a MAC control message and is instructed to enter the DRX mode. When the DRX is configured and the UE is in an active state while in the on- duration interval (see Fig. 1.83), the UE monitors the relevant control channels to detect any downlink allocations and to receive pending transmissions from the serving base sta- tion. If no allocations including paging messages are detected within the on-duration, the UE will enter the inactive interval and will follow the DRX configuration to wake up in the next DRX cycle. During inactivity period, the UE first follows the short DRX cycle, if con- figured, and starts the DRX short cycle timer. After the short cycle timer expires, the UE follows the long DRX cycle. If no short DRX cycle is configured, the UE directly enters the long DRX cycle. It is necessary to optimize the DRX parameters according to the QoS requirements of vari- ous services such as VoIP, web browsing and video streaming, where each has a different traffic model and specific QoS requirements, which may significantly impact the configura- tion of DRX parameters. The services with low delay requirement can be dealt with by acti- vating the UE more frequently to monitor downlink allocations with short DRX cycle setting. While delay-tolerant services can achieve high energy-saving gains by waking up the UE to monitor downlink allocations with long DRX cycle configuration. Therefore, the best trade-off between power saving and responsiveness of the UEs should be made to opti- mally configure DRX parameters. 3GPP releases define the DRX configuration per UE. When multiple data bearers are established, DRX is enabled only when all the data bearers met their corresponding DRX inactivity timer condition, and the shortest DRX cycles 170 Chapter 1 DRX concept UE traffic Inactivity Traffic timer Traffic UE state Active DRX sleep DRX sleep Active On-duration On-duration On-duration DRX cycle DRX sleep DRX sleep DRX cycle DRX cycle DRX cycle LTE DRX Connected Connected C-DRX Connection Data/ signaling User-inactivity 