Performance evaluation of medium access control for multiple-beam antenna nodes in a wireless LAN

We consider the medium access control (MAC) protocol design for nodes in a wireless LAN that use a wide-azimuth switched beam smart antenna system comprised of a multiple beam antenna array. The one-hop performance of carrier sense multiple access (CSMA) as well as slotted aloha for such a system is presented analytically and through simulation. The problem of synchronization of multiple beams in CSMA is investigated in our analysis. Our results show that, under heavy offered load conditions, CSMA is a good choice with nodes that have multiple-beam smart antennas, despite the performance loss due to the beam synchronization, providing a stable throughput that approaches unity and is invariant to fluctuations in the offered load. Slotted Aloha, on the other hand, is capable of higher peak throughput in a narrow range of offered loads as more switched beams are employed, but performance drastically reduces beyond optimum offered loads. We also introduce a method, expanded receive rule (ERR), whereby the tight synchronization among different beams of a receiver node in CSMA is relaxed, which is observed to provide better throughput. Finally, we also present performance results for a 4-way-handshake-type carrier sense mechanism using multiple beam antennas.     

Adaptive Flocking Control with a Minority of Informed Agents

In this paper, we address the flocking problem of multiple dynamic mobile agents with a virtual leader in a dynamic proximity network. To avoid fragmentation, we propose a novel flocking algorithm that consists of both an adaptive controller for followers and a feedback controller for the virtual leader. Based on our algorithm, all agents in the group can form a network, maintain connectivity, and track the virtual leader, even when only a minority of agents have access to the information of the virtual leader. Finally, several convincing simulation results are provided that demonstrate 2‐D flocking of a group of agents using the proposed algorithm.     

Stability and throughput optimization of multichannel slotted non-persistent CSMA protocol

This paper describes two protocols suitable for a multichannel, multiaccess slotted non-persistent CSMA environment for infinite population, analyses their stability and examines their throughput optimization. The common sensing multichannel slotted non-persistent CSMA (CSCSMA) protocol is an extension of the slotted single channel non-persistent CSMA protocol with an appropriate policy for the selection of the channel in which a station (re)transmits. This policy restricts the control information among the stations, and consequently reduces the cost of the station interface. Pakes's Lemma criteria are applied to define sufficient conditions for ergodicity of the Markov chain, which describes the evolution of the busy station population, and to specify regions in which the multichannel system is stable. Control parameters are the retransmission probabilities. Optimization rules are derived which show that the optimal retransmission probabilities may be expressed as a function of the number of busy stations. The separate sensing multichannel slotted non-persistent CSMA protocol (SSCSMA), using a different policy, distinguishes the channels into two groups: the retransmission channel group devoted in collision resolution; and the transmission channel group for the access of free stations. Stability regions are defined using the results for CSCSMA, and rules for optimal allocation of channels among the two groups are derived for improvement of system performance.     

A hybrid access method in a multiple access channel

This paper considers a multiple-access communication channel with an infinite number of users. We show that if a controlled slotted Aloha protocol is used, then messages with variable length will have a negative impact on the average message delay. In order to alleviate this problem, a mixed mode (Hybrid) access method is suggested under which the channel bandwidth is split into two sub-channels, managed under different policies. Messages whose length is less than or equal to a critical value are transmitted in one sub-channel under a slotted Aloha policy. The rest of the messages are sent through a separate portion of the channel bandwidth, using a reservation protocol. We show that under this hybrid access method the average delay of a message is greatly improved.     

A derivation of the steady-state random-access results without the poisson assumption

In this paper the throughput results of the pure aloha multiple-access protocol are derived without the assumptions made in earlier derivations. Furthermore, the derivation is based on a finite number of users, and explicit expressions for the channel traffic rate as well as the throughput rate are obtained in terms of the number of users and their individual rates.     

