Figure 4.11, which introduces some new vocabulary, is an overview of radio protocol structure, which is further described in the next section. It briefly describes the main purpose of the different layers and how they interact with each other. This picture only describes the protocol layering on the eNodeB side, but there, of course, exist similar – or dual – functions and layers on the terminal side.
Starting from the top of the picture, the RRC layer (Radio Resource Control) supports all the signalling procedures between the terminal and the eNodeB. This includes mobility procedures as well as terminal connection management. The signalling from the EPC Control plane (e.g. for terminal registration or authentication) is transferred to the terminal through the RRC protocol, hence the link between the RRC and upper layers.
The PDCP layer (whose main role consists of header compression and implementation of security such as encryption and integrity) is offered to Radio Bearers by E-UTRAN lower layers. Each of these bearers corresponds to a specific information flow such as User plane data (e.g. voice frames, streaming data, IMS signalling) or Control plane signalling (such as RRC or NAS signalling issued by the EPC). Due to their specific purpose and handling, information flows generated by ‘System Information Broadcast’ and ‘Paging’ functions are transparent to the PDCP layer.