Cisco Systems This paper summarizes the recommended correlation between service classes and their usage, with references to their corresponding recommended Differentiated Service Code Points (DSCP), traffic conditioners, Per-Hop Behaviors (PHB) and Active Queue Management (AQM) mechanisms. There is no intrinsic requirement that particular DSCPs, traffic conditioner PHBs and AQM be used for a certain service class, but as a policy it is useful that they be applied consistently across the network.