Cooperation among peers in an ad hoc network to support an energy efficient IM service

Continuous wireless wide area network (WWAN) access for mobile devices in future pervasive systems may be limited by battery power and may generate extensive data telecommunication costs. In this paper, we develop a new message notification protocol (MNP) to enable mobile users to maintain a continuous presence at their instant messaging (IM) server while avoiding long, idle connections. In MNP, mobile users cooperatively share a single message notification channel to reduce users’ telecommunication charges and extend a device’s battery life. A device may turn off its WWAN interface for most of the time to save power, and only needs to contact the IM server when needed. Precise group information does not need to be maintained by a mobile device. Message notification exchanged between the IM server and the peer group is represented by a compressed Bloom filter to further reduce the protocol overhead and provide additional privacy and security. The results of performance evaluation show that the MNP protocol could be able to save significant energy consumed in a mobile device.     

Scalability Analysis of the Hierarchical Architecture for Distributed Virtual Environments

A distributed virtual environment (DVE) is a shared virtual environment where multiple users at their workstations interact with each other over a network. Some of these systems may support a large number of users, for example, multiplayer online games. An important issue is how well the system scales as the number of users increases. In terms of scalability, a promising system architecture is a two-level hierarchical architecture. At the lower level, multiple servers are deployed; each server interacts with its assigned users. At the higher level, the servers ensure that their copies of the virtual environment are as consistent as possible. Although the two-level architecture is believed to have good properties with respect to scalability, not much is known about its performance characteristics. In this paper, we develop a performance model for the two-level architecture and obtain analytic results on the workload experienced by each server. Our results provide valuable insights into the scalability of the architecture. We also investigate the issue of consistency and develop a novel technique to achieve weak consistency among copies of the virtual environment at the various servers. Simulation results on the consistency/scalability trade-off are presented.     

A novel channel-adaptive uplink access control protocol for nomadic computing

We consider the uplink access control problem in a mobile nomadic computing system, which is based on a cellular phone network in that a user can use the mobile device to transmit voice or file data. This resource management problem is important because an efficient solution to uplink access control is critical for supporting a large user population with a reasonable level of quality of service (QoS). While there are a number of recently proposed protocols for uplink access control, these protocols possess a common drawback in that they do not adapt well to the burst error properties, which are inevitable in using wireless communication channels. We propose a novel TDMA-based uplink access protocol, which employs a channel state dependent allocation strategy. Our protocol is motivated by two observations: (1) when channel state is bad, the throughput is low due to the large amount of FEC (forward error correction) or excessive ARQ (automatic repeated request) that is needed and (2) because of item 1, much of the mobile device's energy is wasted. The proposed protocol works closely with the underlying physical layer in that, through observing the channel state information (CSI) of each mobile device, the MAC protocol first segregates a set of users with good CSI from requests gathered in the request contention phase of an uplink frame. The protocol then judiciously allocates channel bandwidth to contending users based on their channel conditions. Simulation results indicate that the proposed protocol considerably outperforms five state-of-the-art protocols in terms of packet loss, delay, and throughput.     

Exponential trajectory tracking control in the workspace of a class of flexible robots

In this paper, we present a control strategy that ensures the exponential stability of the tracking error in the virtual joint space of a class of mechanical systems made up of rigid links that form a chain that ends with a flexible beam. Virtual joints are defined so as to be related kinematically to the workspace. Thus, when the inverse kinematics is nonsingular, trajectory tracking in the virtual joint space is equivalent to trajectory tracking in the workspace. The method proposed in this paper calls for the transformation of the trajectory from the virtual joint space to the joint and deformation space. The robot is a non-minimum-phase system in the virtual joint space. However, this transformation, which involves the dynamics of the flexible part, can be solved using a causal–anticausal iterative approach. The controller is then designed using an input–output feedback linearization scheme, with regard to the joints, and two linear control laws with regard to the joint and to the deformation variable tracking errors. Analysis based on the passivity theorem, hierarchical systems stability, and linear matrix inequalities then allows us to determine the controller gains that ensure that the tracking errors in the virtual joint space are well damped and exponentially stable. Finally, the strategy is validated by simulating a controller that incorporates the proposed laws and that drives a two-link manipulator that has one rigid and one flexible link. The simulation results demonstrate the good performance of the proposed control system. © 1998 John Wiley & Sons, Inc.     

