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Lawrence M. Wein - One of the best experts on this subject based on the ideXlab platform.

  • Scheduling Networks of queues: heavy traffic analysis of a multistation Closed Network
    Operations Research, 1993
    Co-Authors: Philippe Chevalier, Lawrence M. Wein
    Abstract:

    We consider the problem of finding an optimal dynamic priority sequencing policy to maximize the mean throughput rate in a multistation, multiclass Closed queueing Network with general service time distributions and a general routing structure. Under balanced heavy loading conditions, this scheduling problem can be approximated by a control problem involving Brownian motion. Although a unique, Closed-form solution to the Brownian control problem is not derived, an analysis of the problem leads to an effective static sequencing policy, and to an approximate means of comparing the relative performance of arbitrary static policies. Several examples are provided that illustrate the effectiveness of our procedure.

  • Scheduling Networks of Queues: Heavy Traffic Analysis of a Two-Station Closed Network
    Operations Research, 1990
    Co-Authors: J. Michael Harrison, Lawrence M. Wein
    Abstract:

    We consider a multiclass Closed queueing Network with two single-server stations. Each class requires service at a particular station, and customers change class after service according to specified probabilities. There is a general service time distribution for each class. The problem is to schedule the two servers to maximize the long-run average throughput of the Network. By assuming a large customer population and nearly balanced loading of the two stations, the scheduling problem can be approximated by a dynamic control problem involving Brownian motion. A reformulation of this control problem is solved exactly and the solution is interpreted in terms of the queueing Network to obtain a scheduling rule. We conjecture, quite naturally, that the resulting scheduling rule is asymptotically optimal under heavy traffic conditions, but no attempt is made to prove that. The scheduling rule is a static priority policy that computes an index for each class and awards higher priority at station 1 respectively, station 2 to classes with the smaller respectively, larger values of this index. An analytical comparison of this rule to any other static policy is also obtained. An example is given that illustrates the procedure and demonstrates its effectiveness.

Thomas Begin - One of the best experts on this subject based on the ideXlab platform.

  • Higher-order distributional properties in Closed queueing Networks
    Performance Evaluation, 2009
    Co-Authors: Alexandre Brandwajn, Thomas Begin
    Abstract:

    In many real-life computer and Networking applications, the distributions of service times, or times between arrivals of requests, or both, can deviate significantly from the memoryless negative exponential distribution that underpins the product-form solution for queueing Networks. Frequently, the coefficient of variation of the distributions encountered is well in excess of one, which would be its value for the exponential. For Closed queueing Networks with non-exponential servers there is no known general exact solution, and most, if not all, approximation methods attempt to account for the general service time distributions through their first two moments. We consider two simple Closed queueing Networks which we solve exactly using semi- numerical methods. These Networks depart from the structure leading to a product-form solution only to the extent that the service time at a single node is non-exponential. We show that not only the coefficients of variation but also higher-order distributional properties can have an important effect on such customary steady-state performance measures as the mean number of customers at a resource or the resource utilization level in a Closed Network. Additionally, we examine the state that a request finds upon its arrival at a server, which is directly tied to the resulting quality of service. Although the well-known Arrival Theorem holds exactly only for product-form Networks of queues, some approximation methods assume that it can be applied to a reasonable degree also in other Closed queueing Networks. We investigate the validity of this assumption in the two Closed queueing models considered. Our results show that, even in the case when there is a single non-exponential server in the Network, the state found upon arrival may be highly sensitive to higher-order properties of the service time distribution, beyond its mean and coefficient of variation. This dependence of mean numbers of customers at a server on higher-order distributional properties is in stark contrast with the situation in the familiar open M/G/1 queue. Thus, our results put into question virtually all traditional approximate solutions, which concentrate on the first two moments of service time distributions.

Darren Hurley-smith - One of the best experts on this subject based on the ideXlab platform.

  • Virtual Closed Networks : optimised security for autonomous MANETs
    2015
    Co-Authors: Darren Hurley-smith
    Abstract:

