Quality of service (QoS) is an essential element in the enterprise transport infrastructure. The concept of QoS is loosely defined across different enterprise service management layers such that it can be used to evaluate and control the performance of a link, a domain sub-network, or the entire network. There are three aspects of QoS management: control, monitoring, and adjustment. QoS control includes planning and configuring performance attributes such as bandwidth, delay, loss, and traffic priority. An effective QoS planning and configuration function should include a policy registry and enforcement engine leveraging on open interfaces for policy descriptions. The QoS policies dictate the behaviors of the adjustment function. QoS monitoring works hand-in-hand with service monitoring functions depicted in chapter 6 to provide quality indicators of the managed services. The collected and organized indicators permit the adjustment function to arrange different amount of traffic being injected into the network in accordance with the QoS classes. The adjustment function drives vendor specific access facilities for access control in responding to the policy rules.