Journal articles

- titre
- Short-Scale Stochastic Solar Energy Models: A Datacenter Use Case
- auteur
- Sara Alouf, Alain Jean-Marie
- article
*Mathematics*, MDPI, 2020, Queue and Stochastic Models for Operations Research, 8 (12), pp.1--26. ⟨10.3390/math8122127⟩- resume
- Modeling the amount of solar energy received by a photovoltaic panel is an essential part of Green IT research. The specific motivation of this work is the management of the energy consumption of large datacenters. We propose a new stochastic model for the solar irradiance, featuring minute-scale variations and therefore suitable for short-term control of performances. Departing from previous models, we use a weather-oriented classification of days obtained from past observations, to parameterize the solar source. We demonstrate through extensive simulations, using real workloads, that our model outperforms the existing ones in predicting performance metrics related to energy storage.
- DOI
- DOI : 10.3390/math8122127
- Accès au texte intégral et bibtex

- titre
- Performance models for hierarchy of caches: Application to modern DNS caches
- auteur
- Sara Alouf, Nicaise Choungmo Fofack, Nedko Nedkov
- article
*Performance Evaluation*, Elsevier, 2016, 97, pp.57-82. ⟨10.1016/j.peva.2016.01.001⟩- resume
- This paper studies expiration-based caching systems in which caches assign a timer to each content they store and redraw the timer upon a cache miss. The modern Domain Name System (DNS) hierarchy is a valid application case and will be used throughout the paper. We introduce analytical models to study expiration-based caching systems based on renewal arguments. For polytree cache networks, we derive the cache performance metrics and characterize at each cache the aggregate request process, the thinning process and the miss process. A constant TTL policy is proved to maximize/minimize the hit probability if the requests' renewal function is concave/convex. We find that no distribution maximizes the hit probability anywhere in a network of caches. We validate our theoretical findings using real DNS traces (single cache and network cases) and via trace-driven simulations (network case).
- DOI
- DOI : 10.1016/j.peva.2016.01.001
- Accès au texte intégral et bibtex

- titre
- Lifetime and availability of data stored on a P2P system: Evaluation of redundancy and recovery schemes
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
*Computer Networks*, Elsevier, 2014, 64 (1), pp.243-260. ⟨10.1016/j.comnet.2014.02.015⟩- resume
- This paper studies the performance of Peer-to-Peer storage and backup systems (P2PSS). These systems are based on three pillars: data fragmentation and dissemination among the peers, redundancy mechanisms to cope with peers churn and repair mechanisms to recover lost or temporarily unavailable data. Usually, redundancy is achieved either by using replication or by using erasure codes. A new class of network coding (regenerating codes) has been proposed recently. Therefore, we will adapt our work to these three redundancy schemes. We introduce two mechanisms for recovering lost data and evaluate their performance by modeling them through absorbing Markov chains. Specifically, we evaluate the quality of service provided to users in terms of durability and availability of stored data for each recovery mechanism and deduce the impact of its parameters on the system performance. The first mechanism is centralized and based on the use of a single server that can recover multiple losses at once. The second mechanism is distributed: reconstruction of lost fragments is iterated sequentially on many peers until that the required level of redundancy is attained. The key assumptions made in this work, in particular, the assumptions made on the recovery process and peer on-times distribution, are in agreement with the analysis in [1] and in [2] respectively. The models are thereby general enough to be applicable to many distributed environments as shown through numerical computations. We find that, in stable environments such as local area or research institute networks where machines are usually highly available, the distributed-repair scheme in erasure-coded systems offers a reliable, scalable and cheap storage/backup solution. For the case of highly dynamic environments, in general, the distributed-repair scheme is inefficient, in particular to maintain high data availability, unless the data redundancy is high. Using regenerating codes overcomes this limitation of the distributed-repair scheme. P2PSS with centralized-repair scheme are efficient in any environment but have the disadvantage of relying on a centralized authority. However, the analysis of the overhead cost (e.g. computation, bandwidth and complexity cost) resulting from the different redundancy schemes with respect to their advantages (e.g. simplicity), is left for future work.
- DOI
- DOI : 10.1016/j.comnet.2014.02.015
- Accès au texte intégral et bibtex

- titre
- Analysis of power save and its impact on web traffic in cellular networks with continuous connectivity
- auteur
- Sara Alouf, Vincenzo Mancuso, Nicaise Choungmo Fofack
- article
*Pervasive and Mobile Computing*, Elsevier, 2012, 8 (5), pp.646-661. ⟨10.1016/j.pmcj.2012.04.001⟩- resume
- In this work, we analyze the power saving and its impact on web traffic performance when customers adopt the continuous connectivity paradigm. To this end, we provide a model for packet transmission and cost. We model each mobile user's traffic with a realistic web traffic profile, and study the aggregate behavior of the users attached to a base station by means of a processor-shared queueing system. In particular, we evaluate user access delay, download time and expected economy of energy in the cell. Our study shows that dramatic energy saving can be achieved by mobile devices and base stations, e.g., as much as 70%-90% of the energy cost in cells with realistic traffic load and the considered parameter settings.
- DOI
- DOI : 10.1016/j.pmcj.2012.04.001
- Accès au texte intégral et bibtex

- titre
- Analysis of power saving with continuous connectivity
- auteur
- Vincenzo Mancuso, Sara Alouf
- article
*Computer Networks*, Elsevier, 2012, 56 (10), pp.2481-2493. ⟨10.1016/j.comnet.2012.03.010⟩- resume
- Always-on mobile users need high bandwidth channels with negligible access delay and limited power consumption. Such a continuous connectivity mode requires the management of high-speed channels, which can turn into substantial operational costs (i.e., power consumption rate) even in presence of low traffic, unless a power saving mechanism is enforced. In this paper, we analyze the impact of 3GPP-defined power saving mechanisms on the performance of users with continuous connectivity. We develop a model for packet transmission and operational costs. We model each downlink mobile user's traffic by means of an M/G/1 queue, and the base station's downlink traffic as an M/G/1 PS queue with multiple classes and inhomogeneous vacations. The model is validated through packet-level simulations. Our results show that consistent power saving can be achieved in the wireless access network, as high as 75% for mobiles and 55% for base stations.
- DOI
- DOI : 10.1016/j.comnet.2012.03.010
- Accès au texte intégral et bibtex

- titre
- Correction to "Analysis and Optimization of Sleeping Mode in WiMAX via Stochastic Decomposition Techniques" [Sep 11 1630-1640]
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman
- article
*IEEE Journal on Selected Areas in Communications*, Institute of Electrical and Electronics Engineers, 2012, 30 (4), pp.846-846. ⟨10.1109/JSAC.2012.120517⟩- resume
- This note is to correct the authorlist and authors affiliations of [1]. The submitted version of this paper that appeared in the September 2011 issue of the IEEE Journal on Selected Areas in Communications was incomplete. The byline and author affiliations should have appeared as in the current correction. References 1. A. P. Azad, "Analysis and optimization of sleeping mode in WiMAX via stochastic decomposition techniques", IEEE Journal on Selected Areas in Communications, vol. 29, no. 8, pp. 1630-1640, September 2011.
- DOI
- DOI : 10.1109/JSAC.2012.120517
- Accès au texte intégral et bibtex

- titre
- Optimal Control of Sleep Periods for Wireless Terminals
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman, Vivek Borkar, Georgios Paschos
- article
*IEEE Journal on Selected Areas in Communications*, Institute of Electrical and Electronics Engineers, 2011, Energy-Efficient Wireless Communications, 29 (8), pp.1605-1617. ⟨10.1109/JSAC.2011.110910⟩- resume
- We consider a mobile connected to a base station, and study how to optimally schedule shutting off its transceiver. First, we study the model from optimal control perspective. We consider off-times (periods of inactivity) of (controlled) duration. We study the question of scheduling "waking up" instants in which the mobile communicates with the base station and checks whether the inactivity period is over. There is a cost proportional to the delay from the moment the off-time ends until the mobile discovers it, a (small) running cost while the mobile is sleeping and a cost for waking up. We present conditions for optimal sleep periods to be constant and derive the optimal period. For the case that the conditions do not hold, we obtain suboptimal solutions which perform strictly better than the optimal constant one. We then investigate optimality restricted to classes of policies with specific constraints. We adopt the parametric optimization approach which entails cost minimization for a given parameterized policy and selection of the best policy among a class. We then compare the performance of optimal policies, of the proposed suboptimal policies as well as that of standard policies like IEEE 802.16e.
- DOI
- DOI : 10.1109/JSAC.2011.110910
- Accès au texte intégral et bibtex

- titre
- Analysis and Optimization of Sleeping Mode in WiMAX via Stochastic Decomposition Techniques
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman
- article
*IEEE Journal on Selected Areas in Communications*, Institute of Electrical and Electronics Engineers, 2011, 29 (8), pp.1630 - 1640. ⟨10.1109/JSAC.2011.110912⟩- resume
- The paper establishes a general approach for analyzing queueing models with repeated inhomogeneous vacations. The server goes on for a vacation if the inactivity prolongs more than the vacation trigger duration. Once the system enters in vacation mode, it may continue for several consecutive vacations, possibly with a {\em different} probability distribution. We study a simple $M/G/1$ queue, which has the advantage of being tractable analytically. The theoretical model is applied to the problem of power saving for mobile devices in which the sleep durations of a device correspond to the server vacations. Various system performance metrics such as the frame response time and the economy of energy are derived. A constrained optimization problem is formulated to maximize the economy of energy in power save mode with QoS constraints. An illustration of the proposed methods is shown with a WiMAX system scenario to obtain design parameters for better performance. Our analysis allows us not only to optimize the system parameters for a given traffic intensity but also to propose parameters that provide the best performance under worst case conditions.
- DOI
- DOI : 10.1109/JSAC.2011.110912
- Accès au texte intégral et bibtex

