Locally excitatory globally inhibitory oscillator networks

A novel class of locally excitatory, globally inhibitory oscillator networks (LEGION) is proposed and investigated. The model of each oscillator corresponds to a standard relaxation oscillator with two time scales. In the network, an oscillator jumping up to its active phase rapidly recruits the oscillators stimulated by the same pattern, while preventing other oscillators from jumping up. Computer simulations demonstrate that the network rapidly achieves both synchronization within blocks of oscillators that are stimulated by connected regions and desynchronization between different blocks. This model lays a physical foundation for the oscillatory correlation theory of feature binding and may provide an effective computational framework for scene segmentation and figure/ground segregation in real time.     

How noise affects the synchronization properties of recurrent networks of inhibitory neurons

GABAergic interneurons play a major role in the emergence of various types of synchronous oscillatory patterns of activity in the central nervous system. Motivated by these experimental facts, modeling studies have investigated mechanisms for the emergence of coherent activity in networks of inhibitory neurons. However, most of these studies have focused either when the noise in the network is absent or weak or in the opposite situation when it is strong. Hence, a full picture of how noise affects the dynamics of such systems is still lacking. The aim of this letter is to provide a more comprehensive understanding of the mechanisms by which the asynchronous states in large, fully connected networks of inhibitory neurons are destabilized as a function of the noise level. Three types of single neuron models are considered: the leaky integrate-and-fire (LIF) model, the exponential integrate-and-fire (EIF), model and conductance-based models involving sodium and potassium Hodgkin-Huxley (HH) currents. We show that in all models, the instabilities of the asynchronous state can be classified in two classes. The first one consists of clustering instabilities, which exist in a restricted range of noise. These instabilities lead to synchronous patterns in which the population of neurons is broken into clusters of synchronously firing neurons. The irregularity of the firing patterns of the neurons is weak. The second class of instabilities, termed oscillatory firing rate instabilities, exists at any value of noise. They lead to cluster state at low noise. As the noise is increased, the instability occurs at larger coupling, and the pattern of firing that emerges becomes more irregular. In the regime of high noise and strong coupling, these instabilities lead to stochastic oscillations in which neurons fire in an approximately Poisson way with a common instantaneous probability of firing that oscillates in time.     

Analysis of synchronization between two modules of pulse neural networks with excitatory and inhibitory connections

To study the synchronized oscillations among distant neurons in the visual cortex, we analyzed the synchronization between two modules of pulse neural networks using the phase response function. It was found that the intermodule connections from excitatory to excitatory ensembles tend to stabilize the antiphase synchronization and that the intermodule connections from excitatory to inhibitory ensembles tend to stabilize the in-phase synchronization. It was also found that the intermodule synchronization was more noticeable when the inner-module synchronization was weak.     

A population density approach that facilitates large-scale modeling of neural networks: extension to slow inhibitory synapses

A previously developed method for efficiently simulating complex networks of integrate-and-fire neurons was specialized to the case in which the neurons have fast unitary postsynaptic conductances. However, inhibitory synaptic conductances are often slower than excitatory ones for cortical neurons, and this difference can have a profound effect on network dynamics that cannot be captured with neurons that have only fast synapses. We thus extend the model to include slow inhibitory synapses. In this model, neurons are grouped into large populations of similar neurons. For each population, we calculate the evolution of a probability density function (PDF), which describes the distribution of neurons over state-space. The population firing rate is given by the flux of probability across the threshold voltage for firing an action potential. In the case of fast synaptic conductances, the PDF was one-dimensional, as the state of a neuron was completely determined by its transmembrane voltage. An exact extension to slow inhibitory synapses increases the dimension of the PDF to two or three, as the state of a neuron now includes the state of its inhibitory synaptic conductance. However, by assuming that the expected value of a neuron's inhibitory conductance is independent of its voltage, we derive a reduction to a one-dimensional PDF and avoid increasing the computational complexity of the problem. We demonstrate that although this assumption is not strictly valid, the results of the reduced model are surprisingly accurate.     

Secure and self-stabilizing clock synchronization in sensor networks

In sensor networks, correct clocks have arbitrary starting offsets and nondeterministic fluctuating skews. We consider an adversary that aims at tampering with the clock synchronization by intercepting messages, replaying intercepted messages (after the adversary’s choice of delay), and capturing nodes (i.e., revealing their secret keys and impersonating them). We present an efficient clock sampling algorithm which tolerates attacks by this adversary, collisions, a bounded amount of losses due to ambient noise, and a bounded number of captured nodes that can jam, intercept, and send fake messages. The algorithm is self-stabilizing, so if these bounds are temporarily violated, the system can efficiently stabilize back to a correct state. Using this clock sampling algorithm, we construct the first self-stabilizing algorithm for secure clock synchronization in sensor networks that is resilient to the aforementioned adversarial attacks.     

