Material aspects of underwater marine systems in Greece

The material selection for the improvement of the quality of already existing underwater marine systems (including oil platforms) and for the construction of new and unfamiliar underwater sea-structures (including the submerged parts of Wave Energy Converters and Tidal Energy Converters) in many countries like Greece is an essential parameter from a technical, qualitative and economical point of view. The purpose of this review paper is to present a justified analysis of the proposed materials, explaining in more details the advantages of their properties and focusing on the applicable welding issues taking into consideration the previous existing experience from shipbuilding and submarine constructions

Evolving SDN for Low-Power IoT Networks

Software Defined Networking (SDN) offers a flexible and scalable architecture that abstracts decision making away from individual devices and provides a programmable network platform. However, implementing a centralized SDN architecture within the constraints of a low-power wireless network faces considerable challenges. Not only is controller traffic subject to jitter due to unreliable links and network contention, but the overhead generated by SDN can severely affect the performance of other traffic. This paper addresses the challenge of bringing high-overhead SDN architecture to IEEE 802.15.4 networks. We explore how traditional SDN needs to evolve in order to overcome the constraints of low-power wireless networks, and discuss protocol and architectural optimizations necessary to reduce SDN control overhead - the main barrier to successful implementation. We argue that interoperability with the existing protocol stack is necessary to provide a platform for controller discovery and coexistence with legacy networks. We consequently introduce {\mu}SDN, a lightweight SDN framework for Contiki, with both IPv6 and underlying routing protocol interoperability, as well as optimizing a number of elements within the SDN architecture to reduce control overhead to practical levels. We evaluate {\mu}SDN in terms of latency, energy, and packet delivery. Through this evaluation we show how the cost of SDN control overhead (both bootstrapping and management) can be reduced to a point where comparable performance and scalability is achieved against an IEEE 802.15.4-2012 RPL-based network. Additionally, we demonstrate {\mu}SDN through simulation: providing a use-case where the SDN configurability can be used to provide Quality of Service (QoS) for critical network flows experiencing interference, and we achieve considerable reductions in delay and jitter in comparison to a scenario without SDN

Isolating SDN Control Traffic with Layer-2 Slicing in 6TiSCH Industrial IoT Networks

Recent standardization efforts in IEEE 802.15.4-2015 Time Scheduled Channel Hopping (TSCH) and the IETF 6TiSCH Working Group (WG), aim to provide deterministic communications and efficient allocation of resources across constrained Internet of Things (IoT) networks, particularly in Industrial IoT (IIoT) scenarios. Within 6TiSCH, Software Defined Networking (SDN) has been identified as means of providing centralized control in a number of key situations. However, implementing a centralized SDN architecture in a Low Power and Lossy Network (LLN) faces considerable challenges: not only is controller traffic subject to jitter due to unreliable links and network contention, but the overhead generated by SDN can severely affect the performance of other traffic. This paper proposes using 6TiSCH tracks, a Layer-2 slicing mechanism for creating dedicated forwarding paths across TSCH networks, in order to isolate the SDN control overhead. Not only does this prevent control traffic from affecting the performance of other data flows, but the properties of 6TiSCH tracks allows deterministic, low-latency SDN controller communication. Using our own lightweight SDN implementation for Contiki OS, we firstly demonstrate the effect of SDN control traffic on application data flows across a 6TiSCH network. We then show that by slicing the network through the allocation of dedicated resources along a SDN control path, tracks provide an effective means of mitigating the cost of SDN control overhead in IEEE 802.15.4-2015 TSCH networks

Experiences from porting the Contiki operating system to a popular hardware platform

In contrast to original belief, recent work has demonstrated the viability of IPv6-based Wireless Sensor Networks (WSNs). This has led to significant research and standardization efforts with outcomes such as the "IPv6 over Low-Power Wireless Personal Area Networks " (6LoWPAN) specification. The Contiki embedded operating system is an important open source, multi-platform effort to implement 6LoWPAN functionality for constrained devices. Alongside its RFC-compliant TCP/IP stack (uIP), it provides support for 6LoWPAN and many related standards. As part of our work, we have made considerable fixes and enhancements to one of Contiki's ports. In the process, we made significant optimizations and a thorough evaluation of Contiki's memory and code footprint characteristics, focusing on network-related functionality. In this paper we present our experiences from the porting process, we disclose our optimizations and demonstrate their significance. Lastly, we discuss a method of using Contiki to deploy an embedded Internet-to-6LoWPAN router. Our porting work has been made available to the community under the terms of the Contiki license

Stateless multicast forwarding with RPL in 6LowPAN sensor networks

Recent research efforts have resulted in efficient support for IPv6 in Low power Wireless Personal Area Networks (6LoWPAN), with the "IPv6 Routing Protocol for Low power and Lossy Networks" (RPL) being on the forefront as the state of the art routing approach. However, little attention has been paid to IPv6 multicast for networks of constrained devices. The "Multicast Forwarding Using Trickle" (Trickle Multicast) internet draft is one of the most noteworthy efforts, while RPL's specification also attempts to address the area but leaves many questions unanswered. In this paper we expose our concerns about the Trickle Multicast (TM) algorithm, backed up by thorough performance evaluation. We also introduce SMRF, an alternative multicast forwarding mechanism for RPL networks, which addresses TM's drawbacks. Simulation results demonstrate that SMRF achieves significant delay and energy efficiency improvements at the cost of a small increase in packet loss. We have extended the TCP/IP engine of the Contiki embedded Operating System to support both algorithms. Both implementations have been made available to the community. © 2012 IEEE

Atomic-SDN: Is Synchronous Flooding the Solution to Software-Defined Networking in IoT?

The adoption of Software Defined Networking (SDN) within traditional networks has provided operators the ability to manage diverse resources and easily reconfigure networks as requirements change. Recent research has extended this concept to IEEE 802.15.4 low-power wireless networks, which form a key component of the Internet of Things (IoT). However, the multiple traffic patterns necessary for SDN control makes it difficult to apply this approach to these highly challenging environments. This paper presents Atomic-SDN, a highly reliable and low-latency solution for SDN in low-power wireless. Atomic-SDN introduces a novel Synchronous Flooding (SF) architecture capable of dynamically configuring SF protocols to satisfy complex SDN control requirements, and draws from the authors' previous experiences in the IEEE EWSN Dependability Competition: where SF solutions have consistently outperformed other entries. Using this approach, Atomic-SDN presents considerable performance gains over other SDN implementations for low-power IoT networks. We evaluate Atomic-SDN through simulation and experimentation, and show how utilizing SF techniques provides latency and reliability guarantees to SDN control operations as the local mesh scales. We compare Atomic-SDN against other SDN implementations based on the IEEE 802.15.4 network stack, and establish that Atomic-SDN improves SDN control by orders-of-magnitude across latency, reliability, and energy-efficiency metrics