- titre
- Reducing costs and pollution in cellular networks
- auteur
- Vincenzo Mancuso, Sara Alouf
- article
*IEEE Communications Magazine*, Institute of Electrical and Electronics Engineers, 2011, 49 (8), pp.63-71. ⟨10.1109/MCOM.2011.5978417⟩- resume
- Cellular wireless networks are expected to provide high-quality audio and video services while enabling fast and low-cost Internet access to mobile users. The need for green cost-efficient networks is twofold: reduce the service price and preserve the environment. In this work, we discuss the various strategies that help reduce infrastructure costs, power costs, and greenhouse gas (GHG) emissions with no impairments on the quality of network services. These strategies range over a wide area from enhancing the electronics, to developing new energy-aware radio access protocols, to deploying enhanced base stations with tunable capacity. To reduce both capital and operational expenditures, and the GHG footprint, manufacturers propose new compact installation with lightweight antenna systems, very efficient power amplifiers, and efficient hardware and software. The resulting economy can be up to 50 percent or more by reducing the electricity bill, sparing the use of air conditioning, and deploying compact sites that would seldom require maintenance. Recent scientific publications confirm that a very high gain could be achieved by optimizing the use of base stations proactively, and huge additional improvements could be obtained by optimizing power saving mechanisms by leveraging traffic statistics.
- DOI
- DOI : 10.1109/MCOM.2011.5978417
- Accès au texte intégral et bibtex

- titre
- Fitting genetic algorithms to distributed online evolution of network protocols
- auteur
- Sara Alouf, Giovanni Neglia, Iacopo Carreras, Daniele Miorandi, Álvaro Fialho
- article
*Computer Networks*, Elsevier, 2010, 54 (18), pp.3402-3420. ⟨10.1016/j.comnet.2010.06.015⟩- resume
- In this work, we introduce a framework for enabling the on-line evolution of network protocols. The proposed approach is based on the use of techniques and tools drawn from evolutionary computing research, and it enables embedding evolutionary features in the operation of network protocols. In this way, it becomes possible to build a system in which the operation of the network changes at run-time to adapt to the current conditions. As a case study, we apply the proposed framework to the evolution of forwarding schemes in intermittently connected wireless networks. Simulation results are reported to validate the ability of the proposed scheme to converge to the optimal operating point and to explore the various trade-offs deriving from its design and implementation.
- DOI
- DOI : 10.1016/j.comnet.2010.06.015
- Accès au texte intégral et bibtex

- titre
- Optimal estimation of multicast membership
- auteur
- Sara Alouf, Eitan Altman, Chadi Barakat, Philippe Nain
- article
*IEEE Transactions on Signal Processing*, Institute of Electrical and Electronics Engineers, 2003, Signal Processing in Networking, 51 (8), pp.2165-2176. ⟨10.1109/TSP.2003.814461⟩- resume
- This paper addresses optimal online estimation of the size of a multicast group. Three distinct approaches are used. The first one builds on Kalman filter theory to derive the MSE-optimal estimator in a heavy-traffic regime. Under more general assumptions, the second approach uses Wiener filter theory to compute the MSE-optimal linear filter. The third approach develops the best first-order linear filter from which an estimator that holds for any on-time distribution is derived. Our estimators are tested on real video traces and exhibit good performance. The paper also provides guidelines on how to tune the parameters involved in the schemes in order to achieve high-quality estimation while simultaneously avoiding feedback implosion.
- DOI
- DOI : 10.1109/TSP.2003.814461
- Accès au texte intégral et bibtex

- titre
- Forwarders vs. centralized server: an evaluation of two approaches for locating mobile agents
- auteur
- Sara Alouf, Fabrice Huet, Philippe Nain
- article
*Performance Evaluation*, Elsevier, 2002, Performance 2002, 49 (1-4), pp.299-319. ⟨10.1016/S0166-5316(02)00133-5⟩- resume
- This paper evaluates and compares the performance of two approaches for locating an agent in a mobile agent environment. The first approach dynamically creates a chain of forwarders to locate a moving agent, whereas the second one relies on a centralized server to perform this task. Based on a Markov chain analysis, we compute the performance of each scheme (time to reach an agent, number of forwarders) and compare them first with simulations and second with experimental results obtained by using ProActive, a Java library. Depending on the system parameters we identify the best scheme and observe that over a LAN the server yields the best performance, whereas the forwarders yield the best performance over a MAN.
- DOI
- DOI : 10.1016/S0166-5316(02)00133-5
- Accès au texte intégral et bibtex

Conference papers

- titre
- Stochastic Models for Solar Power
- auteur
- Dimitra Politaki, Sara Alouf
- article
*EPEW 2017 - European Performance Engineering Workshop*, Sep 2017, Berlin, Germany. pp.282--297, ⟨10.1007/978-3-319-66583-2_18⟩- resume
- In this work we develop a stochastic model for the solar power at the surface of the earth. We combine a deterministic model of the clear sky irradiance with a stochastic model for the so-called clear sky index to obtain a stochastic model for the actual irradiance hitting the surface of the earth. Our clear sky index model is a 4-state semi-Markov process where state durations and clear sky index values in each state have phase-type distributions. We use per-minute solar irradiance data to tune the model, hence we are able to capture small time scales fluctuations. We compare our model with the on-off power source model developed by Miozzo et al. (2014) for the power generated by photovoltaic panels, and to a modified version that we propose. In our on-off model the output current is frequently resampled instead of being a constant during the duration of the " on " state. Computing the autocorrelation functions for all proposed models, we find that the irradiance model surpasses the on-off models and it is able to capture the multiscale correlations that are inherently present in the solar irradiance. The power spectrum density of generated trajectories matches closely that of measurements. We believe our irradiance model can be used not only in the mathematical analysis of energy harvesting systems but also in their simulation.
- DOI
- DOI : 10.1007/978-3-319-66583-2_18
- Accès au texte intégral et bibtex

- titre
- Performance Evaluation of Train Moving-Block Control
- auteur
- Giovanni Neglia, Sara Alouf, Abdulhalim Dandoush, Sebastien Simoens, Pierre Dersin, Alina Tuholukova, Jérôme Billion, Pascal Derouet
- article
*13th International Conference on Quantitative Evaluation of SysTems (QEST)*, Aug 2016, Quebec City, Quebec, Canada. pp.348-363, ⟨10.1007/978-3-319-43425-4_23⟩- resume
- In moving block systems for railway transportation a central controller periodically communicates to the train how far it can safely advance. On-board automatic protection mechanisms stop the train if no message is received during a given time window. In this paper we consider as reference a typical implementation of moving-block control for metro and quantify the rate of spurious Emergency Brakes (EBs), i.e. of train stops due to communication losses and not to an actual risk of collision. Such unexpected EBs can happen at any point on the track and are a major service disturbance. Our general formula for the EB rate requires a probabilistic characterization of losses and delays. Calculations are surprisingly simple in the case of homogeneous and independent packet losses. Our approach is computationally efficient even when emergency brakes are very rare (as they should be) and can no longer be estimated via discrete-event simulations.
- DOI
- DOI : 10.1007/978-3-319-43425-4_23
- Accès au texte intégral et bibtex

- titre
- ns-3 Based Framework for Simulating Communication Based Train Control (CBTC) Systems
- auteur
- Abdulhalim Dandoush, Alina Tuholukova, Sara Alouf, Giovanni Neglia, Sebastien Simoens, Pascal Derouet, Pierre Dersin
- article
*Workshop on ns-3 (WNS3 '16)*, Jun 2016, Seattle, WA, United States. pp.116-123, ⟨10.1145/2915371.2915378⟩- resume
- In a Communication Based Train Control System (CBTC), a central zone controller server (ZC) exchanges signaling messages with on-board carborne controllers (CC) inside the trains through a wireless technology. The ZC calculates and sends periodically to each train its Limit of Movement Authority (LMA), i.e. how far the train can proceed. A CC triggers an emergency break (EB) if no message is received within a certain time interval to avoid collision. Clearly, it is not desired to have an EB due to signaling messages losses (called spurious EB) and not to real risks for the trains. Quantifying the rate of spurious EBs and predicting correctly CBTC system performance are hard tasks with important industrial relevance. This work aims at filling this gap using simulation to better predict CBTC system performance and avoid extra provisioning before deployment. A typical CBTC system implementation for metro by Alstom Transport is considered. New ns-3 modules (CBTC protocol, Video traffic generator, multi-channel scanning mechanism, 3D antennas patterns) are developed and a piece of existing code is enhanced. The simulation is also used to investigate the dimension of the radio access networks in a realistic environment (specific modems and access point antennas, radio frequencies, train and track models), another aspect also ignored in the previous literature. Last, our approach can be useful to validate some analytical works.
- DOI
- DOI : 10.1145/2915371.2915378
- Accès au texte intégral et bibtex