New conditions for synchronization in dynamical communication networks

This paper investigates synchronization in a typical multi-agent system in which the communication network changes according to the system state. Through building new relationships between a matrix and its associated graph and estimating the diameter of the communication network, we prove that synchronization can be achieved if the speed of agents is bounded by O(n^−β), where n is the number of agents and β is bounded by a constant independent of n, which is much better than the existing bound O(n⁻ⁿ). Some simulations are provided to illustrate the theoretical results.     

Ad Hoc网络时间同步技术研究

通过分析影响时间同步性能的6类网络延时,对几种基本的无线传感器网络时钟同步协议进行了介绍和性能比较,提出了一种基于时延探测机制的AdHoc网络时间同步协议.协议通过构造特定的工作方式及信息格式,将各类时延误差作为一个整体来探测和消除.此外,由于时延探测过程和时间校正过程相互分离,减轻了时延探测过程中对于信息成功交互的时间要求,有利于降低网络的冲突和负担.分析表明,DMC-TS协议实现简单,扩展性好,由于针对所有的延时误差都进行了校正,因此能够提供较好的同步精度.     

Divisive inhibition in recurrent networks

Models of visual cortex suggest that response selectivity can arise from recurrent networks operating at high gain. However, such networks have a number of problematic features: (i) they operate perilously close to a point of instability, (ii) small changes in synaptic strength can dramatically modify the degree of amplification, and (iii) they respond slowly to rapidly changing stimuli. Divisive inhibition, acting through interneurons that are themselves divisively inhibited, can solve these problems without degrading the selectivity of a recurrent network.     

Feedback-based synchronization in system area networks for cluster computing

Many applications in cluster computing require QoS (quality of service) services. Since performance predictability is essential to provide QoS service, underlying systems must provide predictable performance guarantees. One way to ensure such guarantees from network subsystems is to generate global schedules from applications' network requests and to execute the local portion of the schedules at each network interface. To ensure accurate execution of the schedules, it is essential that a global time base must be maintained by local clocks at each network interface. The task of providing a single time base is called a synchronization problem and this paper addresses the problem for system area networks. To solve the synchronization problem, FM-QoS [K. Connelly (1999)] proposed a simple synchronization mechanism called FBS (feedback-based synchronization) which uses built-in flow control signals. This paper extends the basic notion of FM-QoS to a theoretical framework and generalizes it: 1) to identify a set of built-in network flow control signals for synchrony and to formalize it as a synchronizing schedule and 2) to analyze the synchronization precision of FBS in terms of flow control parameters. Based on generalization, two application classes are studied for a single switch network and a multiple switch network. For each class, a synchronizing schedule is proposed and its bounded skew is analyzed. Unlike FM-QoS, the synchronizing schedule is proven to minimize the bounded skew value for a single switch network. To understand the analysis results in practical networks, skew values are obtained with flow control parameters of Myrinet-2000. We observed that the maximum bounded skew of FBS is 5.79/spl mu/sec or less over all our experiments. Based on this result, we came to a conclusion that FBS was a feasible synchronization mechanism in system area networks.     

A simple algorithm for clock synchronization in transputer networks

In a distributed system based on Transputer components there is one clock for each processing element, and the definition of the global system time requires the choice of a hardware or software synchronization method. This paper describes the RING_SYNC algorithm, based on a ring-structured synchronization scheme. RING_SYNC has no provision for fault tolerance, but it introduces little overhead, thanks to the optimization of both the number of messages exchanged at sync time and the resynchronization frequency. The implementation of the algorithm together with the tests performed for measuring the synchronization error and their results are discussed extensively, and some typical applications are pointed out.     

