The structure is based on the principle that the layers and planes are logically independent of each other. Protocol structures in UTRAN terrestrial interfaces are designed according to the same general protocol model. This model is shown in below. The structure is based on the principle that the layers and planes are logically independent of each other and, if needed, parts of the protocol structure may be changed in the future while other parts remain intact.
The protocol structure consists of two main layers, the Radio Network Layer (RNL) and the Transport Network Layer (TNL). All UTRAN-related issues are visible only in the Radio Network Layer, and the Transport Network Layer represents standard transport technology that is selected to be used for UTRAN but without any UTRAN-specific changes.
The Control Plane is used for all UMTS-specific control signaling. It includes the Application Protocol (i.e. RANAP in Iu, RNSAP in Iur and NBAP in Iub), and the Signaling Bearer for transporting the Application Protocol messages. The Application Protocol is used, among other things, for setting up bearers to the UE (i.e. the Radio Access Bearer in Iu and subsequently the Radio Link in Iur and Iub). In the three plane structure the bearer parameters in the Application
Protocol are not directly tied to the User Plane technology, but rather are general bearer parameters. The Signaling Bearer for the Application Protocol may or may not be of the same type as the Signaling Bearer for the ALCAP. It is always set up by O&M actions.
All information sent and received by the user, such as the coded voice in a voice call or the packets in an Internet connection are transported via the User Plane. The User Plane includes the Data Stream(s), and the Data Bearer (s) for the Data Stream(s). Each Data Stream is characterized by one or more frame protocols specified for that interface.
Transport Network Control Plane
The Transport Network Control Plane is used for all control signaling within the Transport Layer. It does not include any Radio Network Layer information. It includes the ALCAP protocol that is needed to set up the transport bearers (Data Bearer) for the User Plane. It also includes the Signaling Bearer needed for the ALCAP. The Transport Network Control Plane is a plane that acts between the Control Plane and the User Plane. The introduction of the Transport Network
Control Plane makes it possible for the Application Protocol in the Radio Network Control Plane to be completely independent of the technology selected for the Data Bearer in the User Plane.