- titre
- A Markovian Queueing System for Modeling a Smart Green Base Station
- auteur
- Ioannis Dimitriou, Sara Alouf, Alain Jean-Marie
- article
*EPEW: European Performance Evaluation Workshop*, Aug 2015, Madrid, Spain. pp.3-18, ⟨10.1007/978-3-319-23267-6_1⟩- resume
- We investigate a model to assess the performance of a base station (BS) fully powered by renewable energy sources. The BS is modeled as a three-queue system where two of them are coupled. One represents accumulated energy, the second is the data queue and the third one serves as a reserve energy queue. This smart BS is able to dynamically adjust its coverage area (thereby controlling the traffic intensity) and to generate signals to the reserve energy queue that trigger the movement of energy units to the main energy buffer. Given the randomness of renewable energy supply and the internal traffic intensity control, our queueing model is operated in a finite state random environment. Using the matrix analytic formalism we construct a five-dimensional Markovian model to study the performance of the BS. The stationary distribution of the system state is obtained and key performance metrics are calculated. A small numerical example illustrates the model and a simplified product-form approximation is proposed.
- DOI
- DOI : 10.1007/978-3-319-23267-6_1
- Accès au texte intégral et bibtex

- titre
- Modeling modern DNS caches
- auteur
- Nicaise Choungmo Fofack, Sara Alouf
- article
*VALUETOOLS - 7th International Conference on Performance Evaluation Methodologies and Tools*, Dec 2013, Turin, Italy. pp.184-193, ⟨10.4108/icst.valuetools.2013.254416⟩- resume
- Caching is undoubtedly one of the most popular solution that easily scales up with a world-wide deployment of resources. Records in Domain Name System (DNS) caches are kept for a pre-set duration (time-to-live or TTL) to avoid becoming outdated. Modern caches are those that set locally the TTL regardless of what authoritative servers say. In this paper, we introduce analytic models to study the modern DNS cache behavior based on renewal arguments. For tree cache networks, we derive the cache performance metrics, characterize at each cache the miss process and the aggregate request process. We address the problem of the optimal caching duration and find that constant TTL is the best only if if inter-request times have a concave CDF. We validate our theoretical findings using real DNS traces (single cache case) and via event-driven simulations (network case). Our models are very robust as the relative error between empirical and analytic values stays within 1% in the former case and less than 5% in the latter case.
- DOI
- DOI : 10.4108/icst.valuetools.2013.254416
- Accès au texte intégral et bibtex

- titre
- On the Minimization of Power Consumption in Base Stations using on/off Power Amplifiers
- auteur
- Angelos Chatzipapas, Sara Alouf, Vincenzo Mancuso
- article
*2011 IEEE Online Conference on Green Communications*, Sep 2011, Online, France. pp.18-23, ⟨10.1109/GreenCom.2011.6082501⟩- resume
- Using energy generated with fossil fuel causes global warming due to the greenhouse effect, which threatens our environment. One of the challenges for New Generation Networks (NGN) is then the reduction of energy consumption, in particular at the BSs (Base Stations) which use about 85% of the total network energy. We contribute to the research with a mathematical model that calculates the total power consumption of a BS and enlightens the way to minimize it. First, we analyze the power consumed at every different component of the BS. Second, based on the cost incurred in turning off the BS's power amplifiers, we show how to decide whether it is convenient to keep the BS idle during those intervals in which no traffic has to be sent, or to turn off the amplifiers. Our model is evaluated by means of numerical examples, and shows that interesting power gain can be obtained under a large spectrum of load conditions.
- DOI
- DOI : 10.1109/GreenCom.2011.6082501
- Accès au texte intégral et bibtex

- titre
- Power save analysis of cellular networks with continuous connectivity
- auteur
- Vincenzo Mancuso, Sara Alouf
- article
*2011 IEEE International Symposium on a World of Wireless, Mobile and Multimedia Networks (WoWMoM)*, Jun 2011, Lucca, Italy. ⟨10.1109/WoWMoM.2011.5986202⟩- resume
- In this paper, we analyze the power save and its impact on web traffic performance when customers adopt the continuous connectivity paradigm. To this aim, we provide a model for packet transmission and cost. We model each mobile user's traffic with a realistic web traffic profile, and study the aggregate behavior of the users attached to a base station by means of a processor-shared queueing system. In particular, we evaluate user access delay, download time and expected economy of energy in the cell. The model is validated through packet-level simulations. Our model shows that dramatic energy save can be achieved by both mobile users and base stations, e.g., as much as 70% of the energy cost due to packet transmission at the base station.
- DOI
- DOI : 10.1109/WoWMoM.2011.5986202
- Accès au texte intégral et bibtex

- titre
- Passive Online RTT Estimation for Flow-Aware Routers Using One-Way Traffic
- auteur
- Damiano Carra, Konstantin Avrachenkov, Sara Alouf, Alberto Blanc, Philippe Nain, Georg Post
- article
*NETWORKING 2010*, May 2010, Chennai, India. pp.109-121, ⟨10.1007/978-3-642-12963-6_9⟩- resume
- With the introduction of new generation high speed routers, and with the help of "flow-aware" traffic management, it becomes possible to improve the Quality of Service for users as well as the network efficiency for ISPs. An example of the "flow-aware" traffic management is the Alcatel-Lucent "Semantic Networking" framework where short-lived flows are processed with high priority and long-lived flows are controlled on a per flow basis. In order to control efficiently the flows, it is useful to know an estimate of the Round Trip Time (RTT). In the present work, we provide an online RTT estimation algorithm which is passive and needs one-way traffic only. The one-way traffic requirement is essential for the application of the algorithm for "flow-aware" traffic management inside the network. To the best of our knowledge, there was no online one-way traffic RTT estimators. Tests on a controlled testbed and on the Internet demonstrate high accuracy of the proposed estimator.
- DOI
- DOI : 10.1007/978-3-642-12963-6_9
- Accès au texte intégral et bibtex

- titre
- Optimal sampling for state change detection with application to the control of sleep mode
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman, Vivek Borkar, Georgios Paschos
- article
*48th IEEE Conference on Decision and Control (CDC 2009)*, Dec 2009, Shanghai, China. pp.1645-1650, ⟨10.1109/CDC.2009.5400669⟩- resume
- This work considers systems with inactivity periods of unknown duration. We study the question of scheduling waking up instants in which a server can check whether the inactivity period is over. There is a cost proportional to the delay from the moment the inactivity period ends until the server discovers it, a (small) running cost while the server is away and also a cost for waking up. As an application to the problem, we consider the energy management in WiMax where inactive mobiles reduce their energy consumption by entering a sleep mode. Various standards exist which impose specific waking-up scheduling policies at wireless devices. We check these and identify optimal policies under various statistical assumptions. We show that periodic fixed vacation durations are optimal and derive the optimal period. We show that this structure does not hold for other inactivity distributions but manage to obtain some suboptimal solutions which perform strictly better than the periodic ones. We finally obtain structural properties for optimal policies for the case of arbitrary distribution of inactivity periods.
- DOI
- DOI : 10.1109/CDC.2009.5400669
- Accès au texte intégral et bibtex

- titre
- Vacation policy optimization with application to IEEE 802.16e power saving mechanism
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman, Vivek Borkar, Georgios Paschos
- article
*2nd IFIP Wireless Days (WD 2009)*, Dec 2009, Paris, France. ⟨10.1109/WD.2009.5449688⟩- resume
- Much research has been devoted to optimizing the power saving mechanism in wireless mobile devices. Recent advances in wireless radio technology facilitate the implementation of various possible sleep policies. One basic question that arises is: which policy performs best under a certain condition? Furthermore, what are the optimal parameters for a given policy? To answer these questions, we formulate an optimization problem, which entails cost minimization for a given parameterized policy and selection of the best policy among a class. We propose a cost function which captures the inherent tradeoff of delay and energy saving. This takes into account the cost of response time due to the extra sleep, the energy saving during the sleep, and the cost for periodic waking up (for listening). As an application, we consider IEEE 802.16e's power saving mechanism. We study various practical policies and check their performance. We show that the constant duration policy is optimal for Poisson inactivity periods, but not for hyper-exponentially distributed inactivity periods. In the policy where vacations are i.i.d. exponential random variables, we derive analytically the optimal control as a function of the expected inactivity period. This result holds for general inactivity periods. Our framework allows us to compare the performance of several optimal and suboptimal practical policies with that of the IEEE 802.16e standard.
- DOI
- DOI : 10.1109/WD.2009.5449688
- Accès au texte intégral et bibtex

- titre
- Distributed gradient optimization for epidemic routing: A preliminary evaluation
- auteur
- Giovanni Neglia, Giuseppe Reina, Sara Alouf
- article
*2nd IFIP Wireless Days (WD 2009)*, Dec 2009, Paris, France. ⟨10.1109/WD.2009.5449659⟩- resume
- In this paper we address the problem of designing adaptive epidemic-style forwarding mechanisms for message delivery in Delay Tolerant Networks. Our approach is based on a new analytical framework for multi-agent optimization through distributed subgradient methods. We investigate how this framework can be adapted to the considered networking problem and perform a preliminary evaluation, which shows promising results in terms of convergence speed.
- DOI
- DOI : 10.1109/WD.2009.5449659
- Accès au texte intégral et bibtex

