Series and Parallel Dependency Links: Vitality for Interdependent Networks of Networks

Abstract: In a network of networks, a dependency link runs from a supplier to the node it serves and binds the supplier's failure to the node's failure. Indices computed from the graph topology leave aside the service each edge carries, but in real interdependent systems that service decides how failure propagates: a pumping station requires electricity and telemetry, and losing either stops it; a district fed by either of two reservoirs survives the loss of one. Building on this observation, we label each node of the network with the service it provides. We then measure how far this qualitative input can change the node's vitality ranking: by fixing the graph topology we show that changing only the labels moves the Mean Operativity Loss from $0.023$ to $0.114$, while a ranking computed from the unlabelled graph topology is blind to the difference. The links give a node its suppliers; the labels group those suppliers into alternatives within one service and joint requirements across services. These labels should belong to the description of an interdependent system alongside the links themselves for a fidelity vitality measure