The NAS ( Non Access Stratum) protocol, is being used to communicate directly between the UE and the MME. It is called Non Access , in order to distinguish it from the AS ( Access Stratum) protocol that carries information over the air between the UE and the eNodeB..
The NAS protocol supports :
- Mobility Management Functionality , also called EPS Mobility Management ( EMM)
- The EPS Session Management (ESM) including:
- User Plane Bearer Activation
- User Plane Modification
- User Plane Deactivation
The interface used for the NAS is the S1-MME interface , and the transport protocol that carries the NAS protocol to the MME is S1-AP.
As an example we can take the procedure of Identification initiation by the network
The network initiates the identification procedure by sending an IDENTITY REQUEST message to the UE and starting the timer T3470 The IDENTITY REQUEST message specifies the requested identification parameters in the Identity type information element.
The UE answers with an IDENTITY RESPONSE
When the MME receives the rsponse it stops timer T3470
More on the NAS protocol can be found in ETSI TS 124 301
LTE; Non-Access-Stratum (NAS)protocol for Evolved Packet System (EPS)