eHMIP:支持无缝切换的层次移动IP协议

移动主机(MH)在域间切换时延迟较长,不能实现快速切换。为支持MH域间的快速切换,该文在层次移动IP(HMIP)协议的基础上,提出了一种扩展的层次移动IP(eHMIP)协议,使得域间切换分组丢失延迟与MH到家乡网络之间的距离无关。以注册信令开销、切换分组丢失延迟为性能评价指标,对基于HMIP和eHMIP协议的切换性能进行分析。仿真实验结果表明:基于eHMIP协议的平均切换分组丢失延迟低于HMIP,但注册信令开销略高于HMIP。     

Cluster-group based trusted computing for mobile social networks using implicit social behavioral graph

Mobile Social Networks (MSNs) facilitate connections between mobile devices, and are capable of providing an effective mobile computing environment for users to access, share, and distribute information. However, MSNs are virtual social spaces, the available information may not be trustworthy to all. Therefore, trust inference plays a critical role for establishing social links between mobile users. In MSNs, users’ transactions will more and more be complemented with group contact. Hence, future usage patterns of mobile devices will involve more group contacts. In this paper, we describe the implicit social behavioral graph, i.e., ego-i graph which is formed by users’ contacts, and present an algorithm for initiating ego-i graph. We rate these relationships to form a dynamic contact rank, which enables users to evaluate the trust values between users within the context of MSNs. We, then, calculate group-based trust values according to the level of contacts, interaction evolution, and users’ attributes. Based on group-based trust, we obtain a cluster trust by the aggregation of inter group-based trust values. Due to the unique nature of MSNs, we discuss the propagation of cluster trust values for global MSNs. Finally, we evaluate the performance of our trust model through simulations, and the results demonstrate the effectiveness of group-based behavioural relationships in MSNs’ information sharing system.     

An energy-efficient video transport protocol for personal cloud-based computing

Recently, we are surrounded by a collection of heterogeneous computing devices such as desktop computers, laptops, smart phones, smart televisions, and tablet PCs. Each device is operated with its own host operating system, has its own internal architecture, and performs its independent tasks with its own applications and data. A common property amongst these devices, namely that of internet-connectivity, allows them to configure personal virtual cloud system by interconnecting each other through an intermediate switching device. The personal cloud service should provide a seamlessly unified computing environment across multiple devices with synchronized data and application programs. As such, it allows users to freely switch their workspace from one device to another while continuing the interaction with the available applications. In order to support video applications, the cloud system should provide seamless video synchronization among the multiple devices. However, we note that the current cloud services do not provide efficient data flow among devices. In this paper, we propose and develop a new reliable transport protocol to support video synchronization for the personal virtual cloud system. In order to cope with battery limitation of many mobile devices in personal virtual cloud system, the proposed protocol is designed to provide energy efficient video communications. Our simulation results show that the proposed protocol can reduce end users power consumption up to 25 % compared to the legacy TCP with given packet loss probabilities and the average length of error bursts.     

Asynchronous reservation protocol for variable-sized messages in a WDM-based local network

In this paper, we propose and analyse an asynchronous reservation protocol for a very high-speed optical LAN using a passive star topology. Each node is equipped with a single tunable transmitter and a single tunable receiver, both of which are tunable over a range of wavelengths. A separate channel, called the control channel, is used to coordinate message transmissions on the other channels, called data channels. We consider random and idle selection schemes for the data channel selection strategy. The proposed protocol can efficiently support variable-sized messages. It is operated asynchronously, i.e. data channels are not slotted and the control channel is slotted with the size of a control packet. Also, the protocol is scalable, hence the network can accommodate a variable number of nodes. Any new node can join the network without network reinitialization. Moreover, with the protocol one can avoid destination conflicts. We analyse its performance by using the equilibrium point analysis (EPA) method, and validate the results by simulation. According to the numerical results, the asynchronous protocol achieves higher throughput than the synchronous one.     