- titre
- Flow Aware Traffic Management
- auteur
- Alberto Blanc, Konstantin Avrachenkov, Sara Alouf, Georg Post
- article
*5th International Workshop on Traffic Management and Traffic Engineering for the Future Internet (EuroNFTraf '09)*, Dec 2009, Paris, France- resume
- TCP congestion control mechanism, while simple and scalable, has several well known limitations: 1) often different flows experience synchronized losses, leading to lower link utilization, and 2) when flows with different Round Trip Times (RTT) share the same bottleneck link, the flows with a smaller RTT will receive a larger share of the capacity. Not only this allocation of capacity is non-optimal in general, but it cannot be modified as long as only drop tail queues and TCP are used. Active Queue Management algorithms aim at improving link utilization by desynchronizing packet losses for different flows, albeit without addressing the fairness issue. Many AQM algorithms have been proposed (like RED, REM, Blue, Stochastic fair blue, just to name a few), but, to the best of our knowledge, none of them is capable of addressing both issues while being easy to configure and supporting different fairness criteria. Another advantage of using AQM algorithms is that routers can use Explicit Congestion Notification, in order to inform a sender that it needs to reduce its sending rate. We propose a new flow-aware traffic management mechanism that aims at addressing the two aforementioned limitations of TCP, while being self-configuring and supporting different fairness criteria. Clearly the fairness criteria has to be
- Accès au texte intégral et bibtex

- titre
- A realistic simulation model for peer-to-peer storage systems
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
*Fourth International ICST Conference on Performance Evaluation Methodologies and Tools (VALUETOOLS '09)*, Oct 2009, Pisa, Italy. ⟨10.4108/ICST.VALUETOOLS2009.7653⟩- resume
- The peer-to-peer (P2P) paradigm have emerged as a cheap, scalable, self-repairing and fault-tolerant storage solution. This solution relies on erasure codes to generate additional redundant fragments of each "block of data" in order to increase the reliability and availability and overcome the churn. When the amount of unreachable fragments attains a predefined threshold, due to permanent departures or long disconnections of peers, a recovery process is initiated to compensate the missing fragments, requiring multiple fragments of data of a given "block" to be downloaded in parallel for an enhanced service. Recent modeling efforts that address the availability and the durability of data have assumed the recovery process to follow an exponential distribution, an assumption made mainly in the absence of studies characterizing the "real" distribution of the recovery process. This work aims at filling this gap and better understanding the behavior of these systems through simulation while taking into consideration the heterogeneity of peers, the underlying network topologies, the propagation delays and the transport protocol. To that end, the distributed storage protocol is implemented in the NS-2 network simulator. This paper describes a realistic simulation model that captures the behavior of P2P storage systems. We provide some experiments results that show how modeling the availability and durability can be impacted by the recovery times distribution which is impacted in turn by the characteristics of the the network and the context.
- DOI
- DOI : 10.4108/ICST.VALUETOOLS2009.7653
- Accès au texte intégral et bibtex

- titre
- Simulation analysis of download and recovery processes in P2P storage systems
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
*21st International Teletraffic Congress (ITC 2009)*, Sep 2009, Paris, France- resume
- Peer-to-peer storage systems rely on data fragmentation and distributed storage. Unreachable fragments are continuously recovered, requiring multiple fragments of data (constituting a ldquoblockrdquo) to be downloaded in parallel. Recent modeling efforts have assumed the recovery process to follow an exponential distribution, an assumption made mainly in the absence of studies characterizing the ldquorealrdquo distribution of the recovery process. This work aims at filling this gap through a simulation study. To that end, we implement the distributed storage protocol in the NS-2 network simulator and run a total of seven experiments covering a large variety of scenarios. We show that the fragment download time follows approximately an exponential distribution. We also show that the block download time and the recovery time essentially follow a hypo-exponential distribution with many distinct phases (maximum of as many exponentials). We use expectation maximization and least square estimation algorithms to fit the empirical distributions. We also provide a good approximation of the number of phases of the hypo-exponential distribution that applies in all scenarios considered. Last, we test the goodness of our fits using statistical (Kolmogorov-Smirnov test) and graphical methods.
- Accès au texte intégral et bibtex

- titre
- Performance analysis of centralized versus distributed recovery schemes in P2P storage systems
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
*International IFIP-TC 6 Networking Conference : Networking 2009*, May 2009, Aachen, Germany. pp.676-689, ⟨10.1007/978-3-642-01399-7_53⟩- resume
- This paper studies the performance of Peer-to-Peer Storage Systems (P2PSS) in terms of data lifetime and availability. Two schemes for recovering lost data are modeled through absorbing Markov chains and their performance are evaluated and compared. The first scheme relies on a centralized controller that can recover multiple losses at once, whereas the second scheme is distributed and recovers one loss at a time. The impact of each system parameter on the performance is evaluated, and guidelines are derived on how to engineer the system and tune its key parameters in order to provide desired lifetime and/or availability of data. We find that, in stable environments such as local area or research laboratory networks where machines are usually highly available, the distributed-repair scheme offers a reliable, scalable and cheap storage/backup solution. This is in contrast with the case of highly dynamic environments, where the distributed-repair scheme is inefficient as long as the storage overhead is kept reasonable. P2PSS with centralized-repair scheme are efficient in any environment but have the disadvantage of relying on a centralized authority. Our analysis also suggests that the use of large size fragments reduces the efficiency of the recovery mechanism.
- DOI
- DOI : 10.1007/978-3-642-01399-7_53
- Accès au texte intégral et bibtex

- titre
- Analysis of an M/G/1 Queue with Repeated Inhomogeneous Vacations with Application to IEEE 802.16e Power Saving Mechanism
- auteur
- Sara Alouf, Eitan Altman, Amar Prakash Azad
- article
*5th International Conference on Quantitative Evaluation of Systems (QEST '08)*, Sep 2008, Saint-Malo, France. pp.27-36, ⟨10.1109/QEST.2008.37⟩- resume
- The goal of this paper is to establish a general approach for analyzing queueing models with repeated in homogeneous vacations. At the end of a vacation, the server goes on another vacation, possibly with a different probability distribution, if during the previous vacation there have been no arrivals. In case there have been one or more arrivals during a vacation then a busy period starts after a warm-up time. In order to get an insight on the influence of parameters on the performance, we choose to study a simple M/G/1 queue (Poisson arrivals and general independent service times) which has the advantage of being tractable analytically. The theoretical model is applied to the problem of power saving for mobile devices in which the sleep durations of a device correspond to the vacations of the server. Various system performance metrics such as the frame response time and the economy of energy are derived. A constrained optimization problem is formulated to maximize the economy of energy achieved in power save mode, with constraints as QoS conditions to be met. An illustration of the proposed methods is shown with a WiMAX system scenario to obtain design parameters for better performance. Our analysis allows us not only to optimize the system parameters for a given traffic intensity but also to propose parameters that provide the best performance under worst case conditions.
- DOI
- DOI : 10.1109/QEST.2008.37
- Accès au texte intégral et bibtex

- titre
- M/G/1 queue with repeated inhomogeneous vacations applied to IEEE 802.16e power saving
- auteur
- Sara Alouf, Eitan Altman, Amar Prakash Azad
- article
*2008 ACM SIGMETRICS International Conference on Measurement and Modeling of Computer Systems*, Jun 2008, Annapolis, Maryland, United States. pp.451-452, ⟨10.1145/1375457.1375516⟩- DOI
- DOI : 10.1145/1375457.1375516
- Accès au texte intégral et bibtex

- titre
- Embedding Evolution in Epidemic-Style Forwarding
- auteur
- Sara Alouf, Iacopo Carreras, Daniele Miorandi, Giovanni Neglia
- article
*IEEE Internatonal Conference on Mobile Adhoc and Sensor Systems (MASS 2007)*, Oct 2007, Pisa, Italy. ⟨10.1109/MOBHOC.2007.4428686⟩- resume
- In this work, we introduce a framework to let forwarding schemes evolve in order to adapt to changing and a priori unknown environments. The framework is inspired by genetic algorithms: at each node a genotype describes the forwarding scheme used, a selection process fosters the diffusion of the fittest genotypes in the system and new genotypes are created by combining existing ones or applying random changes. A case study implementation is presented and its performance evaluated via numerical simulations.
- DOI
- DOI : 10.1109/MOBHOC.2007.4428686
- Accès au texte intégral et bibtex

- titre
- Performance analysis of peer-to-peer storage systems
- auteur
- Sara Alouf, Abdulhalim Dandoush, Philippe Nain
- article
*20th international teletraffic conference on Managing traffic performance in converged networks (ITC '07)*, Jun 2007, Ottawa, Canada. pp.642-653, ⟨10.1007/978-3-540-72990-7_57⟩- resume
- This paper evaluates the performance of two schemes for recovering lost data in a peer-to-peer (P2P) storage systems. The first scheme is centralized and relies on a server that recovers multiple losses at once, whereas the second one is distributed. By representing the state of each scheme by an absorbing Markov chain, we are able to compute their performance in terms of the delivered data lifetime and data availability. Numerical computations are provided to better illustrate the impact of each system parameter on the performance. Depending on the context considered, we provide guidelines on how to tune the system parameters in order to provide a desired data lifetime.
- DOI
- DOI : 10.1007/978-3-540-72990-7_57
- Accès au texte intégral et bibtex

