Ανάλυση της πιθανοτικής διάχυσης της πληροφορίας σε ασύρματα δίκτυα με αλγεβρική θεωρία γράφων

Information dissemination constitutes a large field of study in the scientific community. In the past, the networks consisted mainly of a relatively small number of nodes with a preconstructed structure and hierarchy. Over the years, both their form and their size have been changed radically resulting in unstructured large-scale networks with high node mobility. Therefore, information dissemination in modern networks is a bigger challenge comparing to the past, because of the large scale and the high complexity of this process. Approaches like traditional flooding, are not suitable in modern networks due to the large number of transmitted information messages that waste valuable resources. In order to increase network’s lifetime, it is necessary to disseminate information in a way that minimizes unnecessary transmissions and at the same time maximizes the number of nodes that receives it. The purpose of this dissertation is to find optimized algorithms for disseminating information in modern networks that saves valuable resources and increases network’s lifetime.Η διάχυση πληροφοριών στα δίκτυα, αποτελούσε και αποτελεί ένα από τα μεγαλύτερα πεδία μελέτης της επιστημονικής κοινότητας. Κατά το παρελθόν, τα δίκτυα απαρτίζονταν κυρίως από ένα σχετικά μικρό αριθμό κόμβων με προκαθορισμένη δομή και ιεραρχία. Με το πέρασμα των χρόνων, τόσο η μορφή τους όσο και το μέγεθός τους άλλαξαν ριζικά. Έτσι σήμερα έχουμε δίκτυα πολύ μεγάλου μεγέθους τα οποία χαρακτηρίζονται από υψηλή κινητικότητα των κόμβων τους, από την ανυπαρξία προκαθορισμένης δομής αλλά και από το γεγονός ότι στις περισσότερες των περιπτώσεων υπάρχει ισοτιμία μεταξύ των κόμβων. Η διάχυση επομένως μιας πληροφορίας σε ένα σύγχρονο δίκτυο, αποτελεί μεγαλύτερη πρόκληση συγκριτικά με το παρελθόν, αφού υπεισέρχονται νέοι παράγοντες που αυξάνουν την πολυπλοκότητα της διαδικασίας. Μέθοδοι όπως το κλασσικό flooding δεν μπορούν να εφαρμοστούν στα σημερινά δίκτυα, αφού οι άσκοπες μεταδόσεις μηνυμάτων σπαταλούν πολύτιμους πόρους. Είναι επομένως αναγκαίο, η διάχυση της πληροφορίας να γίνεται με τέτοιο τρόπο ώστε να ελαχιστοποιούνται οι άσκοπες μεταδόσεις και ταυτόχρονα να μεγιστοποιείται ο αριθμός των κόμβων που τελικά την έχουν λάβει. Σκοπός της συγκεκριμένης διατριβής θα είναι η εύρεση βελτιστοποιημένων αλγόριθμων για την διάχυση των πληροφοριών στα σύγχρονα δίκτυα, με τρόπους οι οποίοι θα εξοικονομούν πολύτιμους πόρους, αυξάνοντας έτσι την διαθεσιμότητα των κόμβων και την διάρκεια ζωής του δικτύου

Adapting Probabilistic Flooding in Energy Harvesting Wireless Sensor Networks

Technological advantages in energy harvesting have been successfully applied in wireless sensor network environments, prolonging network’s lifetime, and, therefore, classical networking approaches like information dissemination need to be readdressed. More specifically, Probabilistic Flooding information dissemination is revisited in this work and it is observed that certain limitations arise due to the idiosyncrasies of nodes’ operation in energy harvesting network environments, resulting in reduced network coverage. In order to address this challenge, a modified version of Probabilistic Flooding is proposed, called Robust Probabilistic Flooding, which is capable of dealing with nodes of about to be exhausted batteries that resume their operation after ambient energy collection. In order to capture the behavior of the nodes’ operational states, a Markov chain model is also introduced and—based on certain observations and assumptions presented here—is subsequently simplified. Simulation results based on the proposed Markov chain model and a solar radiation dataset demonstrate the inefficiencies of Probabilistic Flooding and show that its enhanced version (i.e., Robust Probabilistic Flooding) is capable of fully covering the network on the expense of increased termination time in energy harvesting environments. Another advantage is that no extra overhead is introduced regarding the number of disseminated messages, thus not introducing any extra transmissions and therefore the consumed energy does not increase

Implementation of a Topology Independent MAC (TiMAC) Policy on a Low-Cost IoT System

The emerging new paradigm under the fifth generation of wireless communications technologies (5G) and high expectations for massively expanding today’s Internet of Things (IoT) under 5G, are expected to support a large plurality of low-cost devices for an all-increasing number of new IoT applications. Many emerging IoT applications are going to take advantage of techniques and technologies that have high demands from low-cost devices in terms of processing large amounts of data and communication. For example, in systems based on fog computing technology, low-cost devices have to assign some of their limited resources for processing purposes. Considering the drawbacks emerging from using low-cost devices and the fact that many applications are in need for time-constrained approaches, TDMA-based Medium Access Control (MAC) policies need to be revisited and implemented in low-cost devices of today. In this sense, a policy independent of the underlying topology, TiMAC policy, is considered here and is implemented in low-cost devices using 433 MHz RF modules. Even though the implementation is limited by synchronization issues and a small number of nodes, the obtained experimental results demonstrate the potential for employing TDMA-based MAC policies on IoT systems consisting of low-cost devices