3 Protocol Architecture in LTE
3.2 LTE Control Plane Protocol Stack
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LTE control plane protocol stack

NAS: NAS protocol performs mobility management and bearer management functions.

RRC: RRC protocol supports the transfer of the NAS signaling. It also performs functions required for efficient management of the radio resources. The main functions are as follows:

X2AP: X2AP protocol supports UE mobility and SON functions within the E-UTRAN. To support UE mobility, the X2AP protocol provides functions such as user data forwarding, transfer of SN status and UE context release. For SON functions, eNBs exchange resource status information, traffic load information and eNB configuration update information, and coordinate each other to adjust mobility parameters using the X2AP protocol.

S1AP: S1AP protocol supports functions such as S1 interface management, E-RAB management, NAS signaling transport and UE context management. It delivers the initial UE context to the eNB to setup E-RAB(s) and manages modification or release of the UE context thereafter.

GTP-C: GTP-C protocol supports exchange of control information for creation, modification and termination for GTP tunnels. It creates data forwarding tunnels in case of LTE handover.