As the need of network user for high availability is increased, how to access and quantify the availability of networks just becomes a critical issue. This paper deals with the most common protection schemes in ASON (automatic switch optical network) networks, providing for each of them an algebraic formulation of availability analysis. We consider single or multiple link failure scenarios. Availability models are applied to some numerical examples that allow us to compare the different availability degrees granted by each protection technique. The paper highlights some important availability relations between path protection schemes and most relevant network parameters.