# RAN Background Knowledge

[ORAN Overview](#Emerging-Approach-:-Virtualization)
- [Virtualization](#What,-Why-and-How)
- [Virtualization Example](#Virtualization-in-Computer-Network)
- [Virtualization in 5G](#How-5G-Implement-Virtualization?)

[ORAN Definition](#O-RAN-:-What-does-"Open"-Mean?)
- [RAN Evolution](#Radio-Access-Network-Evolution)
- [O-RAN Architecture & Protocol Stack](#O-RAN-Architecture)
- [O-RAN Interfaces](#O-RAN-Interfaces)

## Emerging Approach : Virtualization

### Virtualization : What, Why and How

Let's go back to the time before virtualization. During that period, a computer system would run a single industrial scale software. At that time, increasing capacity, technology adaptations, and system maintenance is a time-consuming, costly, and energy-intensive tasks. Some believed that this approach was challenging and wouldn't yield significant improvements, prompting them to seek better ways to streamline processes related to IT infrastructure changes. Then, virtualization was born.

In simple terms, virtualization involves techniques aimed at enhancing adaptability and simplifying the maintenance of both software and hardware systems. Virtualization can be achieved by adding an abstraction layer above the hardware (firmware), creating an interface that enabling software to run on various hardware systems. With this approach, hardware and software developments can progress independently, facilitating the adaptation to emerging technologies. This approach also allows isolation between software instances, ensuring that disruptions in one system do not affect others, increasing system security and prevent single point failure.


### Virtualization in Computer Network

#### a. Virtual LAN (VLAN)

This is the oldest virtualization approach in computer networks. In simple terms, a VLAN (Virtual Local Area Network) comprises multiple local networks operating on a single local network infrastructure. VLAN enables network segmentation, reduces broadcast domains, and enhances local network security.


#### b. Virtual Private Network (VPN)

In simple words, VPN is a virtualization that make a physically different network act as a single network. VPN using tunneling technique that encapsulate traffic between remote network so the source and destination seems like existing in the same network. This tunneling also increase data-at-transfer security by applying data encryption.


#### c. Software Defined Networking (SDN)

When talking about network virtualization, many people think about SDN in no time. SDN often describe as separation between control-plane and data-plane. SDN provide a centralized platform to control and monitor network behavior. This approach is enabling network management automation that reduce human error and reducing transmission delay as the device need to do is just forward the packet to some interface without calculating route or applying network policies. SDN is commonly deployed in Cloud Network and Data Center.


#### d. Network Function Virtualization (NFV)

Network Function Virtualization is the recently developed virtualization solution for Internet Service Provider Company. NFV is a technique to implementing network function such as load balancer, authentication server, firewall, etc as a software that run on a standardized general purpose hardware. The benefit of this approach is simple deployment and maintenance.


### How 5G Implement Virtualization?

5G Technology use Software Defined Networking and Network Function Virtualization to improve scalability, flexibility, availability and performance. Software Defined Networking implemented by deploying a network function (SMF) to control forwarding devices (UPF). Network Function Virtualization implemented by deploying 5G network functions (AMF,SMF,PCF,etc) as a software that run on a physical server.


## O-RAN : What does "Open" Mean?

### Radio Access Network Evolution

Access Network is consisted of Remote Radio Unit (RRU) that handles Radio Frequency task such as Analog to Digital Conversion and Baseband Unit (BBU) that handle digital signal processing task such as multiplexing and coding.

#### a. Traditional RAN

Traditional RAN has a characteristic where RRU, BBU and antenna located at the same position which makes installation and maintenance is expensive and time consuming.


#### b. Centralized RAN (C-RAN)

Centralized RAN moves the BBU unit to a centralized location (such as cloud). This deployment model simplify BBU maintenance.


#### c. Virtualized RAN (V-RAN)

Virtualized RAN replace a dedicated BBU hardware to a general purpose hardware that run BBU software.


#### d. Open RAN (O-RAN)

Difference between Open RAN and Virtualized RAN is existence of **open interface** between RRU and BBU so a radio infrastructure could built by multiple vendors. This approach reduce technology dependency toward single party and increase competition in radio communication infrastructure.


### O-RAN Architecture


One of the major difference between vRAN and ORAN that Baseband Unit is separated into Distributed Unit and Central Unit. The advantage of this separation is to minimize computation process that led to energy saving and moderate maintenance

#### a. Components

##### 1. O-RU (Radio Unit)

Radio unit is a logical node that perform digital to radio frequency conversion (and vice versa), interract with user equipment network interface using radio wave and serve as gateway to 5G systems.

##### 2. O-DU (Distributed Unit)

Logical node that perform computation to prevent collision in wireless interface

##### 3. O-CU-UP (Central Unit, User Plane)

Logical node that perform computation to ensure a reliable and efficient communication

##### 4. O-CU-CP (Central Unit, Control Plane)

Interract with 5G core network during communication establishment and user equipment handover

##### 5. Near Real-Time Radio Intelligent Controller (Near-RT RIC)

RAN controller that work as real-time system to optimize RAN nodes, with a maximum response time equal to 10 milisecond

##### 6. Non Real-Time Radio Intelligent Controller (Non-RT RIC)

RAN controller that utilize Artificial Intelligence to tackle complex problem, with a response time about 1 second

#### b. Protocol Stack




##### OSI layer and O-RAN Protocol Stack


Most of the RAN protocol are equal to data link layer in OSI model.

