The WDM technology has significantly enhanced the performance and reliability of optical components. Still failures occur. Due to the massive increase of bandwidth supported by fiber networks it becomes extremely important to identify the impact of individual failures may have on the network performance. Contemporary link failures in a WDM optical network results in a very high value of call drop probability (CDP). In a backbone network, a link usually carries a huge amount of data and a low CDP is desirable. This paper focuses on understanding the different parameters that affect the reliability of optical networks with emphasis on failures caused due to the links comprising the network infrastructure. One algorithm has been developed for calculation of link reliability in WDM optical network. The algorithm is implemented in NSFNET, Ring and Mesh topology. The parameters affecting the Link Reliability of the optical network are presented, discussed and compared. The different scenarios under study are based on a national USA network topology i.e. NSFNET.