[11/02/2021] All About RAN and O-RAN (part 2)

# [11/02/2021] All About RAN and O-RAN (part 2) ### Salman's Notes :notebook: --- check out [part 1](https://hackmd.io/@msalmana/rkaZ1Ql-_) if you haven't done so! --- ### RAN Components :sandwich: From the functionalities that were discussed in the first part of my RAN and O-RAN note, ==RAN is "split" into a few physical or virtualized (more on this later) components, which are the CU (Central Unit), DU (Distributed Unit), and RU (Radio Unit).== ![](https://i.imgur.com/4ctZs50.png) *Figure 1. Split-RAN processing pipeline distributed across a Central Unit (CU), Distributed Unit (DU), and Radio Unit (RU).* The Figure above shows how the functional components are grouped together into larger components that are implemented in the same hardware or virtual hardware scope. It is worth noting that these components, the CU, DU, and RU do not always have one-to-one relationships. Usually, there is a one-to-many relationship between the CU and DU, and a one-to-many relationships between the DU and RU, creating a tree-like structure. The connection between the CU and DU is called the **Midhaul** and the connection between the DU and the RU is called the **Fronthaul**. Lastly, we also must not forget that there is a connection between the CU and Mobile Core that connects the Radio-Access Network to the Internet, which is called the **BackHaul**. ### What is SD-RAN? :grey_question: In short, SD-RAN is an architecture design of RAN that follows the SDN principles such as Control and User Plane Separation (CUPS). If we go back to Figure 1 we can see that the CU has two separate components (the RRC and PDCP) that are in the Control Plane and User Plane respectively. ![](https://i.imgur.com/qWcobvR.png) *Figure 2. RRC disaggregated into a Mobile Core facing control plane component and a Near-Real-Time Controller..* ==First, this view of the separation has led to the separation of functions within the RRC block of the CU, into a Control Plane Component and the Near-Real Time RAN Intelligent Controller (Near-RT RIC).== The Control Plane Component takes care of the Control Plane communication between the CU and the Mobile Core. Separated from that connection, ==the Near-RT RIC is responsible for managing the Control Plane communication all throughout the RAN architecture==. It is comprised of many dynamic applications that can be used to configure the transmission of data within the RAN, such as the RF Configuration, Load Balancing, and Handover Control. ### Getting to know O-RAN :smile: The O-RAN is the newest architectural design pattern in Radio Access Networks, with the theme of ==many components being converted to microservices that are hosted or virtualized within a cloud and also pushing for interchangable components, leading to more innovation and a highly competitive market==. Thihs is most easily seen by the O- prefix to the components, noting that they are Open. The O-RAN Alliance and the O-RAN Software Community will build interfaces for the hardware to make sure that they can work together in unison despite coming from different vendors. ![](https://i.imgur.com/aWeaRdl.png) *Figure 3. The O-RAN architecture.* The stages of architectural changes that have led to the O-RAN architecture that has been standardized today, shown by the figure about. First, the architecture has moved horizontally from a single Base Station in the original RAN architecture into many multiple components. It also shows how the CU has been lifted vertically from the Split-RAN approach, following the CUPS that is used in many Software-Defined Networks. Also, the connection between the DU and RU has been converted to an Open Fronthaul Interface which means that it allows for vendor-neutral components communication throughh an API. Lastly, the design takes many functionalities that were previously in the CU and delegates them to other components, making the system more dynamic by being able to swap out components within the Near-RT RIC. A new component has also been added, which is the Non-RT RIC, which has the function of making policies and Machine Learning models that are being used by the Near-RT to manage the RAN system. The names for the Non-RT and Near-RT RIC components are derived from the loops inside the O-RAN architecture. The first control loop is the Non-Real Time loop between the two said components, the O1 interface and the A1 interface, with the Non-RT RIC beng the controller for this loop. It is called Non-Real Time because control operations happen at the speed of >>1s. The second loop is the >10ms loop with the Near-RT RIC and the other components connecting to it. The last loop is inside the O-DU with consists of the Scheduler and the MAC components. This loop is Real Time meaning it runs in <1ms time. ### References - Title: 5G Mobile Networks: A Systems Approach Authors: Larry Peterson and Oguz Sunay Source: https://github.com/SystemsApproach/5G License: CC BY-NC-ND 4.0 Link: https://5g.systemsapproach.org/ran.html - [O-RAN Architecture](https://docs.o-ran-sc.org/en/amber)