Assortativity Decreases the Robustness of Interdependent Networks

It was recently recognized that interdependencies among different networks can play a crucial role in triggering cascading failures and hence system-wide disasters. A recent model shows how pairs of interdependent networks can exhibit an abrupt percolation transition as failures accumulate. We report on the effects of topology on failure propagation for a model system consisting of two interdependent networks. We find that the internal node correlations in each of the two interdependent networks significantly changes the critical density of failures that triggers the total disruption of the two-network system. Specifically, we find that the assortativity (i.e. the likelihood of nodes with similar degree to be connected) within a single network decreases the robustness of the entire system. The results of this study on the influence of assortativity may provide insights into ways of improving the robustness of network architecture, and thus enhances the level of protection of critical infrastructures

The failure tolerance of mechatronic software systems to random and targeted attacks

This paper describes a complex networks approach to study the failure tolerance of mechatronic software systems under various types of hardware and/or software failures. We produce synthetic system architectures based on evidence of modular and hierarchical modular product architectures and known motifs for the interconnection of physical components to software. The system architectures are then subject to various forms of attack. The attacks simulate failure of critical hardware or software. Four types of attack are investigated: degree centrality, betweenness centrality, closeness centrality and random attack. Failure tolerance of the system is measured by a 'robustness coefficient', a topological 'size' metric of the connectedness of the attacked network. We find that the betweenness centrality attack results in the most significant reduction in the robustness coefficient, confirming betweenness centrality, rather than the number of connections (i.e. degree), as the most conservative metric of component importance. A counter-intuitive finding is that "designed" system architectures, including a bus, ring, and star architecture, are not significantly more failure-tolerant than interconnections with no prescribed architecture, that is, a random architecture. Our research provides a data-driven approach to engineer the architecture of mechatronic software systems for failure tolerance.Comment: Proceedings of the 2013 ASME International Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE 2013 August 4-7, 2013, Portland, Oregon, USA (In Print

A survey on multilayer networks modelled to assess robustness in infrastructure systems

The development of modern societies places particular demands on the consistent performance of infrastructure systems. Because multilayer network models are capable of representing the interdependencies between infrastructure components, they have been widely used to analyse the robustness of infrastructure systems. This present study is a systematic review of literature, published since 2010. It aims to investigate how multilayer network models have been used in analysing the robustness of infrastructure systems. According to findings, percolation theory was the most popular method used in about 57% of papers. Regarding the properties, coupling strength and node degree were the most common while directed links and feedback conditions were the least common. The following gaps were identified which provide opportunities for further research. These include the absence of models based on real-world data and the need for models that make fewer simplifying assumptions about complex systems. No papers considered all potential properties, and their effect on boosting or weakening each other’s effect. By considering all properties, the importance of different properties on the robustness of infrastructure systems can be quantified and compared in future studies

Robustness of double-layer group-dependent combat network with cascading failure

The networked combat system-of-system (CSOS) is the trend of combat development with the innovation of technology. The achievement of combat effectiveness requires CSOS to have a good ability to deal with external interference. Here we report a modeling method of CSOS from the perspective of complex networks and explore the robustness of the combat network based on this. Firstly, a more realistic double-layer heterogeneous dependent combat network model is established. Then, the conditional group dependency situation is considered to design failure rules for dependent failure, and the coupling relation between the double-layer subnets is analyzed for cascading failure. Based on this, the initial load and capacity of the node are defined, respectively, as well as the load redistribution strategy and the status judgment rules for the cascading failure model. Simulation experiments are carried out by changing the attack modes and different parameters, and the results show that the robustness of the combat network can be effectively improved by improving the tolerance limit of one-way dependency of the functional net, the node capacity of the functional subnet and the tolerance of the overload state. The conclusions of this paper can provide a useful reference for network structure optimization and network security protection in the military field

Robustness of interrelated traffic networks to cascading failures

The vulnerability to real-life networks against small initial attacks has been one of outstanding challenges in the study of interrelated networks. We study cascading failures in two interrelated networks S and B composed from dependency chains and connectivity links respectively. This work proposes a realistic model for cascading failures based on the redistribution of traffic flow. We study the Barabási-Albert networks (BA) and Erd's-Rényi graphs (ER) with such structure, and found that the efficiency sharply decreases with increasing percentages of the dependency nodes for removing a node randomly. Furthermore, we study the robustness of interrelated traffic networks, especially the subway and bus network in Beijing. By analyzing different attacking strategies, we uncover that the efficiency of the city traffic system has a non-equilibrium phase transition at low capacity of the networks. This explains why the pressure of the traffic overload is relaxed by singly increasing the number of small buses during rush hours. We also found that the increment of some buses may release traffic jam caused by removing a node of the bus network randomly if the damage is limited. However, the efficiencies to transfer people flow will sharper increase when the capacity of the subway network αS > α0

Cascading Failures Analysis Considering Extreme Virus Propagation of Cyber-Physical Systems in Smart Grids

