Mobile sensor networks enable the monitoring of remote and hostile environments without requiring human supervision. Several approaches have been proposed in the literature to let mobile sensors self-deploy over a region of interest. In this paper we study, for the first time, the vulnerabilities of one of the most referenced approaches to mobile sensor deployment, namely the Voronoi-based approach. We show that, by compromising a small number of sensors, an attacker can influence the sensor deployment causing a significant reduction of the monitoring capability of the network. We propose a secure deployment algorithm called SecureVOR. We formally prove that SecureVOR has guaranteed termination and that it allows legitimate sensors to detect the malicious behavior of compromised nodes. We also show by extensive simulations that SecureVOR is able to fulfill the network monitoring goals even in presence of an attack, at the expense of a small performance overhead.

The virtual force approach is at the basis of many solutions proposed for deploying mobile sensors. In this paper we study the vulnerabilities of this approach. We show that by compromising a few mobile sensors, an attacker can influence the movement of other sensors and prevent the achievement of the network coverage goals. We introduce an attack, called opportunistic movement, and give an analytical study of its efficacy. We show that in a typical scenario this attack can reduce coverage by more than 50 percent, by only compromising a 7 percent of the nodes. We propose two algorithms to counteract the above mentioned attack, DRM and SecureVF. DRM is a light-weight algorithm which randomly repositions sensors from overcrowded areas. SecureVF requires a more complex coordination among sensors but, unlike DRM, it enables detection and identification of malicious sensors. We investigate the performance of DRM and SecureVF through simulations. We show that DRM can significantly reduce the effects of the attack, at the expense of an increase in the energy consumption due to additional movements. By contrast, SecureVF completely neutralizes the attack and allows the achievement of the coverage goals of the network even in the presence of localization inaccuracies.

In this paper we point out the vulnerabilities of the virtual force approach to mobile sensor deployment, which is at the basis of many deployment algorithms. For the first time in the literature, we show that some attacks significantly hinder the capability of these algorithms to guarantee a satisfactory coverage. An attacker can compromise a few mobile sensors and force them to pursue a malicious purpose by influencing the movement of other legitimate sensors. We make an example of a simple and effective attack, called Opportunistic Movement, and give an analytical study of its efficacy. We also show through simulations that, in a typical scenario, this attack can reduce coverage by more than 50% by compromising a number of nodes as low as the 7%. We propose SecureVF, a virtual force deployment algorithm able to neutralize the above mentioned attack. We show that under SecureVF malicious sensors are detected and then ignored whenever their movement is not compliant with the moving strategy provided by SecureVF. We also investigate the performance of SecureVF through simulations, and compare it to one of the most acknowledged algorithms based on virtual forces. We show that SecureVF enables a remarkably improved coverage of the area of interest, at the expense of a low additional energy consumption.

Unexpected increases in demand and most of all flash crowds are considered the bane of every Web application as they may cause intolerable delays or even service unavailability. Proper quality of service policies must guarantee rapid reactivity and responsiveness even in such critical situations. Previous solutions fail to meet common performance requirements when the system has to face sudden and unpredictable surges of traffic. Indeed they often rely on a proper setting of key parameters which requires laborious manual tuning, preventing a fast adaptation of the control policies. We contribute an original self-overload control (SOC) policy. This allows the system to self-configure a dynamic constraint on the rate of admitted sessions in order to respect service level agreements and maximize the resource utilization at the same time. Our policy does not require any prior information on the incoming traffic or manual configuration of key parameters. We ran extensive simulations under a wide range of operating conditions, showing that SOC rapidly adapts to time varying traffic and self-optimizes the resource utilization. It admits as many new sessions as possible in observance of the agreements, even under intense workload variations. We compared our algorithm to previously proposed approaches highlighting a more stable behavior and a better performance.

This paper addresses the problems of admission control and server selection in a system consisting of several geographically replicated web servers and several access points. We propose a fully distributed solution in which every access point continuously monitors the availability of all server side resources, using a mixture of active and passive measurements. Based on those measures, each access point autonomously applies its decisions to the requests it receives. Admission control is performed prioritizing requests belonging to already admitted sessions, in order to maximize the chance of successfully terminating ongoing sessions. Furthermore, session information is taken into account when performing a probabilistic request redirection and server choice, in order to improve load balancing and mitigate flash crowd effects. Extensive simulations, performed in compliance with industry standards, show that our method exhibits a stable behavior during overloads and improves service quality in terms of both reduced response time and higher successful session termination.