- titre
- Design and Analysis of an Adaptive Backoff Algorithm for IEEE 802.11 DCF Mechanism
- auteur
- Mouhamad Ibrahim, Sara Alouf
- article
*NETWORKING 2006*, May 2006, Coimbra, Portugal. pp.184-196, ⟨10.1007/11753810_16⟩- resume
- This paper presents an adaptive backoff algorithm for the contention-based Distributed Coordination Function (DCF) of the IEEE 802.11 standard. Relying on on-line measurements of the number of sources, the algorithm, called Adaptive BEB, judiciously sets the size of the minimal contention window to adapt to the congestion level in the shared medium. The paper also provides an extension to Adaptive BEB for enhancing its performance over noisy channels. In this extension, a simple EWMA filter is used to derive a Packet Error Rate estimator. The performance evaluation of our proposal is addressed via simulations.
- DOI
- DOI : 10.1007/11753810_16
- Accès au texte intégral et bibtex

- titre
- Quasi-optimal bandwidth allocation for multi-spot MFTDMA satellites
- auteur
- Sara Alouf, Eitan Altman, Jérôme Galtier, Jean-François Lalande, Corinne Touati
- article
*INFOCOM 2005*, Mar 2005, Miami, United States. pp.560-571, ⟨10.1109/INFCOM.2005.1497923⟩- resume
- This paper presents an algorithm for resource allocation in satellite networks. It deals with planning a time/frequency plan for a set of terminals with a known geometric configuration under interference constraints. Our objective is to maximize the system throughput while guaranteeing that the different types of demands are satisfied, each type using a different amount of bandwidth. The proposed algorithm relies on two main techniques. The first generates admissible configurations for the interference constraints, whereas the second uses linear and integer programming with column generation. The obtained solution estimates a possible allocation plan with optimality guarantees, and highlights the frequency interferences which degrade the construction of good solutions.
- DOI
- DOI : 10.1109/INFCOM.2005.1497923
- Accès au texte intégral et bibtex

- titre
- On the dynamic estimation of multicast group sizes
- auteur
- Sara Alouf, Eitan Altman, Chadi Barakat, Philippe Nain
- article
*Sixteenth International Symposium on Mathematical Theory of Networks and Systems (MTNS2004)*, Jul 2004, Leuven, Belgium- resume
- This paper concerns multicast applications that are interested in the evolution of their membership over time. It covers optimal on-line estimation algorithms for determining the membership of a multicast group. The paper briefly reviews the related work and our own contributions to the field. Using a probabilistic acknowledgement scheme and signal processing's filtering techniques, we have derived MSE-optimal estimators under the assumptions of Poisson subscribers arrival and either exponentially or hyperexponentially distributed lifetime of receivers. Our estimators have been tested through trace-driven simulations using data from real multicast video sessions over which they exhibit very good performance.
- Accès au texte intégral et bibtex

- titre
- Estimating membership in a multicast session
- auteur
- Sara Alouf, Eitan Altman, Chadi Barakat, Philippe Nain
- article
*ACM SIGMETRICS international conference on Measurement and modeling of computer systems - 2003*, Jun 2003, San Diego, United States. pp.250-260, ⟨10.1145/781027.781059⟩- resume
- We propose two novel on-line estimation algorithms to determine the size of a dynamic multicast group. We first use a Wiener filter to derive an optimal estimator for the membership size of the session in case the join process is Poisson and the lifetime of participants is distributed exponentially. We next develop the best first-order linear filter from which we derive an estimator that holds for any lifetime distribution. We apply this approach to the case where the lifetime distribution is hyperexponential. Both estimators hold under any traffic regime. Applying both estimators on real traces corresponding to video sessions, we find that both schemes behave well, one of which performs slightly better than the other in some cases. We further provide guidelines on how to tune the parameters involved in both schemes in order to achieve high quality estimation while simultaneously avoiding feedback implosion.
- DOI
- DOI : 10.1145/781027.781059
- Accès au texte intégral et bibtex

- titre
- Optimal on-line estimation of the size of a dynamic multicast group
- auteur
- Sara Alouf, Eitan Altman, Philippe Nain
- article
*Twenty-First Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2002)*, Jun 2002, New York City, New York, United States. pp.1109- 1118, ⟨10.1109/INFCOM.2002.1019359⟩- resume
- In this paper we propose an efficient on-line estimation algorithm for determining the size of a dynamic multicast group. By using diffusion approximation and Kalman filter, we derive an estimator that minimizes the mean square of the estimation error. As opposed to previous studies, where the size of the multicast group is supposed to be fixed throughout the estimation procedure, we consider a dynamic estimation scheme that updates the estimation at every observation step. The robustness of our estimator to violation of the assumptions under which it has been derived is addressed via simulations. Further validations of our approach are carried out on real audio traces.
- DOI
- DOI : 10.1109/INFCOM.2002.1019359
- Accès au texte intégral et bibtex

- titre
- Forwarders vs. centralized server: an evaluation of two approaches for locating mobile agents
- auteur
- Sara Alouf, Fabrice Huet, Philippe Nain
- article
*ACM SIGMETRICS international conference on Measurement and modeling of computer systems - 2002*, Jun 2002, Marina Del Rey, California, United States. pp.278-279, ⟨10.1145/511334.511379⟩- DOI
- DOI : 10.1145/511334.511379
- Accès au texte intégral et bibtex

- titre
- Inferring network characteristics via moment-based estimators
- auteur
- Sara Alouf, Philippe Nain, Don Towsley
- article
*Twentieth Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2001)*, Apr 2001, Anchorage, Alaska, United States. pp.1045-1054, ⟨10.1109/INFCOM.2001.916298⟩- resume
- In this work we develop simple inference models based on finite capacity single server queues for estimating the buffer size and the intensity of cross traffic at the bottleneck link of a path between two hosts. Several pairs of moment-based estimators are proposed to estimate these two quantities. The best scheme is then identified through simulation
- DOI
- DOI : 10.1109/INFCOM.2001.916298
- Accès au texte intégral et bibtex

Book sections

- titre
- Autonomic Information Diffusion in Intermittently Connected Networks
- auteur
- Sara Alouf, Iacopo Carreras, Álvaro Fialho, Daniele Miorandi, Giovanni Neglia
- article
- Mieso K. Denko and Laurence T. Yang and Yan Zhang.
*Autonomic Computing and Networking*, Springer, pp.411-433, 2009, ⟨10.1007/978-0-387-89828-5_17⟩ - resume
- In this work, we introduce a framework for designing autonomic information diffusion mechanisms in intermittently connected wireless networks. Our approach is based on the use of techniques and tools drawn from evolutionary computing research, which enable to embed evolutionary features in epidemic-style forwarding mechanisms. In this way, it is possible to build a system in which information dissemination strategies change at runtime to adapt to the current network conditions in a distributed autonomic fashion. A case study is then introduced, for which design and implementation choices are presented and discussed. Simulation results are reported to validate the ability of the proposed protocol to converge to the optimal operating point (or close to it) in unknown and changing environments.
- DOI
- DOI : 10.1007/978-0-387-89828-5_17
- Accès au texte intégral et bibtex

- titre
- Quasi-Optimal Resource Allocation in Multi-Spot MFTDMA Satellite Networks
- auteur
- Sara Alouf, Eitan Altman, Jérôme Galtier, Jean-François Lalande, Corinne Touati
- article
- Maggie Xiaoyan Cheng and Yingshu Li and Ding-Zhu Du.
*Combinatorial Optimization in Communication Networks*, 18 (18), Kluwer Academic Publishers, pp.325-365, 2006, Combinatorial Optimization, 0-387-29025-7. ⟨10.1007/0-387-29026-5_13⟩ - resume
- This chapter presents an algorithm for resource allocation in satellite networks. It deals with planning a time/frequency plan for a set of terminals with a known geometric configuration under interference constraints. Our objective is to maximize the system throughput while guaranteeing that the different types of demands are satisfied, each type using a different amount of bandwidth. The proposed algorithm relies on two main techniques. The first generates admissible configurations for the interference constraints, whereas the second uses linear and integer programming with column generation. The obtained solution estimates a possible allocation plan with optimality guarantees, and highlights the frequency interferences which degrade the construction of good solutions.
- DOI
- DOI : 10.1007/0-387-29026-5_13
- Accès au texte intégral et bibtex

Directions of work or proceedings

- titre
- Actes du 10ème Atelier en Évaluation de Performances
- auteur
- Sara Alouf, Alain Jean-Marie
- article
- Alouf, Sara; Jean-Marie, Alain. Jun 2014, Sophia Antipolis, France. pp.36, 2014
- resume
- L'Atelier en Évaluation de Performances est une réunion destinée à faire s'exprimer et se rencontrer les jeunes chercheurs (doctorants et nouveaux docteurs) dans le domaine de la Modélisation et de l'Évaluation de Performances, une discipline consacrée à l'étude et l'optimisation de systèmes dynamiques stochastiques et/ou temporisés apparaissant en Informatique, Télécommunications, Productique et Robotique entre autres. La présentation informelle de travaux, même en cours, y est encouragée afin de renforcer les interactions entre jeunes chercheurs et préparer des soumissions de nouveaux projets scientifiques. Des exposés de synthèse sur des domaines de recherche d'actualité, donnés par des chercheurs confirmés du domaine renforcent la partie formation de l'atelier.
- Accès au texte intégral et bibtex

