In underwater wireless sensor networks (UWSN), acoustic communication naturally introduces challenges such as long propagation delay and high packet loss. The flooding-based routing protocol can address these challenges with its multipath characteristics. As in flooding-based routing, due to multipath propagation mechanism, not only DATA but also ACK messages are transmitted through multiple routes however still some packet loss will degrade the performance. So, to provide high reliability of message delivery, an efficient retransmission mechanism is inevitable. Though, if the network uses conventional transport layer protocol such as TCP, it will suffer a spurious retransmission problem as TCP was originally not designed for the multipath environment. In this paper, we propose route discrimination for flooding-based routing to reduce spurious retransmission in UWSN to solve the limitation. The notion of ACK copies waiting time (ACWT) is utilized which is selectively updated based on the similarity of paths of transmission of ACK message copies. We also improved our previous solution that lacks flexibility to cope with dynamic link error characteristics. Through evaluation, we verified that our new scheme achieves the performance improvements of 14%~84% in terms of retransmission ratio compared to the previous research.

Internet of Underwater Things (IoUT) 1 is a network of networks mostly comprising of Underwater Wireless Sensor Networks (UWSNs) of underwater objects. These sensing nodes provide various services such as disaster prevention and management, tactical surveillance and environmental monitoring. Due to the underwater environmental constraints, these sensor nodes mostly communicate using an acoustic channel which inherits long propagation delay and network congestions. Congestion control schemes of existing protocols such as TCP require transmission of Ack messages for congestion detection which consumes unnecessary bandwidth. In this paper, we propose a reliable lightweight congestion control method for CoAP while using Ack message only in a necessary situation to reduce the use of Ack message. The proposed method adjusts the transmission rate based on the network successful packet delivery ratio of current time. Simulation results showed that our proposed scheme promises enhanced performance that can achieve higher throughput when compared with the congestion control mechanism of default CoAP standard.

최근 쇼핑, 운동, 엔터테인먼트 등의 실내 활동이 증가하는 추세이다. 이에 따라 실내 환경에서 LBS(Location Based Service)를 제공하기 위해 많은 연구들이 진행되었다. 하지만, 기존의 Wi-Fi, Bluetooth 등의 근거리 무선통신 기술을 활용한 LBS는 cm단위의 정밀한 정확도를 요구하는 실내 재난 상황에서 사용하기 어렵다. 반면, UWB(Ultra-Wideband)는 3.1~10.6GHz의 넓은 주파수 대역, 저 전력 통신, 그리고 수 십cm 내외의 높은 정확도로 측위가 가능하다는 점에 착안하여 실내 재난 상황을 가정한 환경에서 활용하기 적합하다고 판단된다. 본 논문에서는 UWB 기술을 바탕으로 화재 진압, 건물 내부 탐색 등을 수행하는 로봇의 실내 위치를 정밀하게 파악할 수 있는 측위 모듈을 구현하고, 이에 대한 성능 평가를 정량적으로 수행 및 분석할 수 있는 소프트웨어의 개발을 목적으로 한다. 또한, 구현된 소프트웨어를 활용한 실험을 통해 데이터를 수집 분석한다.

In Underwater Wireless Sensor Networks, we faced a lot of challenges such as long propagation delay, and high packet loss. However, a flooding based routing protocol can address these challenges with its multipath characteristics. If the network uses a flooding based routing protocol, not only DATA but also ACK messages are transmitted through multiple paths. Although the messages are transmitted through multiple paths, we cannot avoid the packet loss completely. So, in order to provide high reliability of delivering messages, we need a retransmission mechanism. Though, if the network uses conventional transport layer protocol such as TCP, it will cause a spurious retransmission problem because it was not designed for the multipath environment. To overcome this problem, we suggest Path Similarity based Spurious Retransmission Minimization over Flooding based Routing in UWSN. Through our simulations, we verified that our scheme achieves the performance improvements of 10% ~ 45% in terms of retransmission ratio and 20% ~36% in terms of throughput compared to the Jacobson's RTO algorithm.