一种异构传感器网络时间同步的安全性研究

分布式时间同步技术是无线传感器网络（WSNs）应用中的一项重要支撑技术,保证时间同步的安全性至关重要。针对敌对异构网络环境,通过利用存储有高能量的高级传感器,提出了一种高效的异构WSNs时间同步安全方案。Tmote传感器节点实验表明：该方案比现有的同步方案更能有效提高同步精度,减少通信开销和抵御网络的多种攻击。     

Temporal synchronization in mobile sensor networks using image sequence analysis

This paper addresses the problem of estimating the temporal synchronization in mobile sensors’ networks, by using image sequence analysis of their corresponding scene dynamics. Unlike existing methods, which are frequently based on adaptations of techniques originally designed for wired networks with static topologies, or even based on solutions specially designed for ad hoc wireless sensor networks, but that have a high energy consumption and a low scalability regarding the number of sensors, this work proposes a novel approach that reduces the problem of synchronizing a general number $N$ of sensors to the robust estimation of a single line in ${\mathbb {R}}^{N+1}$ . This line captures all temporal relations between the sensors and can be computed without any prior knowledge of these relations. It is assumed that (1) the network’s mobile sensors cross the field of view of a stationary calibrated camera that operates with constant frame rate and (2) the sensors trajectories are estimated with a limited error at a constant sampling rate, both in the world coordinate system and in the camera’s image plane. Experimental results with real-world and synthetic scenarios demonstrate that our method can be successfully used to determine the temporal alignment in mobile sensor networks.     

Average time synchronization in wireless sensor networks by pairwise messages

Most of previous algorithms for time synchronization choose a specific node’s (denoted as a root or leader) local time to be the reference time, which is easily disturbed by many events (e.g. root node’s power down or damage). The Gaussian distribution for the nodes’ local clocks has been reported by a few authors based on laboratory tests, the average of all nodes’ clocks is the best approximation to the ideal time. In this paper, the possibility to realize average time synchronization in wireless sensor networks by pairwise messages exchange is studied, and a simple algorithm (ATSP) is proposed, which synchronizes all the nodes’ clocks to their average. For networks with clock skew, the algorithm compensates the frequencies of nodes to their average also. Using the Lyapunov’s stability theory, convergence analyses and proofs of the algorithm are given. Synchronization error (accuracy) of the algorithm is estimated by using probability theory also, which indicates that the synchronization error of the algorithm is linearly related to the standard deviation of the message delay. Simulations are performed on a 300 nodes network to examine the performance of the algorithm, which verified the theoretical results.     

可变周期的无线传感器时间同步算法

时间同步技术是无线传感器网络应用中的一项关键技术.介绍了几种无线传感器网络的主流同步算法.通过对比,总结了各种算法的优点和限制条件.最后提出了可变周期的无线传感器时间同步算法,该算法利用改进的参考广播同步协议估计时间偏差和速率偏差,并通过估计出来的偏差值动态调整下次同步的周期,达到减少同步次数,节约能量的目的.     

Toward secure and scalable time synchronization in ad hoc networks

Time synchronization is crucial in ad hoc networks. Due to the infrastructure-less and dynamic nature, time synchronization in such environments is vulnerable to various attacks. Moreover, time synchronization protocols such as IEEE 802.11 TSF (Timing Synchronization Function) often suffer from scalability problem.In this paper, we address the security and the scalability problems of time synchronization protocols in ad hoc networks. We propose a novel suite of time synchronization mechanisms for ad hoc networks based on symmetric cryptography. For single-hop ad hoc networks, we propose SSTSP, a scalable and secure time synchronization procedure based on one-way Hash chain, a lightweight mechanism to ensure the authenticity and the integrity of synchronization beacons. The “clock drift check” is proposed to counter replay/delay attacks. We then extend our efforts to the multi-hop case. We propose MSTSP, a secure and scalable time synchronization mechanism based on SSTSP for multi-hop ad hoc networks. In MSTSP, the multi-hop network is automatically divided into single-hop clusters. The secure intra-cluster synchronization is achieved by SSTSP and the secure inter-cluster synchronization is achieved by exchanging synchronization beacons among cluster reference nodes via bridge nodes.The proposed synchronization mechanisms are fully distributed without a global synchronization leader. We further perform analytical studies and simulations on the proposed approaches. The results show that SSTSP can synchronize single-hop networks with the maximum synchronization error under 20 μs and MSTSP 55–85 μs for multi-hop networks, which are, to the best of our knowledge, among the best results of currently proposed solutions for single-hop and multi-hop ad hoc networks. Meanwhile, our approaches can maintain the network synchronized even in hostile environments.     