Habilitation à diriger des recherches

- titre
- From Wireless Networks to Green IT - A Journey Through Stochastic Models and Tools
- auteur
- Sara Alouf
- article
- Networking and Internet Architecture [cs.NI]. Université Côte D'Azur, 2017
- resume
- This manuscript recollects some of my contributions since my PhD defense. These were achieved while I was a researcher at Inria Sophia Antipolis Méditerranée in the Project-Team Maestro. My research activities focus on the modeling and performance evaluation of networks. In fact, this term encompasses a wide variety of situations. One may consider a particular layer in the protocols stack like the access protocol to the communication channels, or focus on the application layer and study overlay networks or cache networks. As a researcher, I first became interested in controlling the access to the medium in wireless networks. Subsequently, I considered on one hand the power save mode of mobile devices and on the other hand the problem of evolutionary routing in networks with reduced connectivity. After studying the energy savings at mobile terminals, I turned my focus on the energy consumption of base stations in cellular networks and on the use of renewable energy sources for their electrical power supply. This line of research has led to the stochastic modeling of solar radiation in order to better take into account renewable energy sources in the performance evaluation of communications networks. Meanwhile, I developed a second line of research at the application level of communications systems. I was first interested in peer-to-peer storage systems. I then studied hierarchical networks of caches such as that of the Domain Name System (DNS). I have also done research work in the framework of partnership projects with private companies. I contributed to a study on the active management of flows at the core of the network (project with the former Alcatel-Lucent Bell Labs, now Nokia) and to the performance evaluation of urban train control based on wireless communications (project with Alstom Transport).
- Accès au texte intégral et bibtex

Patents

- titre
- Binary Search Method And System For Congestion Avoidance
- auteur
- Alberto Blanc, Sara Alouf, Konstantin Avrachenkov, Georg Post
- article
- France, Patent n° : EP2413543. 2010
- resume
- A method for packet flow rate control within a network node (4) receiving at least a traffic flow from a sender (1), comprising the following steps: - computing the average rate at the end of a cycle comprising an initial value of packet number; - updating the value of packet number per cycle in function of the computed average rate with respect to a target flow rate.
- Accès au bibtex

- titre
- Flow Aware Congestion Avoidance Method and System
- auteur
- Alberto Blanc, Sara Alouf, Konstantin Avrachenkov, Georg Post
- article
- France, Patent n° : EP2413542. 2010
- resume
- A method for packet flow-rate control within a network node (4) comprised within a packet-switched communication network connection linking a sender (1) to a receiver (2), the said method comprising the following steps - computing the average arrival rate of packets since the last packet mark or drop event; - sending a congestion signal to the sender as soon as the computed packet arrival rate is above a predefined target rate; - repeating congestion signal sending until the sender receives at least a congestion signal and consequently reduces the sending rate.
- Accès au bibtex

- titre
- Method for Estimating a Round Trip Time of a Packet Flow
- auteur
- Damiano Carra, Konstantin Avrachenkov, Sara Alouf, Philippe Nain, Georg Post
- article
- France, Patent n° : EP2226971, WO/2010/100151. 2009
- resume
- A method for estimating a round trip time, RTT, of a packet flow, comprising the steps of: - computing a periodogram P N based on N samples h 1 , h 2 ..h N of an inter-arrival time signal X between adjacent packets, X(t k ) = h k = t k - t k-1 , using a Lomb-Scargle method, - extracting a fundamental pulsation É 0 out of said periodogram P N using a pattern matching technique, - determining the RTT out of said fundamental pulsation.
- Accès au bibtex

Poster communications

- titre
- Performance Evaluation of Train Moving-Block Control
- auteur
- Giovanni Neglia, Sara Alouf, Abdulhalim Dandoush, Sebastien Simoens, Pierre Dersin, Alina Tuholukova, Jérôme Billion, Pascal Derouet
- article
*Reliability, Safety and Security of Railway Systems*, Jun 2016, Paris, France. 2016- resume
- In this work we provide a model-based analysis of the moving block control and quantify the rate of spurious emergency brakes (EBs). We consider as a reference a typical implementation for metro by Alstom Transport. We derive the general formula for the EB rate, that requires to provide the loss and delay model. The delay model considers processing delays, computation times and communication delays. For the loss model we start with the case when packet losses are independent and homogeneous. By developing the general formula we derive an exact expression for the EB rate. We proceed then with a more elaborate loss model, when losses are no longer independent.As it becomes hard to derive a closed-form expression for the EB rate, we evaluate it using efficient Monte Carlo simulations. The theoretical results are validated via discrete-event simulations, including simulations with ns-3 for which we have developed additional modules for train systems.
- Accès au texte intégral et bibtex

Reports

- titre
- Performance Evaluation of Train Moving-Block Control
- auteur
- Giovanni Neglia, Sara Alouf, Abdulhalim Dandoush, Sebastien Simoens, Pierre Dersin, Alina Tuholukova, Jérôme Billion, Pascal Derouet
- article
- [Research Report] RR-8917, Inria Sophia Antipolis. 2016
- resume
- In moving block systems for railway transportation a central controller periodically communicates to the train how far it can safely advance. On-board automatic protection mechanisms stop the train if no message is received during a given time window. In this report we consider as reference a typical implementation of moving-block control for metro and quantify the rate of spurious Emergency Brakes (EBs), i.e.~of train stops due to communication losses and not to an actual risk of collision. Such unexpected EBs can happen at any point on the track and are a major service disturbance. Our general formula for the EB rate requires a probabilistic characterization of losses and delays. We derive an exact formula for the case of homogeneous and independent packet losses and we use the results of this analysis to design an efficient Monte Carlo method that takes into account correlated losses due to handovers. We validate our approach via discrete-event simulations, including simulations with ns-3 for which we have developed additional modules for train systems. Our approach is computationally efficient even when emergency brakes are very rare (as they should be) and can no longer be estimated via discrete-event simulations.
- Accès au texte intégral et bibtex

- titre
- Non-renewal TTL-based Cache Replacement Policy and Applications: Case of Modern DNS Hierarchy
- auteur
- Nicaise Choungmo Fofack, Sara Alouf
- article
- [Research Report] RR-8414, INRIA. 2013
- resume
- Caching is undoubtedly one of the most popular solution that easily scales up with a world-wide deployment of resources. Records in Domain Name System (DNS) caches are kept for a pre-set duration (time-to-live or TTL) to avoid becoming outdated. "Modern" caches are those that set locally the TTL regardless of what authoritative servers say. In this report, we introduce analytic models to study the modern DNS cache behavior based on renewal arguments. For both the single cache case and the network of caches case, we derive the cache performance metrics and characterize at each cache the miss process and the aggregate request process. We address the problem of the optimal caching duration and find that if inter-request times have a concave CDF, then the deterministic policy is the best. We validate our single cache model using real DNS traces and our network of caches model using event-driven simulations. Our models consider general caching durations and are tested with deterministic, hypo-exponential, exponential and hyper-exponential distributions. Our models are very robust as the relative error between empirical and analytic values stays within 1% in the first case and within 5% at the highest cache level in the network case. Our models successfully predict the CDF of the miss process even when the renewal assumption is not met.
- Accès au texte intégral et bibtex

- titre
- Optimal control of sleep periods for wireless terminals
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman, V. S. Borkar, Georgios Stavrou Paschos
- article
- [Research Report] 2011
- resume
- We consider a mobile connected to a base station, saving energy by shutting off its transceiver, and we try to answer the fundamental question: what is the optimal sleep policy? Firstly, we study the model from optimal control perspective. We consider off-times (periods of inactivity) of unknown duration. We study the question of scheduling ''waking up'' instants in which the mobile communicates with the base station and checks whether the inactivity period is over. There is a cost proportional to the delay from the moment the off-time ends until the mobile discovers it, a (small) running cost while the mobile is sleeping and also a cost for waking up. We show that constant sleep periods are optimal for Poisson arrivals and derive the optimal period. We show that this structure does not hold for other off-time distributions but manage to obtain suboptimal solutions which perform strictly better than the constant one. We finally obtain structural properties for optimal policies for the case of arbitrary distribution of off-times. Technological restrictions often permit a limited set of policies to be implemented. Motivated by this, we investigate classes of policies with specific constraints. What is the optimal policy within each class and what are the optimal parameters for it? To answer these questions, we adopt the parametric optimization approach which entails cost minimization for a given parametrized policy and selection of the best policy among a class. We provide the optimal solution for each class which can be used in closed form or evaluated numerically depending on the case. Our framework allows us to compare the performance of obtained optimal policies, proposed suboptimal policies as well as that of standard policies like IEEE 802.16e.
- Accès au texte intégral et bibtex

