Novella Bartolini

A Walk through Content Delivery Networks

Content Delivery Networks (CDN) aim at overcoming the inherent limitations of the Internet. The main concept at the basis of this technology is the delivery at edge points of the network, in proximity to the request areas, to improve the users perceived performance while limiting the costs. This paper focuses on the main research areas in the field of CDN, pointing out the motivations, and analyzing the existing strategies for replica placement and management, server measurement, best fit replica selection and request redirection.

Push & Pull: autonomous deployment of mobile sensors for a complete coverage

Mobile sensor networks are important for several strategic applications devoted to monitoring critical areas. In such hostile scenarios, sensors cannot be deployed manually and are either sent from a safe location or dropped from an aircraft. Mobile devices permit a dynamic deployment reconfiguration that improves the coverage in terms of completeness and uniformity. In this paper we propose a distributed algorithm for the autonomous deployment of mobile sensors called Push & Pull. According to our proposal, movement decisions are made by each sensor on the basis of locally available information and do not require any prior knowledge of the operating conditions or any manual tuning of key parameters. We formally prove that, when a sufficient number of sensors are available, our approach guarantees a complete and uniform coverage. Furthermore, we demonstrate that the algorithm execution always terminates preventing movement oscillations. Numerous simulations show that our algorithm reaches a complete coverage within reasonable time with moderate energy consumption, even when the target area has irregular shapes. Performance comparisons between Push & Pull and one of the most acknowledged algorithms show how the former one can efficiently reach a more uniform and complete coverage under a wide range of working scenarios.

Handoff and optimal channel assignment in wireless networks

In this paper, a non-preemptive prioritization scheme for access control in cellular networks is analyzed. Two kinds of users are assumed to compete for the access to the limited number of frequency channels available in each cell: the high priority users represent handoff requests, while the low priority users correspond to initial access requests originated within the same cell. Queueing of handoff requests is also considered. The research for the best access policy is carried out by means of a Markov decision model which allows us to study a very wide class of policies which includes some well known pure stationary policies, as well as randomized ones. The cutoff priority policy, consisting in reserving a certain number of channels to the high priority stream of requests, is proved to be optimal within this class while using an objective function in the form of a linear combination of some quality of service parameters, when no queueing device is considered. Numerical results confirm the optimality of the cutoff priority policy when queueing of handoff requests is allowed.

Call admission control in wireless multimedia networks

This paper addresses the call admission control problem for the multimedia services that characterize the third generation of wireless networks. In the proposed model each cell has to serve a variety of classes of requests that differ in their traffic parameters, bandwidth requirements and in the priorities while ensuring proper quality of service levels to all of them. A semi Markov process is used to model multi-class multimedia systems with heterogeneous traffic behavior, allowing for call transitions among classes. It is shown that the derived optimal policy establishes state-related threshold values for the admission policy of handoff and new calls in the different classes, while minimizing the blocking probabilities of all the classes and prioritizing the handoff requests. It is proven that in restrictive cases the optimal policy has the shape of a multi-threshold priority policy, while in general situations the optimal policy has a more complex shape.

Snap and Spread: A Self-deployment Algorithm for Mobile Sensor Networks

The use of mobile sensors is motivated by the necessity to monitor critical areas where sensor deployment cannot be performed manually. In these working scenarios, sensors must adapt their initial position to reach a final deployment which meets some given performance objectives such as coverage extension and uniformity, total moving distance, number of message exchanges and convergence rate. We propose an original algorithm for autonomous deployment of mobile sensors called Snap & Spread . Decisions regarding the behavior of each sensor are based on locally available information and do not require any prior knowledge of the operating conditions nor any manual tuning of key parameters. We conduct extensive simulations to evaluate the performance of our algorithm. This experimental study shows that, unlike previous solutions, our algorithm reaches a final stable deployment, uniformly covering even irregular target areas. Simulations also give insights on the choice of some algorithm variants that may be used under some different operative settings.

Optimal dynamic replica placement in content delivery networks

Content delivery networks (CDN) design entails the placement of server replicas to bring the content close to the users, together with an efficient and content aware request routing. In this paper we address the problem of dynamic replica placement to account for users demand variability while optimizing the costs paid by a CDN provider and the overall performance of the distributed replica servers architecture. We formulate the dynamic replica placement problem as a semi Markov decision process accounting for the traffic conditions, the users level of satisfaction, as well as the costs paid to install, maintain or remove a replica from a site. The proposed model applies to general network topologies and considers realistic constraints on network and servers capacity. The optimal strategy derived by means of the decision model provides the ground for designing a centralized heuristic and is used as a benchmark for the heuristic evaluation. Simulation results show that the proposed heuristic has a behavior close to that of the optimal strategy and achieves very good performance in terms of low average distance from a user to the serving replica, low average number of replicas and high probability of being able to serve a request.

Dynamic replica placement and user request redirection in content delivery networks

The content delivery networks (CDN) paradigm is based on the idea to move third-party content closer to the users transparently. More specifically, content is replicated on servers closer to the users, and users requests are redirected to the best replica in a transparent way, so that the user perceives better content access service. In this paper we address the problem of dynamic replica placement and user requests redirection jointly. Our approach accounts for users demand variability and server constraints, and minimizes the costs paid by a CDN provider without degrading the quality of the user perceived access service. A non-linear integer programming formulation is given for the replica placement and user request redirection problems. The actual solution is obtained by mapping the non-linear integer problem into a series of mixed integer linear problems obtained by linearizing the non-linear constraints of the original problem. Preliminary numerical results show that the proposed solution is capable of effectively limiting the percentage of unsatisfied requests without over-replicating the contents over the CDN servers.

Autonomous Deployment of Self-Organizing Mobile Sensors for a Complete Coverage

In this paper we propose an algorithm for the autonomous deployment of mobile sensors over critical target areas where sensors cannot be deployed manually. The application of our approach does not require prior knowledge of the working scenario nor any manual tuning of key parameters. Our algorithm is completely distributed and sensors make movement decisions on the basis of locally available information. We prove that our approach guarantees a complete coverage, provided that a sufficient number of sensors are available. Furthermore, we demonstrate that the algorithm execution always terminates preventing movement oscillations. We compare our proposal with one of the most acknowledged algorithms by means of extensive simulations, showing that our algorithm reaches a complete and more uniform coverage under a wide range of operating conditions.

Self-* through self-learning: Overload control for distributed web systems

Overload control is a challenging problem for web-based applications, which are often prone to unexpected surges of traffic. Existing solutions are still far from guaranteeing the necessary responsiveness under rapidly changing operative conditions. We contribute an original self-* overload control (SOC) algorithm that self-configures a dynamic constraint on the rate of incoming new sessions in order to guarantee the fulfillment of the quality requirements specified in a service level agreement (SLA). Our algorithm is based on a measurement activity that makes the system capable of self-learning and self-configuring even in the case of rapidly changing traffic scenarios, dynamic resource provisioning or server faults. Unlike other approaches, our proposal does not require any prior information about the incoming traffic, or any manual configuration of key parameters. We ran extensive simulations under a wide range of operating conditions. The experiments show how the proposed system self-protects from overload, meeting SLA requirements even under intense workload variations. Moreover, it rapidly adapts to unexpected changes in available capacity, as in the case of faults or voluntary architectural adjustments. Performance comparisons with other previously proposed approaches show that our algorithm has better performance and more stable behavior.

Autonomous deployment of heterogeneous mobile sensors

In this paper we address the problem of deploying heterogeneous mobile sensors over a target area. We show how traditional approaches designed for homogeneous networks fail when adopted in the heterogeneous operative setting.