1. Emerging Science For Interdependent Complex Networks: “Methodological Breakthrough Needed” Junshan Zhang School of ECEE, Arizona State University NITRD Workshop on Complex Engineered Networks Washington D.C., Sept 20-21, 2012

2. From Individual Networks to Network of Networks • Networked systems: modern world consists of an intricate web of interconnected physical infrastructure and cyber systems, e.g., communication networks, power grid, transportation system, social networks, … • Over the past few decades, tremendous effort on studying individual networks: • Communication networks, e.g., Internet, wireless, sensor nets, … • Complex networks, e.g., E-R graph, small world model, scale-free networks … • Little attention has been paid to interdependent networks: Many networks have evolved to depend on each other, and depend heavily on cyber infrastructure in particular • Need a theoretic foundation for cyber-physical systems and interdependent networks per se ! 2

3. Some Examples for Interdependent networks: Modern Power Grid & Mobile Social Networks • Over past 4~5 years, we have been studying • 1) Smart Grid: Power station operation relies on the • control of nodes in cyber infrastructure; • cyber nodes need power supply from power stations • Q) Which parts are most vulnerable and which other • parts are most resilient? Where are interdependences? • 2) Mobile social networks: Social networks rely on • communication media; and network traffic is affected • by social networking. • What is impact of interdependence on traffic • volume and social networking behavior? • Interdependence: Operations of one network depend • heavily on the functioning of the other network [Nature 2010] 3

4. Methodological challenges:Modeling, Designing and Analyzing Interdependent Networks • I) From model abstraction to problem formulation: How to model different functionality modules and the interdependence? • Often times disparate methods are used for cyber systems and physical systems, e.g., differential equations for power systems whereas graph models for communication networks • Interdependence is asymmetric and depends on local properties, e.g., different components of power grids have different QoS requirements; further, • more difficult to abstract dependence of comm. network on power grid • II) Engineering machineries : incommensurate (fragmented) tools for multi-scale dynamics in spatio-temporal domains and across interdependent subsystems, • - tree-based hierarchical control meets mesh networking • - graphical models meet differential equations, Nonconvexity & stochastic • Percolation theory meets combinatorial optimization • From theory to emulation/simulation to prototype … 4

5. Cyber-Physical Systems (CPS): Two Interdependent Networks A networked system consists of physical network and cyber network Emerging as the underpinning technology for 21th century Applications: smart grid, intelligent transportation system, manufacturing, etc.