- titre
- Lifetime and availability of data stored on a P2P system: Evaluation of recovery schemes
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
- [Research Report] RR-7170, INRIA. 2010, pp.37
- resume
- This report studies the performance of Peer-to-Peer storage and backup systems (P2PSS). These systems are based on three pillars: data fragmentation and dissemination among the peers, redundancy mechanisms to cope with peers churn, and repair mechanisms to recover lost or temporarily unavailable data. Usually, redundancy is achieved either by using replication or by using erasure codes. A new class of network coding (regenerating codes) has been proposed recently. Therefore, we will adapt our work to these three redundancy schemes. We introduce two mechanisms for recovering lost data and evaluate their performance by modeling them through absorbing Markov chains. Specifically, we evaluate the quality of service provided to users in terms of durability and availability of stored data for each recovery mechanism and deduce the impact of its parameters on the system performance. The first mechanism is centralized and based on the use of a single server that can recover multiple losses at once. The second mechanism is distributed: reconstruction of lost fragments is iterated sequentially on many peers until the required level of redundancy is attained. The key assumptions made in this work, in particular, the assumptions made on the recovery process and peer on-times distribution, are in agreement with the analysis in [11] and in [20] respectively. The models are thereby general enough to be applicable to many distributed environments as shown through numerical computations. We find that, in stable environments such as local area or research institute networks where machines are usually highly available, the distributed-repair scheme in erasure-coded systems offers a reliable, scalable and cheap storage/backup solution. For the case of highly dynamic environments, in general, the distributed-repair scheme is inefficient, in particular to maintain high data availability, unless the data redundancy is high. Using regenerating codes overcomes this limitation of the distributed-repair scheme. P2PSS with centralized-repair scheme are efficient in any environment but have the disadvantage of relying on a centralized authority. However, the analysis of the overhead cost (e.g. computation, bandwidth cost and complexity) resulting from the different redundancy schemes with respect to their advantages (e.g. simplicity), is left for future work.
- Accès au texte intégral et bibtex

- titre
- Sleep Mode Analysis via Workload Decomposition
- auteur
- Amar Prakash Azad, Sara Alouf, Eitan Altman
- article
- [Research Report] INRIA. 2010, pp.23
- resume
- The goal of this paper is to establish a general approach for analyzing queueing models with repeated inhomogeneous vacations. The server goes on for a vacation if the inactivity prolongs more than a vacation trigger duration. Once the system enters in vacation mode, it may continue for several consecutive vacation. At the end of a vacation, the server goes on another vacation, possibly with a {\em different} probability distribution, if during the previous vacation there have been no arrivals. However the system enters in vacation mode only if the inactivity is persisted beyond a defined trigger duration. In order to get an insight on the influence of parameters on the performance, we choose to study a simple $M/G/1$ queue (Poisson arrivals and general independent service times) which has the advantage of being tractable analytically. The theoretical model is applied to the problem of power saving for mobile devices in which the sleep durations of a device correspond to the vacations of the server. Various system performance metrics such as the frame response time and the economy of energy are derived. A constrained optimization problem is formulated to maximize the economy of energy achieved in power save mode, with constraints as QoS conditions to be met. An illustration of the proposed methods is shown with a WiMAX system scenario to obtain design parameters for better performance. Our analysis allows us not only to optimize the system parameters for a given traffic intensity but also to propose parameters that provide the best performance under worst case conditions.
- Accès au texte intégral et bibtex

- titre
- Optimal Sampling for State Change Detection with Application to the Control of Sleep Mode
- auteur
- Amar Azad, Sara Alouf, Eitan Altman, Vivek Borkar, Georgios Paschos
- article
- [Research Report] RR-7026, INRIA. 2009
- resume
- This work considers systems with inactivity periods of unknown duration during which the server goes on vacation. We study the question of scheduling ''waking up'' instants in which a server can check whether the inactivity period is over. There is a cost proportional to the delay from the moment the inactivity period ends until the server discovers it, a (small) running cost while the server is away and also a cost for waking up. As an application to the problem, we consider the energy management in WiMax where inactive mobiles reduce their energy consumption by entering a sleep mode. Various standards exist which impose specific waking-up scheduling policies at wireless devices. We check these and identify optimal policies under various statistical assumptions. We show that periodic fixed vacation durations are optimal for Poisson arrivals and derive the optimal period. We show that this structure does not hold for other inactivity distributions but manage to obtain some suboptimal solutions which perform strictly better than the periodic ones. We finally obtain structural properties for optimal policies for the case of arbitrary distribution of inactivity periods.
- Accès au texte intégral et bibtex

- titre
- Vacation Policy Optimization with Application to IEEE 802.16e Power Saving Mechanism
- auteur
- Amar Azad, Sara Alouf, Eitan Altman, Vivek Borkar, Georgios Paschos
- article
- [Research Report] RR-7017, INRIA. 2009
- resume
- Much research has been devoted to optimizing the power saving mechanism in wireless mobile devices. Recent advances in wireless radio technology facilitate the implementation of various possible sleep policies. One basic question that arises is: which policy performs best under a certain condition? Furthermore, what are the optimal parameters for a given policy? To answer these questions, we formulate an optimization problem, which entails cost minimization for a given parameterized policy and selection of the best policy among a class. We propose a cost function which captures the inherent tradeoff of delay and energy saving. This takes into account the cost of response time due to the extra sleep, the energy saving during the sleep, and the cost for periodic waking up (for listening). As an application, we consider IEEE 802.16e's power saving mechanism. We study various practical policies and check their performance. We show that the constant duration policy is optimal for Poisson inactivity periods, but not for hyper-exponentially distributed inactivity periods. In the policy where vacations are i.i.d. exponential random variables, we derive analytically the optimal control as a function of the expected inactivity period. This result holds for general inactivity periods. Our framework allows us to compare the performance of several optimal and suboptimal practical policies with that of the IEEE 802.16e standard.
- Accès au texte intégral et bibtex

- titre
- Passive Online RTT Estimation for Flow-Aware Routers using One-Way Traffic
- auteur
- Damiano Carra, Konstantin Avrachenkov, Sara Alouf, Alberto Blanc, Philippe Nain, Georg Post
- article
- [Research Report] RR-7124, INRIA. 2009
- resume
- With the introduction of the new generation high speed routers, it becomes possible to improve the Quality of Service, the Quality of Experience for users and the network efficiency for ISPs with the help of "flow-aware" traffic management. An example of the "flow-aware" traffic management is the Alcatel-Lucent framework "Semantic Networking," where short-lived and long-lived TCP flows are treated differently. Short-lived flows are processed with high priority and long-lived flows are controlled in a "flow-aware" fashion. To control efficiently the long-lived flows, one needs to know an estimation of the Round Trip Time (RTT). In the present work, we provide an online RTT estimation algorithm which is passive and can deal with a one-way traffic. The one-way traffic requirement is essential for the application of the algorithm for "flow-aware" traffic management inside the network. To the best of our knowledge, there was no online one-way traffic RTT estimators. Tests on the Internet demonstrate high accuracy of the proposed estimator. The results show that, 75% (resp. 99%) of the time, the RTT estimation is within 10% (resp. 20%) of the RTT at the source.
- Accès au texte intégral et bibtex

- titre
- Simulation Analysis of Download and Recovery Processes in P2P Storage Systems
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
- [Research Report] RR-6858, INRIA. 2009, pp.19
- resume
- Peer-to-peer storage systems rely on data fragmentation and distributed storage. Unreachable fragments are continuously recovered, requiring multiple fragments of data (constituting a "block") to be downloaded in parallel. Recent modeling efforts have assumed the recovery process to follow an exponential distribution, an assumption made mainly in the absence of studies characterizing the "real" distribution of the recovery process. This report aims at filling this gap through an empirical study. To that end, we implement the distributed storage protocol in the NS-2 network simulator and run a total of six experiments covering a large variety of scenarios. We show that the fragment download time follows approximately an exponential distribution. We also show that the block download time and the recovery time essentially follow a hypo-exponential distribution with many distinct phases (maximum of as many exponentials). We use expectation maximization and least square estimation algorithms to fit the empirical distributions. We also provide a good approximation of the number of phases of the hypo-exponential distribution that applies in all scenarios considered. Last, we test the goodness of our fits using statistical (Kolmogorov-Smirnov test) and graphical methods.
- Accès au texte intégral et bibtex

- titre
- Distributed Gradient Optimization for Epidemic Routing: a preliminary Evaluation
- auteur
- Giovanni Neglia, Giuseppe Reina, Sara Alouf
- article
- [Research Report] RR-7016, INRIA. 2009
- resume
- In this research report we address the problem of designing adaptive epidemic-style forwarding mechanisms for message delivery in Delay Tolerant Networks. Our approach is based on a new analytical framework for multi-agent optimization through distributed subgradient methods. We investigate how this framework can be adapted to the considered networking problem and we perform a preliminary evaluation, which shows promising results in terms of convergence speed.
- Accès au texte intégral et bibtex

- titre
- Analysis of an M/G/1 queue with repeated inhomogeneous vacations -- Application to IEEE 802.16e power saving
- auteur
- Sara Alouf, Eitan Altman, Amar Prakash Azad
- article
- [Research Report] RR-6488, INRIA. 2008, pp.31
- resume
- This report presents a method for analyzing a queueing model with repeated inhomogeneous vacations. At the end of a vacation, the server goes on another vacation, possibly with a different probability distribution, if during the previous vacation there have been no arrivals. In order to get an insight on the influence of parameters on the performance, we choose to study a simple M/G/1 queue (Poisson arrivals and general independent service times) which has the advantage of being tractable analytically. The theoretical model is applied to the problem of power saving for mobile devices in which the sleep durations of a device correspond to the vacations of the server. Various system performance metrics such as the frame response time and the economy of energy are derived. A constrained optimization problem is formulated to maximize the economy of energy achieved in power save mode, with constraints as QoS conditions to be met. An illustration of the proposed methods is shown with a WiMAX system scenario to obtain design parameters for better performance. Our analysis allows us not only to optimize the system parameters for given traffic intensity but also to propose parameters that provide the best performance under worst case conditions.
- Accès au texte intégral et bibtex

