 the eNodeBs. X2-interface is used mainly for radio resource management purposes, like sensing the handover command between eNodeBs, but also temporary for the purposes of data forwarding before core network tunnels have been updated to route the data for the new eNodeB. 2.2.3 LTE Radio Protocols The Non-Access Stratum (NAS) Signalling is sent via eNodeB from UE to MME. This includes authentication when UE connects to the network. All the radio protocol layers are located in the eNodeB with the following key functionalities: Radio Resource Control (RRC) layer is responsible for the related signalling between UE and eNodeB, all the messages such as configuring the connection, measurements reports and RRC reconfiguration (handover command) are covered by the RRC layer. Compared to HSPA, LTE had less RRC states with only RRC idle and RRC connected states being defined, as shown in Figure 2.9. Packet Data Convergence Protocol (PDCP) handles the security (ciphering and integrity protection) as well as IP header compression operation. The PDCP compared to HSPA (WCDMA) includes now the security functionality and is thus used for control plane signalling as well (with the exception of the broadcasted control information). LTE Small Cell Optimization EPC gateways eNodeB Physical Layer eNodeB Physical Layer S1_MME Physical Layer Figure 2.8 LTE radio architecture WCDMA/HSPA Handover Cell_DCH RRC_CONNECTED Cell_FACH Cell_PCH/ URA_PCH Reselection RRC Connection establishment RRC Connection release Idle mode RRC_IDLE Reselection Figure 2.9 LTE RRC states LTE and LTE Advanced in Releases 8-11 Radio Link Control (RLC) layer covers the retransmission handling if the physical layer retransmission operation fails. For some services, RLC retransmissions are not used due to tight delay budget, such as with Voice over LTE, VoLTE. In such a case, RLC is operated without retransmissions, but RLC layer still handles the packet re-ordering functionality. This is needed since the physical layer retransmissions may end up changing the order