As descried in the diagram below the protocol includes four plans:
U-Plan: User Plane – refers to IQ sample data transferred between BBU and RRU
C-Plan: Control Plane – refers specifically to real-time control between BBU and RRU.
S-Plane: Synchronization Plane – refers to traffic between the BBU and RRU to a synchronization controller which is generally an IEEE 1588 Grand Master.
M-Plane: Management Plane – refers to non-real-time management operations between the BBU and the RRU