End-to-end support for statistical quality of service in heterogeneous mobile ad hoc networks

In heterogeneous mobile ad hoc networks (MANETs), different types of mobile devices with diverse capabilities may coexist in the same network. The heterogeneity of MANETs makes end-to-end support for quality of service (QoS) guarantees more difficult than in other types of networks, not to mention the limited bandwidth and frequent topology changes of these networks. Since QoS routing is the first step toward achieving end-to-end QoS guarantees in heterogeneous MANETs, we propose a QoS routing protocol for heterogeneous MANETs. The proposed protocol, called virtual grid architecture protocol (VGAP), uses a cross-layer approach in order to provide end-to-end statistical QoS guarantees. VGAP operates on a fixed virtual rectilinear architecture (virtual grid), which is obtained using location information obtained from global positioning system (GPS). The virtual grid consists of a few, but possibly more powerful, mobile nodes known as ClusterHeads (CHs) that are elected periodically. CHs discover multiple QoS routes on the virtual grid using an extended version of the open shortest path first (OSPF) routing protocol and an extended version of WFQ scheduling policy that takes into account the wireless channel state. Moreover, VGAP utilizes a simple power control algorithm at the physical layer that provides efficient energy savings in this heterogeneous setting. Simulation experiments show that VGAP has a good performance in terms of packet delivery ratio, end-to-end packet delay, call blocking probability, and network scalability.     

A decentralized control system for cooperative transportation by multiple non-holonomic mobile robots

In this paper, we propose a decentralized control system for transporting a single object by multiple non-holonomic mobile robots. Each agent used in the proposed system has two arms, which can steer around a joint offset from the centre point between two driving wheels. One of these mobile robots acts as a leader, who is assumed to be able to plan and to manipulate the omnidirectional motion of the object by using a resolved velocity control. Other robots, referred to as followers, cooperatively transport the object by keeping a constant relative position with the object using a simple PI control. Different from conventional leader–follower type systems that transport an object by multiple robots in coordination, the present followers can plan an action based on their local coordinate and need no absolute positional information. In addition, as a special case, a system consisting of only two robots is introduced, in which the follower robot not only has an arm length controller to follow the leader but also has a fuzzy controller as an avoidance controller to avoid obstacles or a posture controller to keep a desired posture of the object. Simulation results are given to demonstrate the good performance of the proposed systems.     

含未知信息的轮式移动机器人编队确定学习控制

本文研究含未知信息的轮式移动机器人(wheeled mobile robots,WMR)的编队控制问题.首先,基于领航–跟随法和虚拟结构法,将WMR编队控制问题转化为跟随机器人对参考虚拟机器人的跟踪控制问题.然后,利用径向基函数神经网络(radial basis function neural networks,RBF NN)对WMR的未知系统动态进行学习,以及根据李雅普诺夫稳定性理论设计了稳定的自适应RBF NN控制器和RBF NN权值估计的学习率.依据确定学习理论,闭环系统内部信号在对回归轨迹实现跟踪控制的过程中满足部分持续激励(persistent excitation,PE)条件.随着PE条件的满足,RBF NN权值估计收敛到其理想权值,实现了对未知闭环系统动态的准确学习.最后,利用学习结果设计了RBF NN学习控制器,保证了控制系统的稳定与收敛,实现了闭环稳定性和改进了控制性能,并通过仿真验证了所提控制方法的正确性和有效性.     

