Στην παρούσα διατριβή παρουσιάζονται μεθοδολογίες σχεδιασμού που επιτρέπουν την μείωση την κατανάλωση ενέργειας σε υλοποιήσεις ασύρματων εφαρμογών πολυμέσων. Οι προτεινόμενες μεθοδολογίες βελτιστοποιούν την διαχείριση της μνήμης, η κατανάλωσης της οποίας είναι η κυρίαρχη συνιστώσα της συνολικής κατανάλωσης ισχύος στις εφαρμογές πολυμέσων. Επιπλέον, η διατριβή πραγματεύεται τεχνικές για χαμηλής κατανάλωσης διαμερισμό, ο οποίος είναι απαραίτητος για την εκτέλεση σε πραγματικό χρόνο των υψηλής υπολογιστικής πολυπλοκότητας εφαρμογών πολυμέσων. Στόχο της παρούσας διατριβής αποτελεί και η βελτιστοποίηση, όσον αφορά την κατανάλωση ισχύος, των κυκλωμάτων ασύρματης μετάδοσης των πολυμέσων. Στο πλαίσιο αυτό η διατριβή ασχολείται με τεχνικές διαχείρισης ισχύος, και προτείνει υλοποιήσεις που χρησιμοποιούν τις τεχνικές αυτές. Οι μεθοδολογίες και τεχνικές λαμβάνουν υπόψη και την επιφάνεια ολοκλήρωσης και ταχύτητα εκτέλεσης. Τα ισοζύγια (trade-offs) αναγνωρίζονται, και όπου αυτά υπάρχουν προτείνονται μέθοδοι για την αποδοτική τους επίλυση. / In this Ph.D. dissertation, design methodologies that enable energy minimization of wireless multimedia application realizations are presented. The proposed methodologies mainly optimize the management of the memory subsystem, which role is dominant as far as energy dissipation of multimedia application is concerned. Furthermore, techniques for the low-power partitioning, which is required for the real-time implementation of the highly computational complex multimedia applications, are proposed. Apart from the multimedia processing itself the dissertation, also focuses on the transmition of the multimedia content. To this end, power management techniques applicable for the design of wireless receivers are proposed. The proposed techniques are employed for the design of real-life implementation of low and medium rate receivers. Apart from energy minimization The proposed methodologies and techniques take into account constraints regarding the integration area as well as the processing time. The involved trade-offs are identified, and methods for their efficient exploration are proposed

Ανάλυση λειτουργιών πρωτοκόλλου ΜΙΜΕ για τη διαχείριση ομάδων συζητήσεων (forums) και ηλεκτρονικού ταχυδρομείου (e-mail) σε ιστοσελίδες δικτυακού τόπου / Analysis of features of MIME presentation protocol

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This thesis is concerned with the study of secure wireless routing protocols, which have been deployed for the purpose of exchanging information in an adhoc networking enviromnent. A discrete event simulator is developed, utilising an adaptive systems modelling approach and emergence that aims to assess networking protocols in the presence of adversarial behaviour. The model is used in conjunction with the characteristics that routing protocols have and also a number of cryptographic primitives that can be deployed in order to safeguard the information being exchanged. It is shown that both adversarial behaviour, as well as protocol descriptions can be described in a way that allows for them to be treated as input on the machine level. Within the system, the output generated selects the fittest protocol design capable of withstanding one or more particular type of attacks. As a result, a number of new and improved protocol specifications are presented and benchmarked against conventional metrics, such as throughput, latency and delivery criteria. From this process, an architecture for designing wireless routing protocols based on a number of security criteria is presented, whereupon the decision of using particular characteristics in a specification has been passed onto the machine level.

Ο σκοπός της διπλωματικής εργασίας ήταν η ανάπτυξη εφαρμογής για την προσομοίωση, με χρήση του προγράμματος προσομοίωσης OPNET της λειτουργίας του πρωτοκόλλου IPFIX που χρησιμοποιείται για τη συλλογή και την προώθηση δικτυακών δεδομένων στους σταθμούς διαχείρισης του δικτύου. Το πρωτόκολλο αυτό βοηθάει τους διαχειριστές του δικτύου να παρακολουθήσουν «monitoring» την ροή των δεδομένων εντός του δικτύου. Το πρώτο μέρος της εργασίας περιέχει την μελέτη και την ανάλυση της λειτουργίας του πρωτοκόλλου αυτού σε θεωρητικό επίπεδο με σκοπό την κατανόηση της ανάγκης ύπαρξης ενός ομοιόμορφου προτύπου για την εξαγωγή πληροφοριών από δικτυακές συσκευές. Ταυτόχρονα γίνεται εκτενής αναφορά στο διαδεδομένο Netflow της εταιρείας Cisco πάνω στο οποίο είναι βασισμένο το Πρωτόκολλο Εξαγωγής Πληροφοριών Ροής Ipfix. Στο δεύτερο μέρος της διπλωματικής παρατίθεται η ανάπτυξη ενός δικτύου με σκοπό να επεξηγήσουμε τον τρόπο με τον οποίο παρατηρούνται τα δεδομένα μέσα σε αυτό. Η εκτέλεση της εφαρμογής για διαφορετικά είδη κίνησης και διαφορετική ένταση κίνησης ή και για διαφορετικό τρόπο συλλογής των δεδομένων στο OPNET επιτρέπει την εξαγωγή χρήσιμων συμπερασμάτων για το κόστος και την επίδοση του πρωτοκόλλου. / The goal of this diploma thesis is the development of an application in order to simulate the function of the IPFIX Protocol, which is used to collect and transmit network data to the network management stations. For this purpose OPNET (Optimized Network Engineering Tools) was used. Briefly, IPFIX helps network managers to monitor the flow of data within the network. Throughout the first part of this dissertation emphasis is given to the study and analysis of the protocol in theory, and this way the need for a protocol that is in charge of extracting information out of network devices is underlined. In addition, extensive coverage of the widespread Cisco Netflow protocol has been made since IPFIX is based on it. Throughout the second part of this dissertation a network is presented in order to explain the way data is observed within it. The execution of the application for different, kinds of traffic, traffic intensity and ways of collecting data, using OPNET, enables us to export useful conclusions considering cost and performance of the protocol.

