# Daily Report day 7
::: success
28/07/2021
:::

## 1. Aspects : O-DU and O-CU

:::info
### A. O-CU
CU: The centralized unit software that runs the Radio Resource Control (RRC) and Packet Data Convergence Protocol (PDCP) layers. The gNB consists of a CU and one DU connected to the CU via Fs-C and Fs-U interfaces for CP and UP respectively. A CU with multiple DUs will support multiple gNBs. The split architecture lets a 5G network utilize different distributions of protocol stacks between CU and DUs depending on midhaul availability and network design. It is a logical node that includes the gNB functions like transfer of user data, mobility control, RAN sharing (MORAN), positioning, session management etc., except for functions that are allocated exclusively to the DU. The CU controls the operation of several DUs over the midhaul interface. CU software can be co-located with DU software on the same server on site.

#### O-CU/O-DU/0-RU

#### O-RAN Architecture
Multi-RAT CU
- 4G, 5G, and other protocol processing

DU and RUU
- real-time L2 Functions
- Baseband processing
- radio frequency processing

#### F1 Diagram

#### O-CU Related Components

#### 5G UE RRC States

Radio Resource Control (RRC) is a layer within the 5G NR protocol stack. It exists only in the control plane, in the UE and in the gNB. The behaviour and functions of RRC are governed by the current state of RRC. In 5G NR, RRC has three distinct states: RRC_IDLE, RRC_CONNECTED and RRC_INACTIVE.
For each UE RRC state, applicable functions are clearly defined in the standard. Moreover, it's also defined how state transitions will happen, not only within 5G but also to 2G/3G/4G systems via handover or PLMN/cell reselection.
RRC_INACTIVE is not applicable for Non-Standalone (NSA) mode of operation.

#### 5G QoS Architecture

#### O-CU APIs
- RRC-SDAP
- RRC-PDCP
- SDAP-PDCP
:::

:::info
### B. O-DU
DU: The distributed unit software that is deployed on site on a COTS server. DU software is normally deployed close to the RU on site and it runs the RLC, MAC, and parts of the PHY layer. This logical node includes a subset of the eNodeB (eNB)/gNodeB (gNB) functions, depending on the functional split option, and its operation is controlled by the CU.

#### O-DU Related Components

#### O-DU Internal Interfaces
L1/L2 Interface, 5G FAPI specifications by Small Cell Forum
- P5: PHY configuration interface
- P7: Main data path interface
- P19: RF control interface (O-DU will use an adaptation module at Frontend to handle P19 interface)

L2 Interfaces
- O-DU-OAM-Agent_MAC Interface
- RLC-MAC Interface
- MAC-Scheduler Interface

#### 5G NR RLC
* Transfer of upper layer PDUs in Acknowledged Mode (AM), Unacknowledged Mode (UM) and Transparent Mode ( TM )
* Error correction via retransmission of lost RLC PDUs (AM)
* Segmentation and reassembly of RLC SDUs (UM and AM)
* Re-segmentation of RLC data PDUs when a complete RLC PDU cannot be transmitted (AM).
* Reordering of RLC data PDUs (UM and AM)
:::

:::info
### C. 5G NR Radio Protocol Stack

#### 5G-NR Layer 3 (RRC) Functions:
The main services and functions of the RRC sub layer include:
* Broadcast of System Information related to AS and NAS;
* Paging initiated by 5GC or NG-RAN;
* Establishment, maintenance, and release of an RRC connection between the UE and NG-RAN including Addition, modification, and release of carrier aggregation, Addition, modification, and release of Dual Connectivity in NR or between E-UTRA and NR.
* Security functions including key management;
* Establishment, configuration, maintenance, and release of Signalling Radio Bearers (SRBs) and Data Radio Bearers (DRBs);
* Mobility functions including Handover and context transfer; UE cell selection and reselection and control of cell selection and reselection; Inter-RAT mobility.
* QoS management functions;
* UE measurement reporting and control of the reporting;
* Detection of and recovery from radio link failure;
* NAS message transfer to/from NAS from/to UE.

#### 5G-NR Layer 2 Functions:
The layer 2 of NR is split into the following sub layers:
* Service Data Adaptation Protocol (SDAP)
* Packet Data Convergence Protocol (PDCP)
* Radio Link Control (RLC)
* and Medium Access Control (MAC)

The two figures below depict the Layer 2 architecture for downlink and uplink, where:
* The physical layer offers to the MAC sublayer transport channels;
* The MAC sub layer offers to the RLC sublayer logical channels;
* The RLC sub layer offers to the PDCP sublayer RLC channels;
* The PDCP su blayer offers to the SDAP sublayer radio bearers;
* The SDAP su blayer offers to 5GC QoS flows;
* Comp. refers to header compression and segm. to segmentation;
* Control channels (BCCH, PCCH are not depicted for clarity).

:::

## 2. Summary
:::info
CU: The centralized unit software that runs the Radio Resource Control (RRC) and Packet Data Convergence Protocol (PDCP) layers. The gNB consists of a CU and one DU connected to the CU via Fs-C and Fs-U interfaces for CP and UP respectively. A CU with multiple DUs will support multiple gNBs. The split architecture lets a 5G network utilize different distributions of protocol stacks between CU and DUs depending on midhaul availability and network design. It is a logical node that includes the gNB functions like transfer of user data, mobility control, RAN sharing (MORAN), positioning, session management etc., except for functions that are allocated exclusively to the DU. The CU controls the operation of several DUs over the midhaul interface. CU software can be co-located with DU software on the same server on site.
DU: The distributed unit software that is deployed on site on a COTS server. DU software is normally deployed close to the RU on site and it runs the RLC, MAC, and parts of the PHY layer. This logical node includes a subset of the eNodeB (eNB)/gNodeB (gNB) functions, depending on the functional split option, and its operation is controlled by the CU.
3GPP considered the split concept (DU and CU) for 5G from the beginning of writing its specifications. The DU is responsible for real time layer 1 (L1, physical layer) and lower layer 2 (L2) which contains the data link layer and scheduling functions. The CU is responsible for non-real time, higher L2 and L3 (network layer) functions.
:::

## 3. Comment
In O-DU and O-CU, DU works at Layer 1, namely the Physical Layer and Layer 2, namely the data link. CU works on Layer 2 and layer 3, namely the network layer.

---
# <center>Discussion with Team</center>

1. Discuss about final project