On improving the performance of optimistic distributed simulations

This report investigates means of improving the performance of optimistic distributed simulations without affecting the simulation accuracy. We argue that existing clustering algorithms are not adequate for application in distributed simulations, and outline some characteristics of an ideal algorithm that could be applied in this field. This report is structured as follows. We start by introducing the area of distributed simulation. Following a comparison of the dominant protocols used in distributed simulation, we elaborate on the current approaches of improving the simulation performance, using computation efficient techniques, exploiting the hardware configuration of processors, optimizations that can be derived from the simulation scenario, etc. We introduce the core characteristics of clustering approaches and argue that these cannot be applied in real-life distributed simulation problems. We present a typical distributed simulation setting and elaborate on the reasons that existing clustering approaches are not expected to improve the performance of a distributed simulation. We introduce a prototype distributed simulation platform that has been developed in the scope of this research, focusing on the area of emergency response and specifically building evacuation. We continue by outlining our current work on this issue, and finally, we end this report by outlining next actions which could be made in this field

Emergency response systems for disaster management in buildings

Emergency response operations can benefit from the use of information systems that reduce decision making time and facilitate co-ordination between the participating units. We propose the use of two such systems and evaluate them with a specialised software platform that we have developed for simulation of disasters in buildings. The first system provides movement decision support to evacuees by directing them through the shortest or less hazardous routes to the exit. It is composed of a network of decision nodes and sensor nodes, positioned at specific locations inside the building. The recommendations of the decision nodes are computed in a distributed manner and communicated to the evacuees or rescue personnel in their vicinity. The second system uses wireless-equipped robots that move inside a disaster area and establish a network for two-way communication between trapped civilians and rescuers. They are autonomous and their goal is to maximise the number of civilians connected to the network. We evaluate both proposed information systems in various emergency scenarios, using the specialised simulation software that we developed

Approximate Decoding Approaches for Network Coded Correlated Data

This paper considers a framework where data from correlated sources are transmitted with help of network coding in ad-hoc network topologies. The correlated data are encoded independently at sensors and network coding is employed in the intermediate nodes in order to improve the data delivery performance. In such settings, we focus on the problem of reconstructing the sources at decoder when perfect decoding is not possible due to losses or bandwidth bottlenecks. We first show that the source data similarity can be used at decoder to permit decoding based on a novel and simple approximate decoding scheme. We analyze the influence of the network coding parameters and in particular the size of finite coding fields on the decoding performance. We further determine the optimal field size that maximizes the expected decoding performance as a trade-off between information loss incurred by limiting the resolution of the source data and the error probability in the reconstructed data. Moreover, we show that the performance of the approximate decoding improves when the accuracy of the source model increases even with simple approximate decoding techniques. We provide illustrative examples about the possible of our algorithms that can be deployed in sensor networks and distributed imaging applications. In both cases, the experimental results confirm the validity of our analysis and demonstrate the benefits of our low complexity solution for delivery of correlated data sources