The Transmission Control Protocol (TCP) has been designed for reliable data transport over the Internet. The performance of TCP is strongly influenced by its congestion control algorithms that limit the amount of traffic a sender can transmit based on end-to-end available capacity estimations. These algorithms proved successful in environments where applications rate requirements can be easily anticipated, as is the case for traditional bulk data transfer or interactive applications. However, an important new class of Internet applications has emerged that exhibit significant variations of transmission rate over time. Variable-rate traffic poses a new challenge for congestion control, especially for applications that need to share the limited capacity of a bottleneck over a long delay Internet path (e.g., paths that include satellite links). This thesis first analyses TCP performance of bursty applications that do not send data continuously, but generate data in bursts separated by periods in which little or no data is sent. Simulation analysis shows that standard TCP methods do not provide efficient support for bursty applications that produce variable-rate traffic, especially over long delay paths. Although alternative forms of congestion control like TCP-Friendly Rate Control and the Datagram Congestion Control Protocol have been proposed, they did not achieve widespread deployment. Therefore many current applications that rely upon User Datagram Protocol are not congestion controlled. The use of non-standard or proprietary methods decreases the effectiveness of Internet congestion control and poses a threat to the Internet stability. Solutions are therefore needed to allow bursty applications to use TCP. Chapter three evaluates Congestion Window Validation (CWV), an IETF experimental specification that was proposed to improve support for bursty applications over TCP. It concluded that CWV is too conservative to support many bursty applications and does not provide an incentive to encourage use by application designers. Instead, application designers often avoid generating variable-rate traffic by padding idle periods, which has been shown to waste network resources. CWV is therefore shown to not provide an acceptable solution for variable-rate traffic. In response to this shortfall, a new modification to TCP, TCP-JAGO, is proposed. This allows variable-rate traffic to restart quickly after an inactive (i.e., idle) period and to effectively utilise available network resources while sending at a lower rate than the available rate (i.e., during an application-limited period). The analysis in Chapter five shows that JAGO provides faster convergence to a steady-state rate and improves throughput by more efficiently utilising the network. TCP-JAGO is also shown to provide an appropriate response when congestion is experienced after restart. Variable-rate TCP traffic can also be impacted by the Initial Window algorithm at the start or during the restart of a session. Chapter six considers this problem, where TCP has no prior indication of the network state. A recent proposal for a larger initial window is analysed. Issues and advantages of using a large IW over a range of scenarios are discussed. The thesis concludes by presenting recommendations to improve TCP support for bursty applications. This also provides an incentive for application designers to choose TCP for variable-rate traffic.

Modern computer systems are advancing from multi-core to many-core designs and System-on-chips (SoC) are becoming increasingly complex while integrating a great variety of components, thus constituting complex distributed systems. Such architectures rely on extremely complex communication protocols to exchange data with required performance. Arguing formally about the correctness of communication is an acknowledged verification challenge. This thesis presents a generic framework that formalises the idea of incremental modelling and step-wise verification to tackle this challenge: to control the overall complexity, features are added incrementally to a simple initial model and the complexity of each feature is encapsulated into an independent modelling step. Two main strategies reduce the verification effort. First, models are constructed with verification support in mind and the verification process is spread over the modelling process. Second, generic correctness results for framework components allow the verification to be reduced to discharging local assumptions when a component is instantiated. Models in the framework are based on abstract state machines formalised in higher order logic using the Isabelle theorem prover. Two case studies show the utility and breadth of the approach: the ARM AMBA Advanced High-performance Bus protocol, an arbiter-based master-slave bus protocol, represents the family of SoC protocols; the PCI Express protocol, an off-chip point-to-point protocol, illustrates the application of the framework to sophisticated, performance-related features of current and future on-chip protocols. The presented methodology provides an alternative to the traditional monolithic and post-hoc verification approach.

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