Several decades after A.K. Erlang originated the theory of queues and queueing networks, D.V. Lindley added impetus to the development of this field by determining a recursive relation for waiting times. Part I of this thesis provides a theoretical treatment of single-server and multiserver queues described by the basic Lindley relation and its extensions, which are referred to collectively as Lindley-Loynes equations. The concepts of stability, and minimal and maximal solutions are investigated. The interdependence of theory and practice becomes evident in Part II, where the results of recent and current research are highlighted. While the main aim of the first part of the thesis is to provide a firm theoretical framework for the sequel, the objective in Part II is to derive generalised forms of the Lindley-Loynes equations from different network protocols. Such protocols are regulated by different switching rules and synchronization constraints. Parts I and II of the thesis are preceded by Chapter 0 in which several fundamental ideas (including those on notation and probability) are described. It is in this chapter too that a more detailed overview of the concept of the thesis is provided. / Thesis (M.Sc.)-University of Natal, Durban, 1993.

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The Internet as of 2014 connects billions of devices, and is expected to connect tens of billions by 2020. To meet escalating requirements, networks must be scalable, easy to manage, and be able to efficiently execute programs and disseminate data. The prevailing use of centralized systems and control in, e.g., pools of computing resources, clouds, is problematic for scalability. A promising approach to management of large networks is decentralization, where independently acting network nodes communicate with their immediate neighbors to achieve desirable results at the global level. The research in this thesis addresses three distinct but interrelated problems in the context of cloud computing, networks, and programs running in clouds. First, we show how implementation correctness of active objects can be achieved in decentralized networks using location independent routing. Second, we investigate the feasibility of decentralized adaptive resource allocation for active objects in such networks, with promising results. Third, we automate an initial step of a process for converting programs with thread-based concurrency using shared memory to programs with message passing concurrency, which can then run efficiently in clouds. Specifically, starting from fragments of the distributed object modeling language ABS, we give network-oblivious descriptions of runtime behavior of programs, where the global state is a flat collection of objects and method calls. We then provide network-aware semantics, that place objects on network nodes connected point-to-point by asynchronous message passing channels. By relying on location independent routing, which maps object identifiers to next-hop neighbors at each node, inter-object messages can be delivered, regardless of object mobility among nodes. We establish that network-oblivious and network-aware behavior in static networks correspond in the sense of contextual equivalence. Using a network protocol reminiscent of a two-phase commit for controlled node shutdown, we extend the approach to dynamic networks without failures. We investigate node-local procedures for object migration to meet requirements on balanced allocations of objects to nodes, that also attempt to minimize exchange of object-related messages between nodes. By relying on coin-flips biased on local and neighbor load to decide on migration, and heuristics to capture object communication patterns, we show that balanced allocations can be achieved that make headway towards minimizing communication and latency. Our approach to execution of object-oriented programs in networks relies on message-passing concurrency. Mainstream programming languages generally use thread-based concurrency, which relies on control-centric primitives, such as locks, for synchronization. We present an algorithm for dynamic probabilistic inference of annotations for data-centric synchronization in threaded programs. By making collections of variables in classes accessed atomically explicit, these annotations can in turn suggest objects suitable for encapsulation as a unit of message-passing concurrency. / 2014 års Internet sammankopplar miljarder enheter, och förväntas sammankoppla tiotals miljarder år 2020. För att möta eskalerande krav måste nätverk vara skalbara, enkla att underhålla, och effektivt exekvera program och disseminera data. Den nuvarande användningen av centraliserade system och kontrollmekanismer, t ex i pooler av beräkningsresurser, moln, är problematisk för skalbarhet. Ett lovande angreppssätt för att hantera storskaliga nätverk är decentralisering, där noder som agerar oberoende av varandra genom kommunikation med sina omedelbara grannar åstadkommer gynnsamma resultat på den globala nivån. Forskningen i den här avhandlingen addresserar tre distinkta men relaterade problem i kontexten av molnsystem, nätverk och program som körs i moln. För