Inaccuracy of availability metrics estimated by the Serial-Parallel model

Provisioning QoS network connections with the desired availability relies on estimating the availability of the connections in advance. The redundancy of the protection - which denotes how many failures can be survived - gives a good hint on the availability, thus lower bound estimation can be carried out. If we want to achieve a more accurate approximation, we have to use other methods. The Serial-Parallel availability modeling and calculation method, based on the availability metrics of the components of the connection, offers a fast estimation, with the complexity of O(n) in case of n components. However, the result of this estimation can be inaccurate since the model does not take into account the overlapping of components, i.e., when a component is member of more different series. In this paper we analyze the inaccuracy of the Serial-Parallel method. We prove that the estimated availability is always less than the exact one, define an upper bound onto the inaccuracy of the estimated unavailability and show where does this inaccuracy converge by increasing the availability of the network components.