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In this paper, a novel cross-layer Adaptive Modulation and Coding scheme that optimizes the overall packet loss (by both transmission errors and excessive delays) probability under a given arrival process is developed. To this end, an improved Large Deviations approximation for the fraction of packets that suffer from excessive queuing delay is proposed. This approximation is valid for G/G/1 queues with infinite buffers that are driven by stationary arrival and service processes which satisfy certain conditions. Such models can capture the time correlations in the amount of traffic generated by streaming media sources and the time varying service capacity of a wireless link. Through numerical examples, the proposed AMC policy is shown to achieve a significant reduction in the overall packet loss rate compared to previously proposed schemes. This algorithmic performance gain can be translated into a sizeable decrease in the required transmit power or an analogous increase in the rate of the arrival process, subject to a given maximum packet loss rate Quality of Service constraint. Furthermore, the proposed AMC policy can be combined with ARQ in order to achieve an even lower overall packet loss probability. © 2009 Springer Science+Business Media, LLC.


Kachris C.,Athens Information Technology | Kanonakis K.,Athens Information Technology | Tomkos I.,Athens Information Technology
IEEE Communications Magazine | Year: 2013

Warehouse-scale data center operators need much-higher-bandwidth intra-data center networks (DCNs) to sustain the increase of network traffic due to cloud computing and other emerging web applications. Current DCNs based on commodity switches require excessive amounts of power to face this traffic increase. Optical intra-DCN interconnection networks have recently emerged as a promising solution that can provide higher throughput while consuming less power. This article provides an update on recent developments in the field of ultra-highcapacity optical interconnects for intra-DCN communication. Several recently proposed architectures and technologies are examined and compared, while future trends and research challenges are outlined. © 1979-2012 IEEE.


Katrinis K.M.,Athens Information Technology | Tzanakaki A.,Athens Information Technology
IEEE/ACM Transactions on Networking | Year: 2011

Although the problem of dimensioning an optical transport network is not new, the consideration of signal quality degradation caused by the optical medium calls for revisiting the problem in the context of dimensioning optical wavelength division multiplexing (WDM) networks. This paper addresses the issue of minimum-cost planning of long-reach WDM networks in combination with optoelectronic signal regeneration as a countermeasure for sanitizing the signal quality of lightpaths that are found to be impaired. The commonly used method of placing regenerators proportionally to the physical distance covered by a lightpath is evaluated in a realistic dimensioning scenario and for various heterogeneity degrees of optical equipment, showing that it is plagued with a serious tradeoff between efficacy and cost of regeneration. As a remedy, we propose a novel method for design/dimensioning and regeneration placement for WDM networks that employs impairment-awareness. Through experimentation with real optical network configurations and for varying heterogeneity of optical equipment, the proposed method is shown to break the aforementioned tradeoff, resulting in significant reduction in regeneration effort compared to distance-based regeneration. This is achieved without compromising the signal quality of any of the lightpaths selected by the dimensioning process and with increased cost efficiency. © 2010 IEEE.


Konstantinou N.,Athens Information Technology
IEEE Transactions on Communications | Year: 2012

This paper analyzes and proposes Expowave, a distributed algorithm for the scheduling of an RFID reader network. The behavior of the algorithm is presented in detail, and its performance is evaluated through a set of simulation experiments. It is demonstrated that the algorithm constitutes an efficient approach to the reader anti-collision problem, especially in dense and lively environments. © 2012 IEEE.


Christou I.T.,Athens Information Technology | Christou I.T.,Carnegie Mellon University
IEEE Transactions on Pattern Analysis and Machine Intelligence | Year: 2011

We present a novel optimization-based method for the combination of cluster ensembles for the class of problems with intracluster criteria, such as Minimum-Sum-of-Squares-Clustering (MSSC). We propose a simple and efficient algorithmcalled EXAMCEfor this class of problems that is inspired from a Set-Partitioning formulation of the original clustering problem. We prove some theoretical properties of the solutions produced by our algorithm, and in particular that, under general assumptions, though the algorithm recombines solution fragments so as to find the solution of a Set-Covering relaxation of the original formulation, it is guaranteed to find better solutions than the ones in the ensemble. For the MSSC problem in particular, a prototype implementation of our algorithm found a new better solution than the previously best known for 21 of the test instances of the 40-instance TSPLIB benchmark data sets used in [CHECK END OF SENTENCE], [CHECK END OF SENTENCE], and [CHECK END OF SENTENCE], and found a worse-quality solution than the best known only five times. For other published benchmark data sets where the optimal MSSC solution is known, we match them. The algorithm is particularly effective when the number of clusters is large, in which case it is able to escape the local minima found by K-means type algorithms by recombining the solutions in a Set-Covering context. We also establish the stability of the algorithm with extensive computational experiments, by showing that multiple runs of EXAMCE for the same clustering problem instance produce high-quality solutions whose Adjusted Rand Index is consistently above 0.95. Finally, in experiments utilizing external criteria to compute the validity of clustering, EXAMCE is capable of producing high-quality results that are comparable in quality to those of the best known clustering algorithms. © 2011 IEEE.


