redundancy to achieve high availability. Maintenance actions occur during brief periods of down-time only after a fault indicator activates. Failure is only significant if this occurs during a mission critical period. Modeling and simulation is used to evaluate the theoretical reliability for large systems. The outcome of this kind of model is used to evaluate different design options. A model of the entire system is created, and the model is stressed by removing components. Redundancy simulation involves the N-x criteria. N represents the total number of components in the system. x is the number of components used to stress the system. N-1 means the model is stressed by evaluating performance with all possible combinations where one component is faulted. N-2 means the model is stressed by evaluating performance with all possible combinations where two component are faulted simultaneously. Reasons for unavailability A survey among academic availability experts in 2010 ranked reasons for unavailability of enterprise IT systems. All reasons refer to not following best practice in each of the following areas (in order of importance): Monitoring of the relevant components Requirements and procurement Operations Avoidance of network failures Avoidance of internal application failures Avoidance of external services that fail Physical environment Network redundancy Technical solution of backup Process solution of backup Physical location Infrastructure redundancy Storage architecture redundancy A book on the factors themselves was published in 2003. Costs of unavailability In a 1998 report from IBM Global Services, unavailable systems were estimated to have cost American businesses $4.54 billion in 1996, due to lost productivity and revenues. See also Fault tolerance High-availability cluster Overall equipment effectiveness Reliability, availability and serviceability Resilience (network) Ubiquitous computing Notes References External links Lecture Notes on Enterprise Computing University of Tübingen Lecture notes on Embedded Systems Engineering by Prof. Phil Koopman Uptime Calculator (SLA) System administration Quality control Applied probability Reliability engineering Measurement