This paper tackles the problem of autonomic admission control for web clusters. The main contribution of this work is the proposal of a new session admission algorithm that self-configures a dynamic constraint on the rate of incoming new sessions to guarantee the respect of Service Level Agreements (SLA). Unlike other approaches, our policy does not need any prior information on the incoming traffic, nor any assumption on the probability distribution of request inter-arrival or service time. Furthermore, it does not require any manual configuration or parameter tuning. We performed extensive simulations under a range of operating conditions and compared our algorithm to other previously proposed approaches. The simulations show that our policy rapidly adapts to the given traffic profile and improves service throughput while respecting the response time constraints imposed by the SLAs. It also improves service quality by reducing the oscillations of response time and number of active clients common to other policies.

Abstract Store‐and‐forward communication networks may be designed so as to be free from store‐and‐forward deadlock. This is accomplished by incorporating in the network an acyclic buffer graph on which messages are forwarded, from buffer to buffer, according to all the desired routes. A technique producing minimum buffer graphs for a rather large class of networks is illustrated. Successively, a comparison is made with other well‐known deadlock avoidance techniques, showing that substantial savings can be achieved.

In this paper we study the time slot assignment problem in clusters of SS/TDMA satellite systems interconnected through intersatellite links. We show that the problem of finding an assignment which minimizes the total transmission time is NP-complete, i.e., computationally intractable, even for quite restricted intersatellite link patterns and simplified system models. Successively, we focus our attention on clusters of two satellites, proposing a branch-and-bound optimal algorithm and two fast heuristic algorithms. We investigate the performance of the proposed heuristic algorithms both by a theoretical worst case bound and by simulation trials showing that the produced solutions are close to the optimal on the average.

An important parameter related to the efficiency of algorithms for data transmission on SS/TDMA systems is the number of switching modes they generate. It is shown in this note that time-optimal algorithms produce the maximum number of switchings only if the transmission durationLis exponential inN, whereNis the number of channels. This result explains the good performance exhibited by those algorithms in several simulation results.

Two VLSI structures for the computation of the discrete Fourier transform are presented. The first structure is a pipeline working concurrently on different transforms. It is shown that it matches, within a constant factor, the theoretical lower bounds for area versus data rate. The second structure is a simple modification of the first one; it works on a single transform at a time, and it matches within a constant factor the theoretical area-time lower bounds.

In this paper we consider an SS/TDMA system withMuplink beams andNdownlink beams, where uplink beamihas bandwidth βiand downlink beamjhas bandwidth αj. The maximum traffic which can be handled by the satellite (in any given time slot) is assumed to beK. Multiplexing and demuitiplexing are also assumed. An optimal time slot assignment algorithm to minimize the total transmission time for any given traffic demand matrix is proposed and analyzed. Other system configurations of interest are also discussed.

In this paper we analyze various search schemes in a major/minor loop bubble memory. Specifically, we study balanced tree search, one-sided height-balanced tree search, and one-sided K-Keight-balanced tree search. Two parameters are of interest in the present framework, namely, the number of comparisons and the amount of record movement required for a search. One-sided height- balanced tree search seems to offer the best compromise. Other related issues such as insertion and deletions are also discussed.

In this paper we consider an SS/TDMA system withMuplink beams,Ndownlink beams, andK, 1 \leq K \leq \min (M, N)transponders. An optimal time slot assignment algorithm for anyM, N, K,and any traffic matrix is presented, where optimality means achieving the minimal possible total duration for the given traffic matrix. The number of switching matrices generated by the algorithm is bounded above byN^{2} - N + 1forK = M = NandMN + K + 1otherwise. Extensive simulation results on randomly generated matrices are carried out, showing that the average number of switching matrices generated is substantially lower than the bounds.

In this paper we consider an SS/TDMA system withMuplink beams,Ndownlink beams, andKconnection points. We first assume that the downlink is α times faster than the uplink, and a simple time multiplexing scheme is employed. An optimal time slot assignment algorithm for anyM, N,\alpha, andK,1 \leq K \leq \min (M, \alphaN), and for any traffic matrix is presented, where optimality means achieving the minimal possible total transmission time for the given traffic matrix. The number of switching matrices generated by the algorithm never exceedsMN + K\alpha + 1. Extensive simulation results on randomly generated matrices are carried out, showing that the average number of switching matrices generated is substantially lower than the upper bound. The case when the uplink is faster than the downlink is also considered.

A typical organization of magnetic bubble memories is the major-minor loop structure, where n (minor) loops are connected via switches to another (major) loop, containing the I/O station. We work on a sorted file F, split into subfiles allocated in the minor loops.

A new method of description of a straight line defined on a square grid is presented. The discrete equation of the straight line (desl) is introduced as an extension of the classical Cartesian equation, and applies to straight lines quantized on a grid by the grid intersect method. The desl includes a set of intercepts which can be scanned in proper order to generate the chain for the given straight line.