The non-access stratum (NAS) described in the present document forms the highest stratum of the control plane between UE and MME at the radio interface (reference point "LTE-Uu"; see TS 23.401
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 security is an additional function of the NAS providing services to the NAS protocols, e.g. integrity protection and ciphering of NAS signalling messages.
For the support of the above functions, the following procedures are supplied within this specification:
elementary procedures for EPS mobility management in clause 5; and
elementary procedures for EPS session management in clause 6.
Complete NAS transactions consist of specific sequences of elementary procedures. Examples of such specific sequences can be found in TS 23.401
The NAS for EPS follows the protocol architecture model for layer 3 as described in TS 24.007
; however, due to the objective of EPS to provide the subscriber with a "ready-to-use" IP connectivity and an "always-on" experience, the protocol supports a linkage between mobility management and session management procedures during the attach procedure (see subclause 4.2
Signalling procedures for the control of NAS security are described as part of the EPS mobility management in clause 5
. In addition to that, principles for the handing of EPS security contexts and for the activation of ciphering and integrity protection, when a NAS signalling connection is established, are provided in subclause 4.4
During the EPS attach procedure, the network can activate a default EPS bearer context (i.e. if the UE requests PDN connectivity in the attach request). Additionally, the network can activate one or several dedicated EPS bearer contexts in parallel for PDN connections of IP or Ethernet PDN type. To this purpose the EPS session management messages for the default EPS bearer context activation can be transmitted in an information element in the EPS mobility management messages. In this case, the UE and the network execute the attach procedure, the default EPS bearer context activation procedure, and the dedicated EPS bearer context activation procedure in parallel. The UE and network shall complete the combined default EPS bearer context activation procedure and the attach procedure before the dedicated EPS bearer context activation procedure is completed. If EMM-REGISTERED without PDN connection is not supported by the UE or the MME, then the success of the attach procedure is dependent on the success of the default EPS bearer context activation procedure. If the attach procedure fails, then the ESM procedures also fail.
A UE using EPS services with control plane CIoT EPS optimization can initiate transport of user data via the control plane. For this purpose a UE in EMM-IDLE mode can initiate the service request procedure and transmit the ESM DATA TRANSPORT message in an information element in the CONTROL PLANE SERVICE REQUEST message.
Except for the attach procedure and the service request procedure, during EMM procedures the MME shall suspend the transmission of ESM messages. During the service request procedure the MME may suspend the transmission of ESM messages.
Except for the attach procedure and the service request procedure for UE initiated transport of user data via the control plane, during EMM procedures the UE shall suspend the transmission of ESM messages.