A Study of the Interval Availability and its Impact on SLAs Risk

The obligations that telecommunications providers have with their customers are nowadays clearly specified in SLA contracts. The offered availability during the contract period is one of the most relevant variables in SLAs. Modeling accurately the transient solution posed by the need of considering the interval availability is still an open challenge. A common policy taken to make simpler models is the use of steady state assumptions. Nevertheless, this simplification may put on risk the contract fulfillment as stochastic variations of the measured availability are significant over a typical contract period. This paper makes a theoretical study of the interval availability and propose an approximation to evaluate the cumulative downtime distribution of a system component using renewal theory. We study the evolution of the distribution of the interval availability with the increase of the observation period i.e., duration of the contract, and show its respective impact in the SLA success probability. In addition, using the approximation proposed, we analyze numerically the behavior of the cumulative downtime distribution and the SLA risk under processes that do not follow Markovian assumptions.