Communication networks as smart infrastructure systems play an important role in smart girds to monitor, control, and manage the operation of electrical networks. However, due to the interdependencies between communication networks and electrical networks, once communication networks fail (or are attacked), the faults can be easily propagated to electrical networks which even lead to cascading blackout; therefore it is crucial to investigate the impacts of failures of communication networks on the operation of electrical networks. This paper focuses on cascading failures in interdependent systems from the perspective of cyber-physical security. In the interdependent fault propagation model, the complex network-based virus propagation model is used to describe virus infection in the scale-free and small-world topologically structured communication networks. Meanwhile, in the electrical network, dynamic power flow is employed to reproduce the behaviors of the electrical networks after a fault. In addition, two time windows, i.e., the virus infection cycle and the tripping time of overloaded branches, are considered to analyze the fault characteristics of both electrical branches and communication nodes along time under virus propagation. The proposed model is applied to the IEEE 118-bus system and the French grid coupled with different communication network structures. The results show that the scale-free communication network is more vulnerable to virus propagation in smart cyber-physical grids

Network robustness improvement via long-range links

Abstract Many systems are today modelled as complex networks, since this representation has been proven being an effective approach for understanding and controlling many real-world phenomena. A significant area of interest and research is that of networks robustness, which aims to explore to what extent a network keeps working when failures occur in its structure and how disruptions can be avoided. In this paper, we introduce the idea of exploiting long-range links to improve the robustness of Scale-Free (SF) networks. Several experiments are carried out by attacking the networks before and after the addition of links between the farthest nodes, and the results show that this approach effectively improves the SF network correct functionalities better than other commonly used strategies

Nuevas técnicas para modelizar y analizar la vulnerabilidad de infraestructuras críticas de energía interdependientes

La interdependencia entre las redes de gas y electricidad es motivo de preocupación debido a la creciente utilización del gas para la generación de electricidad en centrales de ciclo combinado y al uso de energía eléctrica de los compresores en la red de gas. Estas redes están sujetas a riesgos de interrupción del suministro derivados de posibles problemas técnicos o amenazas intencionadas. Por lo tanto, resulta conveniente modelizar y analizar la vulnerabilidad de estas infraestructuras críticas de energía interdependientes.En esta tesis doctoral se presenta, en primer lugar, una metodología para analizar conjuntamente los flujos de electricidad y gas. El conjunto de ecuaciones no lineales que representan la operación del sistema de potencia se resuelve utilizando el método de Newton-Raphson, mientras que las ecuaciones en la red de gas se resuelven utilizando el enfoque de transformada análoga-lineal. Se presentan dos casos de estudio para demostrar la simplicidad de la metodología propuesta. Los resultados obtenidos se verifican contra el método Newton-Raphson tradicional con el fin de comprobar la solución alcanzada, encontrando un buen desempeño de la metodología conjunta aplicada. La aplicación del enfoque propuesto permite el análisis de la vulnerabilidad de las infraestructuras energéticas interdependientes. También, se desarrolla una metodología para evaluar la vulnerabilidad estructural de las redes de energía eléctrica y gas acopladas, considerando interdependencias en el proceso de fallos en cascada. La vulnerabilidad se evalúa empleando el índice de desconexión de carga y las medidas de centralidad de vulnerabilidad geodésica e impacto en la conectividad. El estudio muestra una elevada correlación entre el índice de desconexión de carga y el índice de vulnerabilidad geodésica. De esta manera, la teoría de grafos puede usarse como sustituto de los enfoques de flujos de carga que demandan un conocimiento detallado de los parámetros eléctricos e hidráulicos de los sistemas bajo estudio y son computacionalmente más intensivos que los métodos estadísticos de grafos. Como resultado, se propone un nuevo método para estimar la vulnerabilidad de las redes de energía eléctrica y gas conjuntas utilizando el índice de vulnerabilidad geodésica. Asimismo, se estudia el comportamiento de las redes de electricidad y gas natural de España, tanto de manera separada como conjunta. Los resultados muestran que la red de gas natural es menos robusta que la red eléctrica y que la red acoplada es más vulnerable que la red eléctrica ante fallos aleatorios y deliberados. Además, eliminar los nodos más fuertemente conectados de los dos sistemas independientes resultaría una estrategia de ataque eficaz para el rápido colapso de las infraestructuras acopladas interdependientes. Por último, se evalúa la robustez estructural de los planes de expansión de las infraestructuras de electricidad y gas natural en España. Los casos de estudio corresponden a las principales inversiones propuestas por los operadores de los sistemas en 2015-2020. Los resultados demuestran que la construcción de algunas instalaciones para la expansión de ambas redes no mejora la robustez estructural de la red acoplada; sin embargo, cuando se tiene en cuenta todo el programa de inversión se produce una mejora relativa de hasta un 6% con respecto al caso base. La metodología propuesta en esta tesis corrobora que la aplicación de la teoría de grafos es adecuada para analizar la planificación de activos de una infraestructura energética crítica, requiriendo únicamente la topología y el programa de inversiones para evaluar el desempeño de la red acoplada en caso de fallos en cascada. En suma, esta tesis doctoral pone de relieve la importancia de que los sistemas energéticos se aborden como redes acopladas debido a sus fuertes interacciones. Una perturbación en un sistema puede no ser crítica si las infraestructuras están separadas, pero dado que ambas redes son interdependientes, el impacto resultante podría causar fallos en el otro sistema. En otras palabras, las interdependencias aumentan el impacto de las perturbaciones.<br /