The protocols over Uu and NG interfaces are divided into two structures:
User plane protocols
These are the protocols implementing the actual PDU Session service, i.e. carrying user data through the access stratum.
Control plane protocols
These are the protocols for controlling the PDU Sessions and the connection between the UE and the network from different aspects (including requesting the service, controlling different transmission resources, handover etc.). Also a mechanism for transparent transfer of NAS messages is included.
The PDU Session Resource service is offered from SAP to SAP by the Access Stratum. Figure 5.2-1 shows the protocols on the Uu and the NG interfaces that linked together provide this PDU Session Resource service.