Kanonakis K.,Athens Information Technology | Giacoumidis E.,Athens Information Technology | Tomkos I.,Athens Information Technology
Journal of Lightwave Technology | Year: 2012

In this paper, a medium access control (MAC) framework tailored for orthogonal frequency-division multiple access passive optical networks (OFDMA-PONs) is defined which allows exploiting both the subcarrier and time domains in a dynamic and flexible way. A dynamic subcarrier assignment algorithm that respects quality of service by offering decreased average delay is proposed. Moreover, effective methods to improve its performance and reduce its complexity are provided. Additionally, the proposed PON MAC framework for the first time leverages on a cross-layer optimization via adaptive quadrature amplitude modulation (QAM) per optical network unit (ONU). The performance of all schemes is evaluated using computer simulations. © 2012 IEEE.


Alrabadi O.N.,University of Aalborg | Perruisseau-Carrier J.,Ecole Polytechnique Federale de Lausanne | Kalis A.,Athens Information Technology
IEEE Transactions on Antennas and Propagation | Year: 2012

An approach for transmitting multiple signals using a single switched parasitic antenna (SPA) has been recently reported. The idea there is to map the signals to be transmitted onto a set of basis functions that serve as "virtual antennas" in the beamspace (i.e., wavevector) domain. In this paper, we generalize the derivation of the antenna pattern basis functions regarding a three-element SPA of arbitrary radiating elements, within a symmetric array topology, for multiplexing signals in the wavevector domain (using different beampatterns) rather than in the hardware antenna domain with multiple feeding ports. A fully operational antenna system example is modeled, optimized regarding its return loss and the power imbalance between the basis functions, and finally realized. The measurements of the SPA show good agreement with the simulated target values, revealing an accurate design approach to be adopted as a fast SPA prototyping methodology. The SPA has been successfully employed for multiplexing two binary phase-shift-keying (BPSK) datastreams over-the-air, thus paving the way for practically compact and highly efficient MIMO transceiver designs. © 2011 IEEE.


Kachris C.,Athens Information Technology | Tomkos I.,Athens Information Technology
IEEE Communications Surveys and Tutorials | Year: 2012

Data centers are experiencing an exponential increase in the amount of network traffic that they have to sustain due to cloud computing and several emerging web applications. To face this network load, large data centers are required with thousands of servers interconnected with high bandwidth switches. Current data center networks, based on electronic packet switches, consume excessive power to handle the increased communication bandwidth of emerging applications. Optical interconnects have gained attention recently as a promising solution offering high throughput, low latency and reduced energy consumption compared to current networks based on commodity switches. This paper presents a thorough survey on optical interconnects for next generation data center networks. Furthermore, the paper provides a qualitative categorization and comparison of the proposed schemes based on their main features such as connectivity and scalability. Finally, the paper discusses the cost and the power consumption of these schemes that are of primary importance in the future data center networks. © 2012 IEEE.


Kanonakis K.,Athens Information Technology | Tomkos I.,Athens Information Technology
IEEE Journal on Selected Areas in Communications | Year: 2010

Two general approaches have been followed for solving the problem of upstream grant scheduling and wavelength assignment in hybrid WDM/TDMA EPON networks, i.e. the offline and the online one. The latter boasts significantly lower frame delay performance in all cases. Nevertheless, we show that simplistic online schemes do not utilize wavelength resources as efficiently as possible, especially in the case of large differential distances of ONUs from the OLT. We propose and analyze several low-and higher-complexity solutions to overcome those inefficiencies, leading to improved utilization of network capacity and reduced frame delay. All schemes are evaluated and compared using computer simulations. © 2010 IEEE.


Christodoulopoulos K.,Trinity College Dublin | Tomkos I.,Athens Information Technology | Varvarigos E.,University of Patras
IEEE Journal on Selected Areas in Communications | Year: 2013

We consider the problem of serving traffic in a spectrum-flexible optical network, where the spectrum allocated to an end-to-end connection can change so as to adapt to the time-varying required transmission rate. In the proposed framework, each connection is assigned a route and is allocated a reference frequency over that route, using an appropriate Routing and Spectrum Allocation (RSA) algorithm, but the spectrum it utilizes around the reference frequency is allowed to expand and contract to match source rate fluctuations. We propose and analyze three spectrum expansion/contraction (SEC) policies for modifying the spectrum allocated to each connection. The first policy, named the Constant Spectrum Allocation (CSA) policy, allocates a number of spectrum slots for exclusive use by each connection. We also present two policies that enable the dynamic sharing of spectrum slots among connections, named the Dynamic High Expansion-Low Contraction (DHL) and the Dynamic Alternate Direction (DAD) policy. We give exact formulas for calculating the blocking probability for a connection and for the whole network under the CSA policy and provide corresponding approximate analyses under the DHL and DAD policies. We also present a simple iterative RSA algorithm that uses the developed blocking models so as to minimize the average blocking of the network. © 2012 IEEE.

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