    Autonomous mobile platforms (such as Unmanned Aerial Vehicles, also known as UAVs) have become a popular tool in exploration, disaster management, civil-engineering, agricultural and military scenarios. Their endurance, low-cost, high mobility and ability to reduce human involvement in prolonged or hazardous activities have proven attractive to both commercial and military sectors. In such domains, security is required to protect the data, functionality and performance of the Network, making it a vital consideration when developing such systems. Systems capable of independent action, following a human-defined mission without scripting or other forms of direction in the field, are adaptable and effective as a means of achieving individually simple tasks that due to their number and distribution represent complex objectives as a collective. However, such systems must communicate to achieve autonomous function. Efficient distribution of tasks requires significant communication between all members of the Network to determine the nodes most fit to undertake a given task. Mobile ad hoc Networks (MANETs) provide the foundation for such communication, providing a means by which nodes may communicate with other members of the Network, even if they are not in range. Issues arise when considering the security of MANET communication, due to the ease observation, interception and manipulation of data broadcast over such Networks. It is trivial for attackers to perform such actions, due to the open nature of the communication medium. This dissertation presents a novel security framework, which specifically targets autonomous MANET communication. Addressing the open-medium problem by providing a Virtual Closed Network (VCN) environment, Security Using Pre-Existing Routing for MANETs (SUPERMAN) also secures routing and control data, providing confidentiality, integrity and authentication services as a complete solution the Network layer and above. Improvements to the efficiency of communication required by distributed task allocation are proposed (Cluster Form CBBA and Broadcast Enabled Cluster Form CBBA) based on this work, in the interests of optimising the use of Network resources to facilitate the addition of robust security measures suitable for resource constrained MANETs.

  • ICITST - Virtual Closed Networks: A secure approach to autonomous mobile ad hoc Networks
    2015 10th International Conference for Internet Technology and Secured Transactions (ICITST), 2015
    Co-Authors: Darren Hurley-smith, Jodie Wetherall, A. A. Adekunle
    Abstract:

    The increasing autonomy of Mobile Ad Hoc Networks (MANETs) has enabled a great many large-scale unguided missions, such as agricultural planning, conservation and similar surveying tasks. Commercial and military institutions have expressed great interest in such ventures; raising the question of security as the application of such systems in potentially hostile environments becomes a desired function of such Networks. Preventing theft, disruption or destruction of such MANETs through cyber-attacks has become a focus for many researchers as a result. Virtual Private Networks (VPNs) have been shown to enhance the security of Mobile Ad hoc Networks (MANETs), at a high cost in Network resources during the setup of secure tunnels. VPNs do not normally support broadcast communication, reducing their effectiveness in high-traffic MANETs, which have many broadcast communication requirements. To support routing, broadcast updates and efficient MANET communication, a Virtual Closed Network (VCN) architecture is proposed. By supporting private, secure communication in unicast, multicast and broadcast modes, VCNs provide an efficient alternative to VPNs when securing MANETs. Comparative analysis of the set-up overheads of VCN and VPN approaches is provided between OpenVPN, IPsec, Virtual Private LAN Service (VPLS), and the proposed VCN solution: Security Using Pre-Existing Routing for MANETs (SUPERMAN).

Alexandre Brandwajn - One of the best experts on this subject based on the ideXlab platform.

  • Higher-order distributional properties in Closed queueing Networks
    Performance Evaluation, 2009
    Co-Authors: Alexandre Brandwajn, Thomas Begin
    Abstract:

    In many real-life computer and Networking applications, the distributions of service times, or times between arrivals of requests, or both, can deviate significantly from the memoryless negative exponential distribution that underpins the product-form solution for queueing Networks. Frequently, the coefficient of variation of the distributions encountered is well in excess of one, which would be its value for the exponential. For Closed queueing Networks with non-exponential servers there is no known general exact solution, and most, if not all, approximation methods attempt to account for the general service time distributions through their first two moments. We consider two simple Closed queueing Networks which we solve exactly using semi- numerical methods. These Networks depart from the structure leading to a product-form solution only to the extent that the service time at a single node is non-exponential. We show that not only the coefficients of variation but also higher-order distributional properties can have an important effect on such customary steady-state performance measures as the mean number of customers at a resource or the resource utilization level in a Closed Network. Additionally, we examine the state that a request finds upon its arrival at a server, which is directly tied to the resulting quality of service. Although the well-known Arrival Theorem holds exactly only for product-form Networks of queues, some approximation methods assume that it can be applied to a reasonable degree also in other Closed queueing Networks. We investigate the validity of this assumption in the two Closed queueing models considered. Our results show that, even in the case when there is a single non-exponential server in the Network, the state found upon arrival may be highly sensitive to higher-order properties of the service time distribution, beyond its mean and coefficient of variation. This dependence of mean numbers of customers at a server on higher-order distributional properties is in stark contrast with the situation in the familiar open M/G/1 queue. Thus, our results put into question virtually all traditional approximate solutions, which concentrate on the first two moments of service time distributions.

Svetlana Anulova - One of the best experts on this subject based on the ideXlab platform.