The application of Lie - point symmetries in classical and quantum cosmology

In this paper we attempt to demonstrate the importance of symmetries both at the classical and quantum level. The originality of this work rests in the generalization of the theory of symmetries (of the action and the equations of motion) in systems with constraints. All theories that exhibit a gauge freedom, result from actions characterized by singular Lagrangians. The corresponding freedom for General Relativity (GR) is the invariance of the theory under random (but sufficiently smooth) diffeomorphisms. This means that four out of ten of Einstein's equations are constriants for the system. The various cosmological models that can be described by equivalent mechanical systems carry a subset of this freedom, as applications of GR. This led us to search for a method in order to extend the theory from regular systems to singular. In both of them these symmetries are conserved quantities, with the difference that for singular systems this may hold due to the constraints. After the classical treatment where we export these integrals of motion, the next step is to consider them as quantum measurable quantities. By following Dirac's line of thinking we construct the corresponding Hermitian operators defining the eigen-equations of the wave-function. Of course the number of operands that can be used in a representation in this way is bound by the dimension of the configuration space and the requirement of finding a non-trivial solution for the wavefunction. The latter leads to an integrability condition, whose satisfaction determines the operator algebra of the measured physical quantities. In an effort to extend the theory of quantization of individual geometries to broader geometrical classes, we are led to study a system whose wave-function is used to define a generalized probability. The latter is proven to exhibit its probable for extrema over the classical trajectories of the initial cosmological model under consideration. Some examples of quantizing individual geometries and geometrical classes are given.Με αυτή την εργασία επιχειρούμε να καταδείξουμε τη δεσπόζουσα σημασία των συμμετριών τόσο σε κλασσικό όσο και σε κβαντικό επίπεδο. Η πρωτοτυπία της παρούσας εργασίας έγκειται στην επέκταση της θεωρίας για αναζήτηση συμμετριών (της δράσης αλλά και των εξισώσεων κίνησης) σε συστήματα που παρουσιάζουν δεσμούς. Όλες οι θεωρίες που παρουσιάζουν κάποιου είδους ελευθερία βαθμίδας, προκύπτουν από δράσεις που χαρακτηρίζονται από ιδιάζουσες Λαγκρανζιανές. Η αντίστοιχη ελευθερία για τη Γενική Σχετικότητα είναι η αναλλοιώτητα της θεωρίας κάτω από τυχαίες (αλλά επαρκώς ομαλές) αμφιδιαφορίσεις. Αυτό έχει ως συνέπεια τέσσερις εκ των δέκα εξισώσεων του Einsteinv να αποτελούν δεσμούς για το σύστημα. Τα διάφορα κοσμολογικά πρότυπα που μπορούν να προκύψουν ως μηχανικά συστήματα από μια απομείωση των βαθμών ελευθερίας, όντας εφαρμογές της ΓΣ, μεταφέρουν ένα υποσύνολο αυτής της ελευθερίας. Αυτό μας οδήγησε στην αναζήτηση μιας μεθόδου ώστε να επεκταθεί η θεωρία ανεύρεσης συμμετριών από τα ομαλά συστήματα στα ιδιάζοντα. Όπως στα πρώτα, έτσι και στα τελευταία πρόκειται για διατηρήσιμες ποσότητες, με τη διαφορά ότι στη δεύτερη περίπτωση η διατηρησιμότητα μπορεί να οφείλεται και στους δεσμούς.Αφού σε κλασσικό επίπεδο καταφέρουμε να εξάγουμε τα ολοκληρώματα της κίνησης αυτά, το επόμενο βήμα είναι η θεώρησή τους ως κβαντικά μετρήσιμες ποσότητες. Αυτό κατά τον Dirac σημαίνει την αντιστοίχισή τους σε γραμμικούς Ερμιτιανούς τελεστές που ορίζουν εξισώσεις ιδιοκαταστάσεων επί της κυματοσυνάρτησης. Φυσικά το πλήθος των τελεστών που μπορούν να χρησιμοποιηθούν σε μία αναπαράσταση με αυτό τον τρόπο, φράσσεται από τη διάσταση του θεσεογραφικού χώρου και την απαίτηση ανεύρεσης μη τετριμμένης λύσης για την κυματοσυνάρτηση. Η τελευταία μας οδηγεί σε μια συνθήκη ολοκληρωσιμότητας, της οποίας η ικανοποίηση καθορίζει την άλγεβρα των τελεστών των υπό μέτρηση φυσικών ποσοτήτων.Στην προσπάθεια να επεκτείνουμε τη θεωρία, από την κβάντωση μεμονωμένων γεωμετριών σε κλάσεις γεωμετρίας, οδηγούμαστε στη μελέτη ενός συστήματος του οποίου η κυματοσυνάρτηση χρησιμοποιείται για τον ορισμό μιας γενικευμένης πιθανότητας. Η τελευταία αποδεικνύουμε ότι παρουσιάζει τα πιθανά της ακρότατα επί των κλασσικών λύσεων του αρχικού κοσμολογικού προτύπου.Παρατίθενται ορισμένα παραδείγματα κβάντωσης μεμονωμένων γεωμετριών αλλά και κλάσεων γεωμετρίας

Distributed building evacuation simulator for smart emergency management

We describe a distributed simulation tool which addresses the unique needs for the simulation of emergency response scenarios. The simulation tool adopts the multi-agent paradigm, so as to facilitate the modelling of diverse and autonomous agents, and it provides mechanisms for the interaction of the entities that are being simulated. It operates in a distributed fashion to reduce the simulation time required for such large-scale systems. The simulation tool represents the individuals that need to be evacuated, the resources that contribute to the evacuation including human rescuers, and other active resources and entities which may include robots and which can autonomously interact with the environment and with each other and take individual or collaborative decisions. We illustrate the tool with an application and compare the results for both centralized and distributed execution. Our results also show the significant reduction in execution time that is achieved for different degrees of distribution of the simulator on multiple servers