e sum of operational periods is divided by number of failures. Different flavors of MTBF based on the failure scenario include mean time between system aborts (MTBSA) and mean time between critical failures (MTBCF). MTBF = (downtime - uptime)/number of failures Service downtime: This is the time interval during which a service is unavail- able. High availability of systems is now a mandatory requirement while offering most services and a specific value of maximum service downtime is specified as part of the contractual agreement. This attribute is similar to MTTR, except that repair and recovery usually deals with a system and its Fundamentals of EMS, NMS and OSS/BSS elements, while service downtime looks at the entire service offering without any reference to the systems that make the service possible. Service order completion time: There is a time gap between placing an order and completing of the order. This time gap is required to do the provisioning activity for activating the service. The customer placing the order is provided with an order completion time. This helps the customer to know when the services will be ready for use or when to check back on the status. There could also be price packages associated with order completion time. Some custom- ers might be in urgent need of a service and can request premium processing of the order. Rather than waiting for all orders that were placed by other customers before the current order to be processed, the order with premium processing is given a higher priority and will be processed as soon as possible leading to shorter service order completion time. Some of the attributes monitored by customer QoS/SLA management mod- ule are: QoE: Quality of experience is also referred to as quality of user experience and perceived quality of service. It is a measure of the end user's experience with the vendor. QoE is usually identified using a survey and by getting feedback from the customer. It is different from QoS, which is contractual and can be directly measured and t