# O-DU and O-CU
[TOC]
## O-DU
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. O-DU is divided into two parts: O-DU High and O-DU Low. O-DU High communicates with O-DU Low on the FAPI interface. O-DU High consists of RLC and MAC. Meanwhile, O-DU Low consist of High PHY.


O-DU High modules are developed as shown in the below diagram.


From this image above, we can see that O-DU High modules not only consist of RLC and MAC, but also DU APP.
O-DU High Modules:
- DU APP
This module configures and manages all the operations of O-DU. It interfaces with external entities as follows:
- OAM: DU APP interacts with OAM on the O1 interface for configuration, alarms and performance management.
- O-CU: DU APP interacts with O-CU for RAN functionalities over the F1 interface which is built on SCTP. Control messages are exchanged on the F1-C interface and data messages on the F1-U interface.
- RIC: DU APP interacts with RIC on E2 interface over SCTP.

- RLC
RLC provides services for transferring the control and data messages between MAC layer and O-CU (via DU App). 5G NR RLC UL and 5G NR RLC DL are the sub modules of this module that implement uplink and downlink functionality respectively. RLC can do error correction, segmentation, reassembly, re-segmentation, and re-ordering.

- MAC
MAC 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:
- Multiplexing/Demultiplexing
- Scheduling Information Reporting
- Handling UE Priority
- Error Correction (HARQ)
- Logical Channel Prioritization

## O-CU
O-DU High communicates O-CU on the F1AP interface. The control message exchanges are on F1-C while data message exchanges are on F1-U interfaces.


## O-DU and O-CU Functional Blocks
**O-DU Functional Blocks**

O-DU High functional blocks are as follows:
- F1 Control Plane interface handling modules (F1AP handler)
F1AP handler used to handle cell state manager, UE state manager, F1AP interface manager, resource manager, MAC interface, and RLC interface.
- F1 User Plane interface handling modules (F1-U)
- Data Plane Application: Handles DL Data Packets.
- eGTPU encaps/decaps: performs eGTPU header encapsulation/decapsulation at F1-U.
- Downlink Flow Control: Data Delivery Status generation with feedback received from RLC.
- RLC Protocol modules (NR-RLC)
- MAC Protocol modules (NR-MAC)
- L2 MAC Scheduler (NR Scheduler)
- O-DU-OAM-Agent

**O-CU Functional Blocks**

For Functional Blocks in O-CU, there are 2 parts:

**1. O-CU-CP**
O-CU-CP handles control plane functionality of O-CU.
- O-CU-CP-OAM-Agent: used for configuration and control management, performance counter management, and fault management.
- Cell Procedure Management: manages the cell level procedures.
- gNB Procedure Management: manages the non-UE associated NGAP/XnAP/F1AP procedures.
- UE Procedure Management: manages the UE access control and signaling procedures.
- O-CU-UP Control: configures and controls the CU-UP entities as per the E1 interface, also invoked primarily by the UE Procedure Management.
- RRC/NGAP/XnAP/F1AP Encoder and Decoder: encodes and decodes all RRC/NGAP/XnAP/F1AP contents.
- RRC States
- RRC Idle: no data transfer, doesn’t have RRC context, no core network connection, and device-controlled mobility.
- RRC Connected: UE context is stored at the network, UE mobility is network controlled, UE location is known at cell level.
- RRC Inactive: proposed in 5G as the low activity UE state

**2. O-CU-UP**
O-CU-UP handles user plane functionality of O-CU
- O-CU-UP-OAM-Agent: used to configuration and control management, performance counter management, and fault management.
- eGTPu: handles the eGTPU protocol stack.
- NR PDCP: transfer UP/CP data, maintains PDCP SNs of 12/18 bits for DRBs and 12 bits for SRBs.
- SDAP: used for flow to DRB mapping.

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# Comment
Although the functional split concept was introduced for 5G, to get full interoperability and cost benefits, it must be applied to RAN for 2G, 3G and 4G as well. When the RAN is opened up horizontally, it could bring in a new range of low-cost radio players, and it gives mobile operators a choice to optimize deployment options for specific performance requirements at much better cost.

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