Telcos are adapting their business to address the rapid changing technology landscape (Moodley and van Olst 2011). Telcos require a flexible architecture to allow seamless adaptation and to leverage these new technologies to gain a competitive advantage (Moodley and van Olst 2011). This research is focused on the transport stratum as an extension to the OSA/Parlay gateway. The proposed OSA/Parlay Network Architecture and Interface has been designed. The OSA/Parlay Network Interface is characterised by openness, simplicity, API based, QoS support and technology independence. The OSA/Parlay Network Architecture features simplicity, technology independence, QoS mechanisms; call admission control; intelligent routing and supporting both federation of telcos and interoperation of legacy technologies. The OSA/Parlay Network Architecture and Interface has been demonstrated over a Java based Distributed Processing Environment (DPE) using CORBA. These architectural concepts and principles are demonstrated in a simulated environment and illustrate the Next Generation Network architectural characteristics. The research contribution therefore achieves an open architecture allowing for 3rd party application developers while also ensuring that call and service requests are provisioned end-to-end with guaranteed application level QoS in the transport network. The OSA/Parlay Network Architecture and Network Interface is synthesised from existing architectural standards and provides the following benefits. It is an extension of the OSA/Parlay standard by including the OSA/Parlay Network Architecture and Network Interface realises the OSA/Parlay next generation network. Both the OSA/Parlay Network Interface and the Network Architecture is specified in a technology agnostic manner. This ensures that the architecture remains future proof as it is not reliant on any particular technology. The long sought after Application level QoS is integrated into the architecture. The periodic network state updates inform the central Connection Coordinator object of both topological network changes as well as current performance of the constituent parts of the network. Intelligent routing of connections is achieved by adapting the Dijkstra algorithm to compute the best path based on dynamic network performance and is tested against QoS requirements, while the call admission control decision naturally allows for load balancing of connection paths within the network. This QoS mechanism achieves the goal of guaranteed QoS for a call admitted into the network.

Digital Twins (DTs) are software representations of physical systems (PSs) that interact with other entities on behalf of their real-world counterparts. To ensure accurate representation and effective interaction, DTs must remain synchronized with their PSs through timely updates—a process known as DT synchronization. This thesis addresses key challenges related to DT synchronization to optimize performance metrics, including the synchronization period and Age of Information (AoI). In the first part, we address the challenge of optimally placing DTs on execution servers (ESs) to minimize both the data request-response delay experienced by applications and the synchronization period between PSs and their DTs, while satisfying communication and computation constraints. We formulate the DT placement problem in two ways. First, we model it as an integer quadratic program (IQP) aiming to minimize the maximum application response delay subject to maximum data age target constraints at the DTs and the application server. Due to the NP-completeness of the problem, we develop practical polynomial-time approximation algorithms that offer trade-offs between application latency and data age targets. Second, we tackle the Minimum Synchronization Period (MSP) problem by modeling it as a multi-commodity quickest flow evacuation problem, considering synchronization data and processing tasks as network flows with flow dependent edge delays. This innovative approach allows us to use well-established techniques from flow network theory to efficiently find the quickest flow solution. An unsplittable flow rounding procedure ensures that each DT is assigned to a single ES. Simulation results demonstrate the effectiveness of our proposed algorithms in both methods, compared to optimal solutions serving as lower bounds. In the second part, we address DT migration in vehicular systems, where maintaining acceptable AoI is challenging due to high mobility and frequent handoffs between cellular domains. We formulate the optimal initiation time for migrating a vehicle's DT as a Markov decision process, aiming to minimize the time-averaged AoI at the DT. An online optimal migration initiation algorithm is proposed using dynamic programming and optimal stopping problem. We also develop a more computationally intensive adaptive version of this algorithm, which recalculates the decision policy at each time step for improved performance. Additionally, we introduce a best-in-expectation algorithm that offers a balance between computational efficiency and AoI performance. These algorithms are compared with heuristic approaches, such as immediate migration and migration at