Radio Resource Control (RRC) in 5G NR
Before discussing RRC directly, let us build some context!
In the 5G NR, Data and Control planes are clearly segregated.
The Data or User plane (U-plane) is responsible for carrying user data via protocol stack as shown in Figure 1 (i.e. SDAP, PDCP, RLC, MAC, PHY).
The Control plane (C-plane) is further divided into two categories:
The first C-plane category is one that runs between the Core Network (5GC) and the user device. It shares control signals via NAS messaging (See the figure above). This device-to-core C-plane functionality is responsible for connection setup, mobility, security etc, and operates between the AMF in the 5GC and the device. (We will discuss NAS messaging in some other blog. But in short, NAS messages are also sent via RRC, but gNB cannot read them. NAS messages are solely between the core and the device)
The second C-plane functionality runs between gNB and the device and is known as RRC (RRC = Radio Resource Control). It terminates at the gNB and has no direct link with the core (but has a transparent link of course… everything is connected to the core as CORE IS THE BIG BOSS!).
Note in the figure above that there is an RRC layer both in the gNB and the device. RRC is responsible for handling RAN-related C-plane procedures like:
