.. This work is licensed under a Creative Commons Attribution 4.0 International License.
.. SPDX-License-Identifier: CC-BY-4.0
-
+##################
O-DU High Overview
-*********************
+##################
+**********************
O-DU High Architecture
-======================
+**********************
O-DU implements the functional blocks of L2 layer of a 5G NR protocol stack in SA(StandAlone) mode.
These layers primarily include NR MAC, NR Scheduler and NR RLC layers.
Figure 1 - O-DU High Architecture Diagram
+==============================
O-DU High Thread Architecture
--------------------------------
+==============================
As shown in Figure 1, there are multiple entities within O-DU High. Modules sharing a
given color belong to one thread. O-DU architecture can be defined at a thread
- Thread 8: O1
+=================
O-DU High Modules
---------------------------
+=================
DU APP
-^^^^^^^^^^^^^^^^^^
+************
+
This module configures and manages all the operations of O-DU.
It interfaces with external entities as follows:
- ASN.1 Codecs contain ASN.1 encode/decode functions which are used for System information, F1AP and E2AP messages.
5G NR RLC
-^^^^^^^^^^^^^^^^^^
+************
+
This module provides services for transferring the control and data messages
between MAC layer and O-CU (via DU App).
uplink and downlink functionality respectively.
5G NR MAC
-^^^^^^^^^^^^^^^^^^
+************
+
This module uses the services of the NR physical layer to send and receive data
on the various logical channels.
Functions of the 5G NR MAC module are as follows:
O-DU Utility and Common Functions
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+*********************************
+
These modules contain platform specific files and support O-DU High functionality and message exchanges.
O1 Module
-^^^^^^^^^^
+==========
.. figure:: ODU-O1-Arch.jpg
:width: 554
- Netopeer server: Serves the northbound SMO/OAM Netconf requests.
-
-O-DU-High Interfaces
-======================
-
+**********************
+O-DU High Interfaces
+**********************
This section describes the other modules that O-DU High interfaces with, as shown in below diagram.
+***********************
O-DU High functionality
-========================
-
+***********************
+===============================
Cell Up and Broadcast Procedure
---------------------------------
+===============================
This section describes the cell-up procedure within O-DU High.
- The 5G NR MAC mutiplexes the PDU and sends SSB/SIB1 packets towards the O-DU Low through the Lower MAC.
-
+=====================
UE Related Procedure
------------------------
-
+=====================
The O-DU High supports
- RRC Release
+================================
Closed Loop Automation Procedure
------------------------------------
+================================
This section describes the closed loop automation procedure within O-DU High.
7. On receiving cell bring up command from SMO, the complete Cell bring up and UE attach procedure will be repeated (as explained in above sections)
-
+===================================
O1 Netconf get-alarm list procedure
------------------------------------
+===================================
This section describes the *Health Status Retrieval* scenario of O-DU High health-check. It enables a northbound client(SMO) to retrieve the health of the O-DU High based on the last self-check performed. The alarm-list is provided as the response to the request via O1 Netconf interface.
.. figure:: ODU-O1-GetAlarmListFlow.jpg
:width: 869
- :alt: Figure 6 O1 get alarm-list flow
+ :alt: Figure 7 O1 get alarm-list flow
Figure 7 - O1 get alarm-list flow
- The callback function fetches the alarm list from Alarm Manager and sends it back to the client (SMO/OAM) via Netconf interface.
-
+==========================
Network Slicing procedure
---------------------------
+==========================
This section describes the Network Slicing feature within O-DU High.
- Scheduler updates the received Slice Configuration in its DB and sends back the Slice Reconfiguration Response to MAC and further MAC forwards it to DU APP. Scheduler applies the optimized RRM policies for the dedicated slice.
-
+==========================
Idle Mode Paging procedure
----------------------------
-
+==========================
This section describes the Idle Mode Paging procedure within O-DU High.
As seen in the Figure 9,
-- When a Paging is received from CU and the Cell to be Paged is UP then DU APP will calculate Paging Frame(PF) and i_s(Index of Paging Ocassion/Slot) and groups the Paging of UEs falling on same PF/SFN together and stores in its Cell's Databse.
+- When a Paging is received from O-CU and the Cell to be Paged is UP then DU APP will calculate Paging Frame(PF) and i_s(Index of Paging Ocassion/Slot) and groups the Paging of UEs falling on same PF/SFN together and stores in its Cell's Databse.
- When a Slot Indication for SFN is received then DU APP extracts the Paging of all UEs whose PF is ahead by PAGING_DELTA and builds Paging RRC PDU. DU APP sends the same via DL PCCH Indication to MAC.
- MAC forwards the PAGE to PHY in TX_Data.Request.
+==============================
+Inter-DU Handover within O-CU
+==============================
+
+This section describes the handling of inter-DU handover of a UE within O-DU High.
+
+.. figure:: Inter_DU_Handover_Within_OCU.png
+ :width: 600
+ :alt: Inter-DU Handover withing O-CU
+
+ Figure 10 - Inter_DU Handover call flow
+
+Assumption: UE is RRC connected with DU and PDU data session is active.
+
+- The UE sends Measurement Report message to the source O-DU. This message is sent from O-DU to O-CU in the UL RRC MESSAGE TRANSFER message over F1AP interface.
+
+- Based on UE Measurement Report, O-CU makes a handover decision to another cell belonging to the target O-DU.
+
+- The O-CU sends a UE CONTEXT MODIFICATION REQUEST message to source O-DU to query the latest configuration.
+
+- The DU APP in source O-DU responds with a UE CONTEXT MODIFICATION RESPONSE message that includes latest full configuration information.
+
+- The O-CU sends a UE CONTEXT SETUP REQUEST message to the target O-DU to create an UE context and setup one or more data bearers. The UE CONTEXT SETUP REQUEST message includes Hand-overPreparationInformation. At target O-DU, DU APP sends UE Create Request to MAC and RLC layers to create the UE context with radio resources and receives UE Create Response from the respective protocol layers.
+
+- The target O-DU responds with a UE CONTEXT SETUP RESPONSE message if the target O-DU can admit resources for the handover.
+
+- The O-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source O-DU, which includes RRCReconfiguration message towards the UE. The O-CU also indicates the source O-DU to stop the data transmission for the UE.
+
+- The source O-DU forwards the received RRCReconfiguration message to the UE and then sends the UE Reconfiguration Request to MAC/Scheduler and RLC layer and get the UE Reconfiguration Response from the respective protocol layers.
+
+- The source O-DU responds to the O-CU with UE CONTEXT MODIFICATION RESPONSE message.
+
+- UE triggers Random Access procedure at the target O-DU. This is a contention free random access if UE was informed about its dedicated RACH resources in RRC Reconfiguration message.
+
+- Once Random Access procedure with target O-DU is complete, the UE responds to the target O-DU with a RRCReconfigurationComplete message.
+
+- The target O-DU sends UL RRC MESSAGE TRANSFER message to O-CU to convey the received RRCReconfigurationComplete message.
+
+- The downlink and uplink data packets are sent to/from the UE through the target O-DU.
+
+- The O-CU sends UE CONTEXT RELEASE COMMAND message to the source O-DU.
+
+- The source O-DU sends UE DELETE REQUEST to MAC/RLC layers to release the UE context and receives UE DELETE RESPONSE message.
+
+- The source O-DU responds to O-CU with UE CONTEXT RELEASE COMPLETE message.
+
+=============================
+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 configuration 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.
+
+***********************
OSC Testcases Supported
-=========================
+***********************
The O-DU High partially supports below use-cases:
Code Review / o-du / l2.git / blobdiff