handoff, as well as an offline algorithm providing a theoretical lower bound on the average AoI. Performance evaluations show that our proposed algorithms significantly enhance the efficiency of DT migrations while minimizing the time-averaged AoI compared to other methods. / Dissertation / Candidate in Philosophy / Digital Twins (DTs) are virtual replicas of real-world Physical Systems (PSs), such as mobile devices, vehicles, or smart cities. These digital counterparts are hosted by network servers. They mirror the state and behavior of their physical versions in real time, allowing them to interact with other devices or applications on behalf of their PSs. For a DT to effectively mirror and reflect any changes in its PS, it must consistently remain synchronized through timely updates, which consume the network resources. As a result, the placement of DTs on network servers affects the quality of the DTs. The problem becomes challenging when placing the DTs of a large number of PSs, and is further complicated when the PSs are mobile. This thesis tackles some key challenges towards optimal DT placements. \begin{enumerate} \item Optimizing Synchronization Timing and Placement: We investigate how to optimally place DTs within the network infrastructure to minimize synchronization delay. To achieve this, we develop algorithms that efficiently assign DTs to servers, balancing the need for timely updates, quick application responses, and the amount of network resources. \item Enhancing DT Migration in Vehicular Systems: Vehicles are constantly on the move. Therefor, the PS-DT synchronization delay varies with the PS locations, and at some point, it is better to migrate the DT to a different server. We develop algorithms that decide when to initiate the migration to minimize costs associated with the migration.

Clock synchronization in a wireless sensor network (WSN) is essential as it provides a consistent and a coherent time frame for all the nodes across the network. Typically, clock synchronization is achieved by message passing using carrier sense multiple access (CSMA) for media access. The nodes try to synchronize with each other, by sending synchronization request messages. If many nodes try to send messages simultaneously, contention-based schemes cannot efficiently avoid collisions which results in message losses and affects the synchronization accuracy. Since the nodes in a WSN have limited energy, it is required that the energy consumed by the clock synchronization protocols is as minimum as possible. This can be achieved by reducing the duration for which the clock synchronization protocols execute. Synchronous clock synchronization protocols in WSNs execute the clock synchronization process at each node, roughly during the same real-time interval, called synchronization phase. The duration when there is no synchronization activity is called the synchronization interval. Synchronization phases are divided into synchronization rounds. The energy consumed by these protocols depends on the duration of the synchronization phase and how frequently the synchronization phase is executed. Hence, to minimize the energy consumption by each node, the duration of synchronization phase should be as small as possible. Due to different drift rates of the clocks, the synchronization phases at different nodes drift apart and special techniques are required to keep them in sync. An existing protocol, called improved weighted-average based clock synchronization (IWICS) uses a pullback technique to achieve this. If a message from (i + 1)th synchronization round is received by a node still executing the ith synchronization round, the receiving node reduces its next synchronization interval to ensure greater overlap in the synchronization rounds. The reduction in overlap is a gradual and continuous phenomenon, and so, it can be detected and dealt with continuously. In this thesis, first, we make use of TDMA-based MAC protocols, instead of CSMA, to deal with the problem of message losses. We discuss the challenges of using TDMA-based MAC protocols for clock synchronization and how to overcome these challenges. Second, The IWICS protocol calculates the virtual drift rate which we use to modify the duration of the synchronization interval so that there is more overlap between the synchronization phases of neighbouring nodes. We refer to this technique as drift rate correction. Finally, we propose a different pullback technique where the pullback detection is carried out in each of the synchronization phase as opposed to the old pullback mechanism where it would be detected only when an out-of-round synchronization message is received. The proposed pullback technique when applied to the current synchronization interval ensures that the synchronization phases, that follow the current synchronization interval, are better synchronized with each other. As a result of this, we are able to reduce the duration of synchronization phases further. The IWICS protocol with all these modifications incorporated is termed as the TIWICS (TDMA-based IWICS) protocol. Simulation and experimental results confirm that the TIWICS protocol performs better in comparison to the existing protocols.