det första visar vi hur implementationskorrekthet för aktiva objekt kan åstadkommas i decentraliserade nätverk med hjälp av platsoberoende routning. För det andra undersöker vi genomförbarheten i decentraliserad adaptiv resursallokering för aktiva objekt i sådana nätverk, med lovande resultat. För det tredje automatiserar vi ett initialt steg i en process för att konvertera program med trådbaserad samtidighet och delat minne till program med meddelandebaserad samtidighet, som då kan köras effektivt i moln. Mer specifikt ger vi, med utgångspunkt i fragment av modelleringsspråket ABS baserat på distribuerade objekt, nätverksomedvetna beskrivningar av körningstidsbeteende för program där det globala tillståndet är en platt samling av objekt och metodanrop. Vi ger därefter nätverksmedvetna semantiker, där objekt placeras på nätverksnoder sammankopplade från punkt till punkt av asynkrona kanaler för meddelandetransmission. Genom att vid varje nod använda platsoberoende routning, som associerar objektidentifierare med grannoder som är nästa hopp, kan meddelanden mellan objekt levereras oavsett hur objekt rör sig mellan noder. Vi etablerar att nätverksomedvetet och nätverksmedvetet beteende i statiska nätverk stämmer överens enligt kontextuell ekvivalens. Genom att använda ett nätverksprotokoll som påminner om en tvåstegsförpliktelse, utökar vi vår ansats till felfria dynamiska nätverk. Vi undersöker nodlokala procedurer för objektmigration för att möta krav på balanserade allokeringar av objekt till noder, som också försöker minimera utbyte av objektrelaterade meddelanden mellan noder. Genom att använda oss av slantsinglingar viktade efter lokal last och grannars last för att besluta om migration, och tumregler för att fånga kommunikationsmönster mellan objekt, visar vi att balanserade allokeringar, som gör framsteg mot att minimera kommunikation och tidsfördröjning, kan uppnås. Vår ansats för exekvering av objektorienterade program i nätverk använder meddelandebaserad samtidighet. Vanligt förekommande programspråk använder sig generellt av trådbaserad samtidighet, som kräver kontrollcentrerade mekanismer, som lås, för synkronisering. Vi presenterar en algoritm som med dynamisk probabilistisk analys härleder annoteringar för datacentrerad synkronisering för trådade program. Genom att göra samlingar av variabler i klasser som läses och skrivs atomiskt explicita, kan sådana annoteringar antyda vilka objekt som är lämpliga att kapsla in som en enhet i meddelandebaserad samtidighet. / <p>QC 20140929</p>

From the smallest microcomputer to the largest mainframe computer, data communication capabilities are and will be a major factor in the success and usefulness of the computer systems. The need for larger systems to process data created on the small computers, the need to move subsets of data to small computers access larger computers for other capabilities, and the need for growth in hardware capability are all major reasons that data communication is so important to today's technology.As data communication is the backbone of today's technology, it will play a leading role in governing the changes in the technology of the future. Hence the knowledge of data communication becomes a very important aspect.In this thesis emphasis has been given to simulate the lower layers of communication protocols using Ada as the base language. The language Ada is chosen because of it's capability of providing parallel processing, which is an inherent property of the data communication. Ada is a complex, general purpose programming language that provides an excellent concurrent programing facility called task, that is based on rendezvous concept.Simulation is an excellent way to teach & understand concepts, which otherwise becomes infeasible to understand. Now a days more and more communication details is embedded in the hardware, and hence what is going inside the computer, to provide communication between two computers or just plain data communication, is becoming transparent to the user. However, this does not eliminate the need to know about the functional characteristics of the protocols, which are the rules to provide an effective and reliable communication environment, rather the need to understand protocols is increasing as more and more computer vendors are coming out with their own products. Thanks to the ISO (International Standard Organization), which has come with the standard for the Computer to Computer Communication, but still protocol analysis and design will play a very important role. The field of protocol conversion has grown tremendously in order to provide communication across heterogeneous systems using message communication will to the field of protocol which now a days is an different protocols.Here in this study, communication backbone is simulated in Ada using its concurrent processing capabilities and it is tried to show how a data or aMessage communication will actually take place. This study can serve as a blue print to improve this in order to cater to the field of protocol analysis and protocol conversion, which now a days is an important facet of the data communication industry. / Department of Computer Science