Today, quality of service (QoS) is the foundation for deploying voice over IP. As IP applications started to proliferate, QoS started to emerge as the way to prioritize between different applications. The normal approach to QoS takes a network perspective. A network link may have poor QoS, but if it is carrying hundreds of voice calls, it is not clear what that means for the performance of a specific user's call. The view only comes from taking a user perspective. Moving from managing network availability to managing network performance also opens up other new dimensions.
Sourced: CRM Daily, http://www.crm-daily.com