Abstract
The IEEE 802.15.4 standard provides a flexible communication support for low-rate wireless personal area networks) applications. When active, the beacon-enabled mode provides a real-time communication to the supported application by adopting a guaranteed time slot (GTS) mechanism. However, this mechanism permits only up to seven real-time communicating devices. One way to deal with this limitation is to share the communication opportunities among the periodic tasks, by skipping some of the task activations in a controlled way. One of the widely accepted periodic task models that allows skips in periodic activations is the (m,k)-firm model. Motivated by this problem, this paper proposes the use of a dynamic GTS scheduling approach based on the (m,k)-firm task model, to deal with the GTS starvation problem. The proposed scheduling approach is based on pre-defined spins of the originally defined (m,k)-firm pattern. The use of an exact schedulability analysis test ensures that for each admitted message stream, at least m messages will be transmitted within each window of k consecutive deadlines. The schedulability analysis may be executed in polynomial time and therefore can be used as an online admission test for GTS requests. The effectiveness of the approach has been assessed both by a set of simulations and an experimental evaluation.

Abstract
An important feature for the design of algorithms for Wireless Sensor Networks is the estimation of its lifetime. To design communication protocols and data collection algorithms with low power consumption, a key factor is the ability to measure the actual consumption in the sensor node. This ability enables the verification of theoretical/simulated models, allowing an accurate estimation of the network longevity. This paper proposes a system for measuring the energy consumption levels of sensor nodes in normal operation, allowing the estimation of the real lifetime for each node, enabling the optimization of energy aware protocols. Presented data is related to the energy consumption of a MicaZ node and to the behavior of the full discharge of a battery pack. A set of polynomial functions describing this energy discharge are also presented that can be easily inserted into the battery models of network simulators to obtain a more realistic behavior.

Abstract

Abstract
When camera-enabled sensors are deployed for visual monitoring, a new set of innovative applications is allowed, enriching the use of wireless sensor network technologies. In these networks, energy-efficiency is a highly desired optimization issue, mainly because transmission of images and video streams over resource-constrained sensor networks is more stringent than transmission of conventional scalar data. Due to the nature of visual monitoring, that follows a directional sensing model, camera-enabled sensors may have different relevancies for the application, according to the desired monitoring tasks and the current sensors' poses and fields of view. Exploiting this concept, each data packet may be associated with a priority level related to the packet's origins, which may be in turn mapped to an energy threshold level. In such way, we propose an energy-efficient relaying mechanism where data packets are only forwarded to the next hop if the associated energy threshold level is below the current energy level of the relaying node. Thus, packets from low-relevant source nodes will be silently dropped when the current energy level of intermediate nodes run below the pre-defined thresholds. Doing so, energy is saved potentially prolonging the network lifetime. Besides the sensing relevancies of source nodes, the relevance of DWT subbands for reconstruction of original images is also considered. This allows the creation of a second level of packet prioritization, assuring a minimal level of image quality even for the least relevant source nodes. We performed simulations for the proposed relaying mechanism, assessing the expected performance over a traditional relaying paradigm.

Abstract
Wireless sensor networks are being widely used to support time-critical applications, even knowing that previous versions of the IEEE 802.15.4 standard have a limited capacity to ensure a timely communication service. Its main limitation is the reduced number of available GTS slots. More recently, the IEEE 802.15.4e LLDN mode has been released to address this issue. It provides static TDMA-based communication, with a larger number of available slots to support time-critical communication. However, it does not provide any guidelines on how to allocate slots, in order to guarantee the message real-time requirements. In this paper, we propose a methodology to allocate LLDN slots based on the traffic load imposed by the supported message streams. The proposed allocation scheme is able to handle message streams with periodicities that are not multiple of the beacon interval, and also to support static and dynamic prioritizing of message streams.

Abstract
IEC 61850 is a communication standard for electrical Substation Automation Systems (SAS). It defines both the information model and services used for communication between Intelligent Electronic Devices (IEDs) in a substation. The adoption of this standard brings several advantages for the design and operation of substations. The abstract data models defined in IEC 61850 can be mapped upon application protocols, such as MMS, GOOSE or SMV. These protocols can run upon TCP/IP networks or upon specific high speed Ethernet LANs, in order to match the timing requirements associated to protective relaying mechanisms. For the specific case of GOOSE messages, the standard specifies the use of VLANs (Virtual LANs) with priority tagging (IEEE 802.1q) to implement separate virtual networks with the appropriate message priority levels, in order to ensure the specified response times. The lack of adequate simulation models that enable the response time assessment of both SMV and GOOSE messages is one of the shortcomings of available simulation tools. In this paper, we propose simulation models for the IEC 61850 communication standard, targeting application that use GOOSE and SMV messages. This simulation models has been built upon OMNeT++/INET. The simulation results obtained from a typical IEC 61850 communication scenario show the effectiveness of the developed models. Some of these results have been experimentally validated.