New Architectures for ubiquitous networksuse and adaptation of internet protocols over wireless sensor networks

Resumen

This thesis focuses on the study of low-resource demanding protocols, communication techniques and software solutions to evaluate, optimise and implement Web service in WSNs. We start analysing the Web service architectures in order to choose the most appropriate for the constraints of WSNs, which is REST. Based on this analysis, we review the state-of-the-art of protocols that allows implementing REST Web services. To this end, we adopt the IEEE 802.15.4 standard for the physical and data-link layers, 6LoWPAN for the network layer and CoAP for the application layer. 6LoWPAN defines two forwarding techniques, which are called mesh under (MU) and route over (RO). It also provides a mechanism to fragment packets, which is called 6LoWPAN fragmentation. In part of the thesis, we study the effects that MU and RO have on communications using 6LoWPAN fragmentation. In particular, MU does not prevent forwarding unnecessary fragments and out-of-order delivery, which could lead to an inefficient use of bandwidth and a growth of energy consumption. We propose, then, a novel technique able to improve the performance of MU with fragmented packets, which we refer to as controlled mesh under (CMU). The results of a performance evaluation in a real WSN show that CMU is able to enhance the performance of MU by reducing its packet loss and end-to-end delay. In 6LoWPAN fragmentation, the loss of a fragment forces the retransmission of the entire packet. To overcome this limitation, CoAP defines blockwise transfer. It splits the packet into blocks and sends each one in reliable transactions, which introduces a significant communication overhead. We propose a novel analytical model to study blockwise and 6LoWPAN fragmentation, which is validated trough Monte Carlo simulations. Both techniques are compared in terms of reliability and delay. The results show that 6LoWPAN fragmentation is preferable for delay-constrained applications. For highly congested networks, blockwise slightly outperforms 6LoWPAN fragmentation in terms of reliability. CoAP defines the observe option to allow a client to register to a resource exposed by a server and to receive updates of its state. Existing QoS in the observe option supports partially timeliness. It allows specifying the validity of an update but it does not guarantee its on-time delivery. This approach is inefficient and does not consider applications, i.e. e-health, that requires the delivery of an update within a deadline. With this limitation in mind, we design and evaluate a novel mechanism for update delivery based on priority. The evaluation proves that implementing a delivery order improves the delay and delivery ratio of updates. Our proposal is also able to reduce the energy consumption allowing clients to express the class of updates that they wish to receive. In part of this thesis, we present our original library for TinyOS, which we referred to as TinyCoAP, and the design and implementation of a CoAP proxy. We compare TinyCoAP to CoapBlip, which is the CoAP implementation distributed with TinyOS. TinyCoAP proves to be able to reach a high code optimization and to reduce the impact over the memory of WSN nodes. The evaluation includes also the analysis of the CoAP reliability mechanism, which was still uncovered in the literature. As a novelty, we also compare CoAP with HTTP considering different solutions for the transport layer protocol such as UDP and persistent TCP connections. The CoAP proxy enables Web applications to transparently access the resources hosted in CoAP devices. It supports long-lived communications by including the WebSocket protocol. It also supports Web applications that use the traditional HTTP long-polling technique. Finally, one of the main contributions of the proxy design is the proposal of a standard URI path format to be used by Web applications to access to a CoAP resource.