We talk over and over again about the complex Long Term Evolution (LTE) and its many components. One of these components, known as the non-access stratum (NAS), is highest stratum of the control plane between user experience (UE) and Mobility Management Entity (MME) at the radio interface, as reported by LteWorld.Basically, NAS supports signaling and traffic between those two elements. LTE NAS also handles attach and tracking area update, authentication and security.
The 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.
The following functions exist in the non-access stratum:
- Mobility management
- Call control
- Session management
- Identity management
Even UMTS has NAS. To break it down, NAS messages are mainly for mobility, session management and security. Within the security field, there are two aspects where NAS comes into play: (1) integrity and (2) ciphering. EMM includes determining UE’s location, user’s authentication, confidentiality, and connection management. Procedure is a group of NAS messages exchange with specific purpose. There are two kinds of EMM elementary procedures, both common and specific procedures.
EMM specific procedures include attach, detach and TAU. As its name suggests, UE invokes attach procedure to attach to the LTE network and to detach from LTE network, known as the detach procedure. These procedures are invoked when the UE is powered ON (attach) or OFF (detach). Also, when UE enters the LTE coverage area (attach) or exits the LTE coverage area (detach). Most times, the UE and the network both are backward compatible. So UE may perform combined attach/detach for both the new LTE network and the legacy GPRS/UMTS network. Supporting a combination procedure like this is optional.
Normal attach and combined attach are different only in their presence of few optional IEs. There are three different types of UEs. The PS only mode UE works exclusively with EPS, synonymous for LTE. UEs are not mobile handsets but more specifically a USB dongle or PC card, and they never perform combined procedures. “CS/PS mode 1 UE” and “CS/PS mode 2 UE” are dual mode. “If UE is under coverage of both LTE and legacy GPRS/UMTS, then Mode 1 UE prefers non-EPS (GPRS/UMTS) and mode 2 prefers EPS(LTE) service. However, they can attach to both networks, (1) EPS network and (2) non-EPS network.”
So, how do these NAS messages travel and get from one place to another? NAS messages are carried by RRC. In UMTS, RRC is terminating at RNC. LTE does not have RNC so RRC terminates at eNB. NAS, however, terminates at MME.
“NAS messages are between UE and MME. RRC and NAS both provides data integrity. Ciphering is mandatory for RRC, but is optional for NAS.”
NAS can have at least two FSMs, including ESM FSM and EMM FSM. EMM FSM has seven states: EMM Null, EMM DeRegistered, EMM DeRegistered initiated, EMM Registered, EMM Registered initiated, EMM TAU initiated, EMM Service Request initiated. Out of these seven states, 4Gwireless jobs reports, most of them are transient states. EMM FSM has major two states only, and they are EMM DeRegistered and EMM Registered. They correspond to UE when it is powered OFF and UE powered ON. ECM can have its FSM. For EMM Registered state, ECM FSM has two states: ECM idle and ECM connected, also known as EMM Idle and EMM Connected respectively. [1]
[1] LTE NAS – 1, 4Gwirelessjobs.com
