Open RAN    

 

 

 

Topology

Following is general topology showing the connectivity among Open RAN components. You can take this as a logical connection rather than physical connection. In physical deployment, these layers (i.e, DU, CU, Core Network) are connected in other structure like ring topology.

A few points to be noticed in this topology are :

  • Each CU may be connected to multiples of DUs
  • Each DU may be connected to multiple of RUs
  • The distance between RU and DU is relatively short (less than 20 km) and timing requirement on this link is stringent (less than 500 us).
  • The distance between DU and CU is relatively long (20~40 km) and timing requirement on this link is less stringent (1.5~10 ms)

The diagram shown below illustrates an Open RAN (O-RAN) deployment topology, focusing on the layout and connectivity between different components within a radio access network. This topology is designed to improve network flexibility and scalability by disaggregating the traditional RAN components into distinct functional entities connected via standard interfaces.

 Here’s a breakdown of the deployment:

Core Components

  • Core Network: This is the central part of the telecommunications network that provides various services to customers who are connected by the access network. It manages data routing, mobility, user sessions, and connectivity to external networks.
  • CU (Central Unit): The CU is split into two main functions:
    • CU Control Plane (CU-CP): Handles control plane functionalities such as session management and signaling.
    • CU User Plane (CU-UP): Manages user plane data processing like packet routing and forwarding.

Access Network Components

  • DU (Distributed Unit): Positioned closer to the network edge, the DU performs real-time baseband processing tasks. It’s responsible for functions such as error correction, encryption, and the formation of radio frames.
  • RU (Radio Unit): The RU handles radio frequency functions, including digital-to-analog conversion, amplification, and transmission/reception of signals over the air.

Connectivity and Latency

  • Distance and Latency Metrics:
    • CU to DU: The distance can reach up to 40 km with a latency requirement of about 1 ms, which is crucial for maintaining the timing and coordination required for high-quality network performance.
    • DU to RU: This connection typically needs to be shorter, less than 20 km, with a very stringent latency requirement of less than 0.5 ms, especially critical for maintaining effective radio signal transmission and timing.

Network Interconnects

  • L2 Switches: These are Layer 2 networking devices used to connect DUs and RUs within the network. They handle data transfer within the same network segment by using MAC addresses to forward data to the correct destination.
  • eCPRI/ROE: Stands for enhanced Common Public Radio Interface/Radio Over Ethernet. This is a key technology used in O-RAN for transmitting radio signals over a packet-switched network. eCPRI specifically reduces the bandwidth required to transport digital radio signals between the RU and DU.

Network Design

  • The topology demonstrates a split architecture where the radio access network is broken down into separate entities (CU, DU, RU), each specializing in different tasks. This modular approach enhances network efficiency and flexibility.
  • Multiple paths between the CUs and DUs indicate a robust design intended to ensure redundancy and high availability.
  • The integration of eCPRI and Layer 2 switches highlights the use of modern transport technologies to meet the stringent latency and bandwidth requirements of advanced mobile networks like 5G.

Reference