media gateway (MGW) goes down due to installation failure of a software module Fundamentals of EMS, NMS and OSS/BSS on the MGW, a set of event logs are generated. The CA will create a log showing loss of connection with MGW and the MGW, which is a different network ele- ment, will generate its own log specifying loss of connection with the CA. There will also be logs in the MGW specifying failure of a software module on MGW and many other logs corresponding to the status of the links that connect the CA and MGW. While at EMS level only collection of data from a specific element like CA or MGW occurs, at the NMS level log data from both these elements are available. So the event logs from different network elements can be correlated and in this scenario the logs from the CA and MGW are correlated and the actual problem can be identified, which is not possible at the EMS level by just a connec- tion down log at CA. Knowledge of the actual problem at the NMS level, leads to the capability of creating applications that can take corrective actions when a predefined problem occurs. The fault logs generated at the network element is also used in layers above the network layer. The time duration for which a link is down can be used as input information for a service assurance application in the service layer, and this can be used to provide a discount in the billing module on the business management layer. There is a separate chapter that details the functions handled in the network management layer and this chapter is more about giving the reader a good under- standing of a network management application and the usual modules required while implementing a network management system. 11.2 Network Management System The network management system as the name signifies is used to manage a network. It provides a holistic view of the entire network. The different activities performed in a typical NMS when it is started and before any maintenance command can be issued are: Discovery of elements in the network: This can happen in