- titre
- Performance Analysis of Centralized versus Distributed Recovery Schemes in P2P Storage Systems
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
- [Research Report] RR-6769, INRIA. 2008, pp.14
- resume
- This report studies the performance of Peer-to-Peer Storage Systems (P2PSS) in terms of data lifetime and availability. Two schemes for recovering lost data are modeled through absorbing Markov chains and their performance are evaluated and compared. The first scheme relies on a centralized controller that can recover multiple losses at once, whereas the second scheme is distributed and recovers one loss at a time. The impact of each system parameter on the performance is evaluated, and guidelines are derived on how to engineer the system and tune its key parameters in order to provide desired lifetime and/or availability of data. We find that, in stable environments such as local area or research laboratory networks where machines are usually highly available, the distributed-repair scheme offers a reliable, scalable and cheap storage/backup solution. This is in contrast with the case of highly dynamic environments, where the distributed-repair scheme is inefficient as long as the storage overhead is kept reasonable. P2PSS with centralized-repair scheme are efficient in any environment but have the disadvantage of relying on a centralized authority. Our analysis also suggests that the use of large size fragments reduces the efficiency of the recovery mechanism.
- Accès au texte intégral et bibtex

- titre
- Evolutionary Epidemic Routing
- auteur
- Sara Alouf, Iacopo Carreras, Daniele Miorandi, Giovanni Neglia
- article
- [Research Report] RR-6140, INRIA. 2007, pp.19
- resume
- We introduce a framework which allows forwarding schemes to evolve in order to adapt to changing and a priori unknown environments. The framework is inspired by genetic algorithms: at each node a genotype describes the forwarding scheme used, a selection process fosters the diffusion of the fittest genotypes in the system and new genotypes are created by combining existing ones or applying random changes. This framework is illustrated through a simple case study and simulations are undertaken to evaluate its performance.
- Accès au texte intégral et bibtex

- titre
- P2P storage systems modeling, analysis and evaluation
- auteur
- Abdulhalim Dandoush, Sara Alouf, Philippe Nain
- article
- [Research Report] RR-6392, INRIA. 2007, pp.30
- resume
- This Report characterizes the performance of peer-to-peer storage systems in terms of the delivered data lifetime and data availability. Two schemes for recovering lost data are modeled and analyzed: the first is centralized and relies on a server that recovers multiple losses at once, whereas the second is distributed and recovers one loss at a time. For each scheme, we propose a basic Markovian model where the availability of peers is exponentially distributed, and a more elaborate model where the latter is hyper-exponentially distributed. Our models equally apply to many distributed environments as shown through numerical computations. These allow to assess the impact of each system parameter on the performance. In particular, we provide guidelines on how to tune the system parameters in order to provide desired lifetime and/or availability of data. One important outcome of our analysis is that a simplifying exponential assumption on the peers availability leads to incorrect evaluation of the performance achieved. Thereby, the more elaborate model is necessary to capture the true behavior of peer-to-peer storage systems
- Accès au texte intégral et bibtex

- titre
- Performance Analysis of Peer-to-Peer Storage Systems
- auteur
- Sara Alouf, Abdulhalim Dandoush, Philippe Nain
- article
- [Research Report] RR-6044, INRIA. 2006, pp.15
- resume
- This report evaluates and compares the performance of two schemes for recovering lost data in a peer-to-peer (P2P) storage systems. The first scheme is centralized and relies on a server that recovers multiple losses at once, whereas the second one is distributed. By representing the state of each scheme by an absorbing Markov chain, we are able to compute their performance in terms of the delivered data lifetime and data availability. Numerical computations are provided to better illustrate the impact of each system parameter on the performance. Depending on the context considered, we provide guidelines on how to tune the system parameters in order to provide a desired data lifetime.
- Accès au texte intégral et bibtex

- titre
- Un algorithme d'allocation de bande passante satellitaire
- auteur
- Sara Alouf, Eitan Altman, Jérôme Galtier, Jean-François Lalande, Corinne Touati
- article
- [Rapport de recherche] RR-5172, INRIA. 2004
- resume
- Ce rapport présente un algorithme d'allocation de ressources pour les réseaux satellitaires. Il s'agit de prévoir un plan d'allocation en temps/fréquence pour un ensemble de terminaux ayant une configuration géométrique définie et soumis à des contraintes d'interférence. On cherche à minimiser la taille du plan de fréquences tout en garantissant que toutes les demandes des terminaux, en termes de bande passante et pour différents types, sont satisfaites. L'algorithme proposé repose sur deux techniques principales: la génération de configurations admissibles pour les contraintes d'interférence par des heuristiques, la programmation mixte linéaire/entière utilisant la génération de colonnes. La solution obtenue permet de prévoir un plan d'allocation admissible avec des garanties d'optimalité et permet aussi de mettre en évidence les configurations d'interférences qui entravent la génération de bonnes solutions.
- Accès au texte intégral et bibtex

- titre
- Forwarders vs. Centralized Server : An Evaluation of two Approaches for Locating Mobile Agents
- auteur
- Sara Alouf, Fabrice Huet, Philippe Nain
- article
- RR-4440, INRIA. 2002
- resume
- This paper evaluates and compares the performance of two approaches for locating an agent (e.g. a code) in a mobile agent environment. The first approach dynamically creates a chain of forwarders to locate a moving agent whereas the second one relies on a centralized server to perform this task. Based on a Markov chain analysis, we compute the performance of each scheme (time to reach an agent, number of forwarders) and compare them first with simulations and second with experimental results obtained by using ProActive, a Java library. Depending on the system parameters we identify the best scheme and observe that within a LAN the server yields the best performan- ce whereas the forwarders yield the best performance within a MAN.
- Accès au texte intégral et bibtex

- titre
- Estimating Membership in a Multicast Session
- auteur
- Sara Alouf, Eitan Altman, Chadi Barakat, Philippe Nain
- article
- RR-4391, INRIA. 2002
- resume
- We propose two novel on-line estimation algorithms to determine the size of a dynamic multicast group. We first use a Wiener filter to derive an optimal estimator for the membership size of the session in case the join process is Poisson and the lifetime of participants is distributed exponentially. We next develop the best first-order linear filter from which we derive an estimator that holds for any lifetime distribution. We apply this approach to the case where the lifetime distribution is hyperexponential. Both estimators hold under any traffic regime. Applying both estimators on real video traces, we find that both schemes behave well, one of which performs slightly better than the other in some cases. We further provide guidelines on how to tune the parameters involved in both schemes in order to achieve high quality estimation while simultaneously avoiding feedback implosion.
- Accès au texte intégral et bibtex

- titre
- Optimal On-Line Estimation of the Size of a Dynamic Multicast Group
- auteur
- Sara Alouf, Eitan Altman, Philippe Nain
- article
- RR-4329, INRIA. 2001
- resume
- In this paper we propose an optimal on-line estimation algorithm for determini- ng the size of a dynamic multicast group. By using diffusion approximation and Kalman filter, we derive an estimator that minimizes the mean square of the estimation error. As opposed to previous studies, where the size of the multicast group is supposed to be fixed throughout the estimation procedure, we consider a dynamic estimation scheme that updates the estimation at every observation step. The robustness of our estimator to violation of the assumptions under which it has been derived is addressed via simulation- s. Further validations of our approach are carried out on real audio traces.
- Accès au texte intégral et bibtex

Theses

- titre
- Parameter estimation and performance analysis of several network applications
- auteur
- Sara Alouf
- article
- Networking and Internet Architecture [cs.NI]. Univeristé Nice Sophia Antipolis, 2002. English
- resume
- In this thesis, we have looked at several modeling problems to estimate some parameters or to evaluate the performance of some mechanisms. The first subject studied is about unicast applications performing rate control to adapt to network conditions. To that end, we have proposed to estimate the cross traffic intensity and the buffer size at the bottleneck of a connection. We have developed two inference models based on the $M/M/1/K$ and $M/D/1/K$ queues, and derived eleven schemes for estimating the above-mentioned characteristics. The performance of these schemes have been evaluated and compared using ns-2 simulations, and the best scheme has been identified. The second subject investigated is about the on-line estimation of the membership of dynamic multicast groups. We first model the multicast group by an $M/M/\infty$ queue and show that under heavy traffic the estimation problem can be solved using a Kalman filter. We next use the same model, but with a general traffic regime, and derive the estimator using Wiener filter theory. We last design an efficient estimator using the $M/H_2/\infty$ queue model. The performance of these estimators have been evaluated over simulations driven by both synthetic and real session traces. The third subject studied is about location mechanisms in a mobile agent environment. There are two widely used mechanisms to ensure communications between components of applications. We have proposed Markovian models to evaluate the cost of these mechanisms in terms of response times. We have validated our models both via simulations and experiments over a LAN and a MAN. We were then able to use the models to formally compare the performance of both mechanisms.
- Accès au texte intégral et bibtex