unctioning. The next-generation networks will be comprised of domains of varying technology at various levels - IP over ATM, ATM over optical, packet voice over IP, IP over cable, etc. A fiber cut in the optical network will possibly generate alarm events at several layers; the optical layer itself, the ATM layer due to loss of signal and the TCP/IP layer due to loss of session. These faults/alarms will originate independently from multiple Chapter 8 - Next-Generation OSS network elements and then be collected by Fault Collectors using standard protocols such as SNMP traps or syslog. Fault collectors perform alarm suppression and filter out some erroneous alarms. For example, a fiber cut in an optical ring may trigger a fault event but the network may re-route traffic around the fault. The next-generation fault management systems must be aware of the intelligent nature of NGN. Since one fault collector may not be able to detect alarms received by another, all fault events are forwarded to a higher-level fault manager. At the higher level, faults are filtered, aggregated and the root cause of the fault is determined. Fault events from the legacy network via the legacy OSS are also combined here. In order to determine the root cause, the fault manager needs information on the topology of the network. This information is obtained from the integrated NMS database. Performance violation events from a performance management system also flow into the fault manager. Based on the analysis, the faulty network resource is identified. The next step is to determine whether the fault was affected service. This facility-to-service correlation uses the NMS database to identify the logical and physical design of the customer service. For proactive customer service, trouble tickets can automatically be created for all affected customers and services. Figure 14 shows the various fault management functions in a NGN OSS. The next- generation fault management systems must be flexible enough SO users can define their own rules and crit