance increase. The hyperpolarizations are likely to be inhibitory postsynaptic potentials (PSPs). The behavior of the intracellularly recorded inhibitory PSPs (IPSPs) closely parallels that of the feed forward inhibitory loop in the neural network described by Rind and Bramwell. 2. The preference of the LGMD for approaching versus receding objects remains over a wide range of starting and finishing distances. The response to object approach, measured both as membrane potential and spike rate, remains single peaked with starting distances of between 200 and 2,100 mm, and approach speeds of 0.5-2 m/s. These results confirm the behavior predicted by the neural network described by Rind and Bramwell but contradicts the findings of Rind and Simmons, forcing a re-evaluation of the suitability of some of the mechanical visual stimuli used in that study. 3. For depolarization of the LGMD neuron to be maintained or increased throughout the motion of image edges, the edges must move with increasing velocity over the eye. Membrane potential declines before the end of edge motion with constant velocities of edge motion. 4. A second identified neuron, the LGMD2 also is shown to respond directionally to approaching objects. In both the LGMD and LGMD2 neurons, postsynaptic inhibition shapes the directional response to object motion.

Vector combined preisach model with a biaxial switching operator for simulation of unoriented thin-film media

The elements of a vector Preisach model for unoriented thin-film media are described. The implementation consists of adding a biaxial anisotropy switching function to the classic Preisach algorithm. The model was developed to simulate high-squareness thin-film recording media. The utility of the model is demonstrated by the simulations of the usual VSM curves: the major loop, forward and reverse remanence loops, and the δM curve, as well as the response to rotating fields. These results are compared with experimental measurements made on an unoriented thin-film hard disk material.

Linearisation of power amplifiers using RF feedback

A power amplifier linearisation scheme using RF feedback is proposed. Stability compensation is provided using Cartesian baseband circuits. The method incorporates all the signal processing elements in the forward loop of the system as a means of eliminating nonlinear errors and noise introduced by the modulator/demodulator circuits.

Multi-Variable Control of an Integrated Propulsion and Airframe System

The development of an integrated control scheme to enhance the performance of a generic interconnected multi-variable dynamical system, consisting of a turbofan engine and an airframe, in the presence of predominantly destructive dynamical interactions over the flight envelope is considered in this paper. The control scheme consists of two components: a simple static forward loop or feedback loop precompensator to improve the interactions followed by a forward or feedback loop controller to improve the performance. The system must be tolerant to soft and hard output sensor failures by means of analytic redundancy only. A control methodology to satisfy the above specifications is presented here. Necessary and sufficient conditions are presented in order to achieve a stable closed-loop performance of the overall system by tuning every loop separately, that is decentralized stability.

Cutting numbers for the forward loop backward hop network

Traditionally, network reliability is often measured by the connectivity of the underlying graph. Recently, a finer measure which counts the number of pairs whose communication is blocked by faulty components, the so-called cutting number, has attracted much attention. Masuyama, and Masuyama and Ichimori computed the cutting numbers for the popular forward loop backward hop computer network for a small number of faulty nodes. Unfortunately, their results contain many errors. In this paper we give a complete analysis for the cutting numbers with double-node and triple-node failures.

Chapter 10 Chemical signalling and striatal interneurones

Publisher Summary The majority of neostriatal neurones are mediumsized spiny neurones, which have diameters between 10 and 20 pm. These cells were originally believed to be interneurones; however, retrograde transport studies established them as projection neurones. Parallel studies showed that GABA inhibition (or more strictly disinhibition) controls the majority of the functional activity in the basal ganglia. In addition to the medium-sized spiny cells, the larger aspiny cells, 20–60 pm diameters, constitute some 4%– 5 % of all striatal neurones, and these are now known to be short axoned interneurones, although there are occasional reports of aspiny projection neurones. These interneurones integrate activity between output pathways through the striatum providing feed forward and feedback loops. In addition, interneurones may represent relatively specific targets for cortical or thalamic inputs. This chapter discusses some of the recent work exploring the phenotypes of striatal interneurones and present evidence indicating that nitric oxide may be a novel intercellular messenger in the striatum modulating the effects of glutamate on N-methyl-D-aspartate (NMDA) receptors expressed by striatal neurones. Stimulation of NMDA receptors in vivo activates local acetylcholine (ACh) release as does local application of substance P and substance P agonists. The exquisite sensitivity of cholinergic neurones to substance P and NMDA suggests that these neurones represent an important target both for local striatal axon collaterals containing substance P, and thalamic excitatory inputs releasing glutamate.

Computer-aided manipulation of mason's formula and its applications

This paper presents a new method to determine the symbolic gain of a linear system via Mason's formula. A combined searching algorithm based on the breadth-first and depth-first searching algorithms in a directed graph is devised to find the forward paths and loops in the linear system. Mason's formula is then determined via touching-non-touching matches of forward paths and loops in the system. A SPICE-like system Description language (SDL) is also proposed to describe the linear system so that this new algorithm can be put into execution in a computer language. In comparison with other approaches, this method is more straightforward and needs no complicated matrix operations. the completeness of this new method had already been guaranteed by Mason's original formula. It is also believed that Mason's formula has a new interpretation through this new approach.

Animal Behavior and Posture Control under Weightlessness

A variety of animal behavior under microgravity has been found in the experiments using parabolic aircraft flights and in space. Forward looping in fish and tadpoles is contrast to backward looping of pigeons, while frogs and hamsters extend their extremities with the head turning backward. Postures of monkeys and man, on the other hand, are different from those of above animals. Loss of vestibular input must be common to all of these animals and there may be some similarities among the species in the sensory conflict against it. Advancing animals experiments in space, it is important to make further progress in comparative neurology particularly regarding the posture control and behavior of animals.