Communication networks are an indispensable component in the smart grid power systems by providing the essential information exchange functions among the electrical devices that are located distributively in the grid. In particular, wireless networks will be deployed widely in the smart grid for data collection and remote control purposes. In this paper, we model the smart grid wireless networks and present the communication delay analysis in typical wireless network deployment scenarios in the grid. As the time critical communications are coupled with the power system protections in the smart grid, it is important to understand the delay performance of the smart grid wireless networks. Our results provide the delay bounds that can help design satisfactory wireless networks to meet the demanding communication requirements in the smart grid.
Energy efficient communication is a critical research problem in large-scale multihop wireless networks because of the limited energy supplies from batteries. We investigate in this paper the minimum energy required to fulfill various information delivery goals that correspond to the major communication paradigms in large wireless networks. We characterize the minimum energy requirement in two steps. We first derive the lower bounds on the energy consumption for all the possible solutions that deliver the information as required. We then design routing schemes that accomplish the information delivery tasks by using an amount of energy comparable to the lower bounds. Our work provides the fundamental understandings of energy needs and the efficient solutions for energy usages in major communication scenarios, which contribute to the rational dimensioning and wise utilization of the energy resources in large wireless networks.
The smart grid features ubiquitous interconnections of power equipments to enable two-way flows of electricity and information for various intelligent power management applications, such as accurate relay protection and timely demand response. To fulfill such pervasive equipment interconnects, a full-fledged communication infrastructure is of great importance in the smart grid. There have been extensive works on disparate layouts of communication infrastructures in the smart grid by surveying feasible wired or wireless communication technologies, such as power line communications and cellular networks. Nevertheless, towards an operable, cost-efficient and backward-compatible communication solution, more comprehensive and practical understandings are still urgently needed regarding communication requirements, applicable protocols, and system performance. Through such comprehensive understandings, we are prone to answer a fundamental question, how to design, implement and integrate communication infrastructures with power systems. In this paper, we address this issue in a case study of a smart grid demonstration project, the Future Renewable Electric Energy Delivery and Management (FREEDM) systems. By investigating communication scenarios, we first clarify communication requirements implied in FREEDM use cases. Then, we adopt a predominant protocol framework, Distributed Network Protocol 3.0 over TCP/IP (DNP3 over TCP/IP), to practically establish connections between electric devices for data exchanges in a small-scale FREEDM system setting, Green Hub. Within the real-setting testbed, we measure the message delivery performance of the DNP3-based communication infrastructure. Our results reveal that diverse timing requirements of message deliveries are arguably primary concerns in a way that dominates viabilities of protocols or schemes in the communication infrastructure of the smart grid. Accordingly, although DNP3 over TCP/IP is widely considered as a smart grid communication solution, it cannot satisfy communication requirements in some time-critical scenarios, such as relay protections, which claim a further optimization on the protocol efficiency of DNP3.
The performance of a wireless multimedia sensor network (WMSN) is tightly coupled with the pose of individual multimedia sensors. In particular, orientation of an individual multimedia sensor (direction of its sensing unit) is of great importance for the sensor network applications in order to capture the entire image of the field. In this paper, we study the problem of self-orientation in a wireless multimedia sensor network, that is finding the most beneficial pose of multimedia sensors to maximize multimedia coverage with occlusion-free viewpoints. We first propose a distributed algorithm to detect a node's multimedia coverage and then determine its orientation, while minimizing the effect of occlusions and total overlapping regions in the sensing field. Our approach enables multimedia sensor nodes to compute their directional coverage, provisioning self-configurable sensor orientations in an efficient way. Simulations show that using distributed messaging and self-orientation having occlusion-free viewpoints significantly increase the multimedia coverage.
Recent studies on mobility-assisted schemes for routing and topology control and on mobility-induced link dynamics have presented significant findings on the properties of a pair of nodes (e.g., the intermeeting time and link life time) or a group of nodes (e.g., network connectivity and partitions). In contrast to the study on the properties of a set of nodes rather than individuals, many works share a common ground with respect to node mobility, i.e., independent mobility in multihop wireless networks. Nonetheless, in vehicular ad hoc networks (VANETs), mobile devices installed on vehicles or held by humans are not isolated; however, they are dependent on each other. For example, the speed of a vehicle is influenced by its close-by vehicles, and vehicles on the same road move at similar speeds. Therefore, the gap between our understanding of the impact of independent mobility and our interest in the properties of correlated mobility in VANETs, along with the real systems altogether, declare an interesting question. How can we measure the internode mobility correlation, such as to uncover the node groups and network components, and explore their impact on link dynamics and network connectivity? Bearing this question in mind, we first examine several traces and find that node mobility exhibits spatial locality and temporal locality correlations, which are closely related to node grouping. To study the properties of these groups on the fly, we introduce a new metric, i.e., dual-locality ratio (DLR), which quantifies mobility correlation of nodes. In light of taking spatial and temporal locality dimensions into account, the DLR can be used to effectively identify stable user groups, which in turn can be used for network performance enhancement.
Spectrum monitoring is a prerequisite in dynamic access regulation, policy enforcement, as well as spectrum database establishment. In this paper, we introduce the dimension of geographical space into the spectrum monitoring problem, and studied deployment strategies of multiple monitors, in terms of coverage time and cost. The monitoring problem is modeled as a 3-d continuous sweep coverage problem, whose solution space is then reduced by effectively dividing the spectra-location space, in order to achieve a small coverage time. The cost minimization is then formulated as a Multiple Traveling Salesman problem (MTSP), which is NP-hard. By observing the structure of the strategy space, we propose a solution that attains a reasonable cost, without applying complex optimization algorithms.
In this paper, we study the critical phase transition time of large-scale wireless multi-hop networks when the network topology experiences a partition due to increasing random node failures. We first define two new metrics, namely the last connection time and first partition time. The former is the last time that the network keeps a majority of surviving nodes connected in a single giant component; while the latter is the first time that the remaining surviving nodes are partitioned into multiple small components. Then we analyze the devolution process in a geometric random graph of n nodes based on percolation-theory connectivity and obtain the sufficient condition under which the graph is percolated. Based on the percolation condition, the last connection time and first partition time are found to be on the same order. Particularly, when the survival function of node lifetime is exponential, they are on the order of log(log n); while if the survival function is Pareto, the order is (log n)1/ρ, where ρ is the shape parameter of Pareto distribution. Finally, simulation results confirm that the last connection time and first partition time serve as the lower and upper bounds of the critical phase transition time, respectively. Further, an interesting result is that the network with heavy-tailed survival functions is no more resilient to random failures than the network with light-tailed ones, in terms of critical phase transition time, if the expected node lifetimes are identical.
To determine the static and dynamic properties of similar materials for engineering rock mass, static and dynamic triaxial tests were conducted on similar engineering rock mass materials. The materials comprised barite powder, quartz sand, gypsum powder, water, and laundry detergent, based on an optimal design. The relationship between deviator stress and an axial strain was investigated under various loading and confining pressure. The test results reveal that the deformation characteristics of similar materials gradually develop from brittleness to ductility with an increase in applied confining pressure, and the static and dynamic elastic moduli show an increasing trend. The hysteresis loop area of similar materials becomes larger with an increase in dynamic stress, suggesting a greater damping ratio of similar materials. The results of this study provide a basic reference for the characteristics model test, and the findings can guide the numerical simulation of the stability analysis of super large-span flat cavern in future studies.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)