Open RAN Architecture: Disaggregation and Functional Splits
Open RAN Architecture
The Disaggregated Architecture of Open RAN
In traditional RAN setups, one vendor often provides all the network components. Open RAN breaks this model up into three main parts:
- RU (Radio Unit): Handles radio signals to and from devices. It lives near antennas.
- DU (Distributed Unit): Does real-time processing (like encoding and decoding). It’s usually near the edge.
- CU (Centralized Unit): Handles slower, less time-sensitive tasks (like mobility management). It can run in a regional data center or cloud.
This split allows different vendors to provide different pieces, offering flexibility and innovation.
What’s a "Functional Split"?
A functional split defines where the work is divided between the RU, DU, and CU. Different splits have trade-offs in terms of latency, bandwidth, complexity, and decide where parts of the signal processing happen in the network. The O-RAN Alliance is a group that creates rules for how Open RAN works, including how to split these tasks.
- Split 7.2x is one of the most popular — it balances real-time performance and interoperability.
- Fronthaul carries data between RU and DU.
- Midhaul connects DU and CU.
Why Does This Matter:
- The split affects speed, cost, and how complex the network is.
- Lower-level splits (more work in the RU): faster performance (low latency), needs more bandwidth between RU and DU (called Fronthaul)
- Higher-level-splits (more work in the DU or CU): Might be slower (higher latency), uses less bandwidth
What is Fronthaul?
You already learned that fronthaul links RU to DU. Here’s why that matters in Open RAN:
- Fronthaul is the connection between the RU and DU.
- In Open RAN, this connection is open and standardized, thanks to specs like Option 7-2x from the O-RAN Alliance.
- This means different vendors' equipment can work together, unlike in old networks where it was locked to one company’s design.
Control Plane (C-Plane) and User Plane (U-Plane) Separation
CUPS: Splitting Network Brain and Body
- CUPS stands for Control and User Plane Separation — a smart way to split up network tasks.
- In Open RAN, the CU is further split into CU-CP (Control Plane) and CU-UP (User Plane) to facilitate this.
- It’s used in Open RAN to make networks faster, smarter, and easier to manage.
What is the Control Plane (C-Plane):
- Think of it as the brain of the network.
- It handles: Signaling (sending instructions), Managing connections, Tracking users as they move, Enforcing rules and settings.
- In Open RAN, the CU (Central Unit) is split: CU-CP = Control Plane part, CU-UP = User Plane part
What is the User Plane (U-Plane):
- This is the muscle of the network — it moves your data!
- It handles: Sending and receiving user data, Encrypting data, Making sure quality of service stays high
Why CUPS is useful:
- Independent Scaling: You can grow the C-Plane or U-Plane separately depending on demand.
- Example: During rush hour, you can boost the U-Plane to handle more traffic without touching the C-Plane.
- Flexible Deployment: The U-Plane can be placed closer to users to reduce delays (latency).
- Better Use of Resources: The network can run more efficiently, which can save money.
- Stronger Network: If one part fails, the other can keep working, making the network more reliable.
Logical Interfaces within the Open RAN Architecture
How Open RAN Parts Talk to Each Other:
- To help different parts of the Open RAN system work together, there are special standard interfaces (or connections). These make sure everything stays compatible so everything stays compatible, no matter who builds the parts.
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