Spares Inventory Sizing for End-to-End Service Availability

A useful strategy for maintaining service, or restoring service quickly, in the event of network element failure, is to maintain a stock of field replaceable units (FRUs) to replace the failed unit(s). The first key decision variable in a sparing plan is the desired level of service to be maintained in case of failure(s). This is called the service continuity objective. In past practice [1], service continuity objective usually referred to a desired value of stockout probability, i.e., the probability of not having a spare available when it is needed. This is no longer adequate since service providers are now more interested in a sparing strategy more closely tied to end-to-end availability objectives, from which service-level agreements are derived. This paper proposes a model for sizing the spare inventory to meet an end-to-end service availability requirement. We use an Internet access network to illustrate our approach.