LTE Non-Access-Stratum (NAS) Protocol

in NAS, Specification, LTE The non-access stratum (NAS) is highest stratum of the control plane between UE and MME at the radio interface. Main functions of the protocols that are part of the NAS are the support of mobility of the user equipment (UE) and the support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).
NAS control protocol performs followings:
EPS bearer management;
Authentication;
ECM-IDLE mobility handling;
Paging origination in ECM-IDLE;
Security control.
Protocol specification
3GPP TS 24.301 - Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3