The Non-Access Stratum (NAS) protocol in LTE is a set of signaling protocols responsible for establishing and maintaining the connection between the User Equipment (UE) and the core network. NAS functions include:
- Mobility Management (MM): Handles mobility-related tasks like location registration, paging, and handovers.
- Session Management (SM): Manages the establishment, maintenance, and release of data sessions (bearers).
Non-Access Stratum (NAS) Protocol.
EPS Mobility Management and Connection Management States.
In the NAS layer of LTE, two key EPS Mobility Management (EMM) states define the status of a UE (User Equipment) concerning mobility management procedures like Attach and Tracking Area Update:
- EMM-DEREGISTERED: The UE is not attached to the network. There is no active context, routing information, or location information available from the MME’s (Mobility Management Entity) perspective.
- EMM-REGISTERED: The UE is successfully attached to the network. The MME has the necessary location and routing information to page the UE and manage mobile terminating connections. The UE also has an active PDN (Packet Data Network) connection and EPS (Evolved Packet System) security context.
Additionally, there are two Evolved Packet Core (EPC) Connection Management (ECM) states indicating the status of signaling connectivity between the UE and the EPC:
- ECM-IDLE: No active NAS signaling connection exists between the UE and the network. The UE’s location is known based on the current Tracking Area List, which comprises all tracking areas where the UE has successfully registered. In this state, the UE may perform cell selection or reselection as needed.
- ECM-CONNECTED: An active NAS signaling connection exists. The UE enters this state upon sending or the MME receiving one of the following NAS messages: Attach Request, Tracking Area Update Request, Service Request, or Detach Request. In this state, the set of EPS bearers is synchronized between the UE and the network.
The ECM and EMM states can exist independently of each other. For example, a UE can be in ECM-IDLE but still be EMM-REGISTERED, indicating no active signaling connection yet being registered to the network. Conversely, a UE can be ECM-CONNECTED while EMM-DEREGISTERED during initial registration procedures, transitioning to EMM-REGISTERED upon successful attachment to the network.