Network survivability gauges the ability of a network to support the committed Quality of Services (QoS) continuously in the presence of various failure scenarios. We perceive that both performance and availability are integral components of survivability. Therefore, we propose a composite model for survivability that consists of performance and availability analysis. A analytical technique is presented to find the excess loss due to failure (ELF) when the system is operating in gracefully

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... ded states. An algorithm is proposed to carry out the availability analysis of the network even when the available paths between nodes are non-disjoint. These two models are combined to construct a hierarchical model to evaluate the network survivability performance. We consider single and multiple link failures, and also node failures. A WDM network with wavelength conversion is used as an example for this evaluation.