##### Service Data Application Protocol (SDAP)

SDAP is responsible to map QoS flag set in the data header to Downlink Radio Bearer (logical link that allocated for each QoS flow)

##### Radio Resource Control (RRC)

RRC is responsible to execute session establishment or termination between UE and Core network. RRC mostly interract with Acess and Mobility Function at Core Network.

##### Packet Data Convergence Protocol (PDCP)

PDCP is responsible for header compression. Header compression could improve efficiency by reducing number of bits transmitted. As the number of transmitted bits is smaller, the communication reliability will improve.

##### Radio Link Control (RLC)

RLC is responsible for data buffering and retransmission. When a data is damaged during transmission, RAN system will re-send the data that buffered before.

##### Media Access Control (MAC)

MAC is responsible for transmission scheduling. The wireless interface could only send data to a device at a time, so there must be a mechanism to control when the data will be transmitted to a proper UE.

##### Physical Layer (PHY)

PHY layer is responsible for transforming digital signal data to analog frequency data. PHY computation including fast fourien transform (or inverse), modulation and ADC/DAC conversion.

### O-RAN Interfaces


As stated in above diagram, each O-RAN components are connected by interface. Each interface carries different information between components.

#### A1 Interface

A1 Interface refers to logical link that connect Non-RT RIC and Near-RT RIC. Non-RT RIC will deliver ML model update and policies through A1 interface. On the other hand, Near-RT RIC will return ML model and policy feedback to improve O-RAN system.

#### E2 Interface

E2 Interface refers to logical link that connect near-RT RIC and E2 node (explained using image below). Near-RT RIC will control O-RAN component through E2 interface and each O-RAN component will send operational data through this interface.


#### O1 Interface

O1 interface is almost similar to E2 interface. The major difference is that O1 has access to low level system to control software update, fault detection and disaster recovery.

> Note:
> To clearify everything, E2 interface is analogous to shell terminal where every command are issued while O1 interface is similar to BIOS that only accessed when something wrong happened.

#### O2 Interface

O2 interface is logical link that connect non-RT RIC to underlying infrastructure that host O-RAN system (O-Cloud). Non-RT RIC will send infrastructure resource management command (such as upscale, downscale) through O2 Interface.

### ORAN Functional Split

#### Overview

ORAN Functional Split is a concept introduces by 3GPP related to the division of tasks performed by the Radio Unit (RU), Central Unit (CU), and Distributed Unit (DU). This concept addresses which protocols are executed by a component to achieve a network metric (such as latency,throughput,etc). It allows flexibility in the implementation of 5G networks based on use cases for different regions.

#### Common Functional Split Options


Based on the picture, there are 8 options to deploy ORAN. Basically the naming convention of each split are based on **Where to deploy each RAN Protocols**.

##### Split 2

The split 2 is deployment option where RRC/SDAP and PDCP are hosted by CU while the rest of the lower RAN protocol stack is hosted near the RU, including DU. It means there are Distributed Unit located physically distributed.

##### Split 5

The split 5 deployment option where RRC/SDAP, PDCP, RLC and High MAC are hosted by CU while Low MAC, entire PHY and RF are hosted by DU/RU. High MAC protocol responsible for transmission scheduling while Low MAC protocol responsible for error recovery (HARQ)

##### Split 7

The split 7 is deployment option where Low PHY and RF hosted by RU while the upper RAN protocol hosted by centarlized and virtualized CU/DU. High PHY protocol are responsible for CRC attachment, coding, and modulation while Low PHY are responsible for performing iFFT to translate data into signal.

##### Split 8

The split 8 is deployment option where the RU only performing Digital/Analog conversion while the rest of the process are performed by CU/DU in seperate geographical location.

#### Trade Offs


Hosting most of RAN protocol stack near the RU has several benefit such as lower latency and higher throughput. This is happen due to most of the transmission process(scheduling, CRC calculation, buffering, modulation) performed near radio unit and focused only on those calculation (the device doesn't do anything else other than transmission task). This options are optimized for real-time usecases such as gaming and internet of things. The drawback of this configuration is higher maintenance cost (time, money, energy) and difficult to scale up and down due to increasing number of infrastructure.

While hosting most of RAN protocol in the cloud will reduce deployment and maintenance cost as the ISP only need to monitor and control limited number of device (few of server that run RAN software). The other advantage of this configuration is high scalability because the usage of cloud infrastructure that highly automated. The drawback of this option is higher latency because the need of addition tranmission between CU/DU and RU (using IP or other protocol to ensure reliability) and non-focused hardware (server that run RAN might processing non-RAN tasks). This option usually deployed for web browsing traffic, file transfer,and other non-real time usecases.