Who is Who on Twitter–Spammer,Fake or Compromised Account? A Tool to Reveal True Identity in Real-Time

Social networks once being an innoxious platform for sharing pictures and thoughts among a small online community of friends has now transformed into a powerful tool of information, activism, mobilization, and sometimes abuse. Detecting true identity of social network users is an essential step for building social media an efficient channel of communication. This paper targets the microblogging service, Twitter, as the social network of choice for investigation. It has been observed that dissipation of pornographic content and promotion of followers market are actively operational on Twitter. This clearly indicates loopholes in the Twitter’s spam detection techniques. Through this work, five types of spammers-sole spammers, pornographic users, followers market merchants, fake, and compromised profiles have been identified. For the detection purpose, data of around 1 Lakh Twitter users with their 20 million tweets has been collected. Users have been classified based on trust, user and content based features using machine learning techniques such as Bayes Net, Logistic Regression, J48, Random Forest, and AdaBoostM1. The experimental results show that Random Forest classifier is able to predict spammers with an accuracy of 92.1%. Based on these initial classification results, a novel system for real-time streaming of users for spam detection has been developed. We envision that such a system should provide an indication to Twitter users about the identity of users in real-time.     

Latency-aware virtual desktops optimization in distributed clouds

Distributed clouds offer a choice of data center locations for providers to host their applications. In this paper, we consider distributed clouds that host virtual desktops which are then accessed by users through remote desktop protocols. Virtual desktops have different levels of latency-sensitivity, primarily determined by the actual applications running and affected by the end users’ locations. In the scenario of mobile users, even switching between 3G and WiFi networks affects the latency-sensitivity. We design VMShadow, a system to automatically optimize the location and performance of latency-sensitive VMs in the cloud. VMShadow performs black-box fingerprinting of a VM’s network traffic to infer the latency-sensitivity and employs both ILP and greedy heuristic based algorithms to move highly latency-sensitive VMs to cloud sites that are closer to their end users. VMShadow employs a WAN-based live migration and a new network connection migration protocol to ensure that the VM migration and subsequent changes to the VM’s network address are transparent to end-users. We implement a prototype of VMShadow in a nested hypervisor and demonstrate its effectiveness for optimizing the performance of VM-based desktops in the cloud. Our experiments on a private as well as the public EC2 cloud show that VMShadow is able to discriminate between latency-sensitive and insensitive desktop VMs and judiciously moves only those that will benefit the most from the migration. For desktop VMs with video activity, VMShadow improves VNC’s refresh rate by 90% by migrating virtual desktop to the closer location. Transcontinental remote desktop migrations only take about 4 min and our connection migration proxy imposes 13 μs overhead per packet.     

Finite‐time consensus tracking for harmonic oscillators using both state feedback control and output feedback control

This paper investigates the distributed finite‐time consensus‐tracking problem for coupled harmonic oscillators. The objective is to guarantee a team of followers modeled by harmonic oscillators to track a dynamic virtual leader in finite time. Only a subset of followers can access the information of the virtual leader, and the interactions between followers are assumed to be local. We consider two cases: (i) The followers can obtain the relative states between their neighbors and their own; and (ii) Only relative outputs between neighboring agents are available. In the former case, a distributed consensus protocol is adopted to achieve the finite‐time consensus tracking. In the latter case, we propose a novel observer‐based dynamic protocol to guarantee the consensus tracking in finite time. Simulation examples are finally presented to verify the theoretical analysis. Copyright © 2012 John Wiley & Sons, Ltd.     

时隙ALOHA协议下的网络化控制系统协同设计

针对网络化控制系统中信道容量有限的问题,本文提出一种基于时隙ALOHA通信协议的控制与通信协同设计方法.将控制系统的采样周期划分为若干等长度的时隙,在每个时隙中,系统的分布式传感器通过时隙ALOHA协议来随机竞争接入网络.由于在不同的采样周期各个传感器的接入状态不同,整个状态反馈控制系统将在若干子系统之间进行切换.据此,本文建立了离散的切换系统模型,并利用分段李雅普诺夫函数方法和平均驻留时间技术得到了能够保证系统指数稳定的充分条件.然后,给出能够保证控制系统稳定所需的信道吞吐率的界限,进而得到了时隙ALOHA协议中的最大重传次数与控制系统衰减率的定量关系.通过上述方法,本文建立了控制-通信协同设计的框架结构,可将控制器的增益矩阵和时隙ALOHA通信协议进行协同设计.最后,通过仿真验证了本文所提出的协同设计方法的有效性.