+=============================
+Discontinuous reception (DRX)
+=============================
+
+This section describes the Discontinuous reception (DRX) feature within O-DU High.
+
+
+.. figure:: Discontinuous_reception.PNG
+ :width: 600
+ :alt: Discontinuous reception flow
+
+ Figure 11 - Discontinuous reception flow
+
+- The connected mode DRX is used to improve UE's battery power consumption. This allows UE to be active for a certain amount of time to monitor PDCCH. UE shall become active or inactive based on the DRX timers.
+
+- When UE is created at O-DU during RRC connection setup procedure, DU APP forwards the default DRX configurationĀ to MAC, who then passes it to SCH as part of UE configuration request. SCH stores these configuration and will use it to calculate the start time and expiry time of various DRX timers. But these timers will only start after UE is RRC connected.
+
+- O-DU may receive modified DRX-configuration in UE CONTEXT SETUP REQUEST from O-CU. DU APP forwards it to MAC who forwards it to SCH as part of UE reconfiguration request. In this case, SCH will stop all DRX timers, re-calculate the start time and expiry time of various timers based on updated configuration and restart the drx-onDurationTimer.
+
+- Along with long cycle, DRX in O-DU high also supports short cycle which is enabled if short cycle configuration is recived in UE CONTEXT SETUP REQUEST.
+
+- DRX timers supported in ODU-High are drx-onDurationTimer, drx-InactivityTimer, drx-ShortCycleTimer, drx-HARQ-RTT-TimerDL, drx-RetransmissionTimerDL, drx-HARQ-RTT-TimerUL and drx-RetransmissionTimerUL.
+
+- UE is active when any of the following timers is running: drx-onDurationTimer, drx-InactivityTimer, drx-RetransmissionTimerDL or drx-RetransmissionTimerUL, else the UE is considered as inactive.
+
+- Initially, drx-onDurationTimer is started based on long cycle length. While drx-onDurationTimer or drx-InactivityTimer are running, UE becomes active to monitor PDCCH and send data in UL/DL. When drx-InactivityTimer expires, drx-ShortCycleTimer starts. While drx-ShortCycleTimer is running, drx-onDurationTimer is started based on short cycle length. Once drx-ShortCycleTimer expires, long cycle length is used again. Refer to figure 12 below for detailed working of these timers.
+
+.. figure:: Drx_Onduration_Inactive_ShortCycle_Timer.png
+ :width: 600
+ :alt: onDurationTimer,InactivityTimer,ShortCycleTimer flow
+
+ Figure 12 - onDurationTimer,InactivityTimer,ShortCycleTimer flow
+
+- If HARQ is received/sent, drx-HARQ-RTT-TimerDL or drx-HARQ-RTT-TimerUL is started. On its expiry drx-RetransmissionTimerDL or drx-RetransmissionTimerUL will start. While it is running, UE becomes active for retransmission of data in DL/UL. Refer to figure 13 and 14 below for detailed working of these timers.
+
+.. figure:: Drx_Dl_Harq_Retransmission_Timer.png
+ :width: 600
+ :alt: HARQ-RTT-TimerDL, RetransmissionTimerDL flow
+
+ Figure 13 - DL Harq Retransmission Timers flow
+
+.. figure:: Drx_Ul_Harq_Retransmission_Timer.png
+ :width: 600
+ :alt: HARQ-RTT-TimerUL, RetransmissionTimerUL flow
+
+ Figure 14 - UL Harq Retransmission Timers flow
+
+- If O-DU receives DRX configuration release indicator IE as a part of UE CONTEXT MODIFICATION REQUEST from O-CU, DU APP will forward this indicator to MAC which forwards it to SCH as part of UE reconfiguration request. In this case SCH stops all DRX timers, deletes DRX configuration and marks UE as active by default.