Synchronization in networks of excitatory and inhibitory neurons with sparse,random connectivity

In model networks of E-cells and I-cells (excitatory and inhibitory neurons, respectively), synchronous rhythmic spiking often comes about from the interplay between the two cell groups: the E-cells synchronize the I-cells and vice versa. Under ideal conditions-homogeneity in relevant network parameters and all-to-all connectivity, for instance-this mechanism can yield perfect synchronization. We find that approximate, imperfect synchronization is possible even with very sparse, random connectivity. The crucial quantity is the expected number of inputs per cell. As long as it is large enough (more precisely, as long as the variance of the total number of synaptic inputs per cell is small enough), tight synchronization is possible. The desynchronizing effect of random connectivity can be reduced by strengthening the E --> I synapses. More surprising, it cannot be reduced by strengthening the I --> E synapses. However, the decay time constant of inhibition plays an important role. Faster decay yields tighter synchrony. In particular, in models in which the inhibitory synapses are assumed to be instantaneous, the effects of sparse, random connectivity cannot be seen.     

Robust gamma oscillations in networks of inhibitory hippocampal interneurons

Recent experiments suggest that inhibitory networks of interneurons can synchronize the neuronal discharge in in vitro hippocampal slices. Subsequent theoretical work has shown that strong synchronization by mutual inhibition is only moderately robust against neuronal heterogeneities in the current drive, provided by activation of metabotropic glutamate receptors. In vivo neurons display greater variability in the interspike intervals due to the presence of synaptic noise. Noise and heterogeneity affect synchronization properties differently. In this paper we study, using model simulations, how robust synchronization can be in the presence of synaptic noise and neuronal heterogeneity. We find that stochastic weak synchronization (SWS) (i.e. when neurons spike within a short interval from each other, but not necessarily at each period) is produced with at least a minimum amount of noise and that it is much more robust than strong synchronization (i.e. when neurons spike at each period). The statistics produced by the SWS population discharge are consistent with previous experimental data. We also find robust SWS in the gamma-frequency range (20-80 Hz) for a stronger synaptic coupling compared with previous models and for networks with 10-1000 neurons.     

无线传感器网络泛洪时间同步协议安全算法

针对无线传感器网络泛洪时间同步协议(FTSP)可能遭受到基于发送时间标攻击的问题,提出一种安全算法,在FTSP中添加异常漂移率检测器和恶意节点ID号过滤器,同时为进一步提高FTSP的同步精度,减小FTSP受到恶意攻击后对时间同步的影响,改进泛洪时间同步协议中计算时钟漂移率的公式,对其进行加权处理。仿真结果表明:该安全算法能够有效地防御基于发送时间标的恶意攻击,减小了计算开销和恶意攻击对算法的影响,鲁棒性增强,同时提高了同步精度,达到了稳定准确的同步效果。     

Fast barrier synchronization in wormhole k-ary n-cube networks with multidestination worms

This paper presents a new approach to implement fast barrier synchronization in wormhole k-ary n-cubes. The novelty lies in using multidestination messages instead of the traditional single destination messages. Two different multidestination worm types, gather and broadcasting, are introduced to implement the report and wake-up phases of barrier synchronization, respectively. Algorithms for complete and arbitrary set barrier synchronization are presented using these new worms. It is shown that complete barrier synchronization in a k-ary n-cube system with e-cube routing can be implemented with 2n communication start-ups as compared to 2nlog₂k start-ups needed with unicast-based message passing. This leads to an asymptotic improvement by a factor of log₂k. Simulation results for different system and architectural parameters indicate that the new framework can reduce barrier synchronization cost considerably compared to the unicast-based scheme. For arbitrary set barrier, an interesting trend is observed where the synchronization cost keeps on reducing beyond a certain number of participating nodes. The framework demonstrates potential for supporting fast barrier synchronization in large wormhole-routed systems.     

Modeling the effect of node synchronization times in ultra-wideband wireless networks

Ultra-wideband wireless (UWB) can provide the physical layer for high-throughput personal area networks. When UWB is used for communication between many nodes, relatively long acquisition times are needed when dropping and re-establishing wireless links between the nodes. This paper describes the development and use of mathematical and simulation models to investigate the impact of dropping and reacquiring links between nodes on average packet delay; we also consider the performance of the alternative strategy of forwarding packets through intermediate nodes without breaking the established wireless links. The work presented here assumes that no specific MAC layer protocol, such as WiMedia UWB MAC, is in operation. The paper describes the models, explains the selection of modeling parameters used, compares the average packet delay for a network of three simple UWB nodes and for a ring of ten UWB nodes and explains the use of these results for network design engineers.