Sign in Subscribe

nas layer in lte

Last updated on 


In the context of LTE (Long-Term Evolution), the NAS (Non-Access Stratum) layer is a crucial component of the protocol stack that operates above the Access Stratum (AS) and below the IP layer in the LTE protocol architecture. Let's delve into the technical details of the NAS layer in LTE:

1. Functionality of NAS:

The NAS layer provides services related to the establishment, maintenance, and release of the UE (User Equipment) connections. Its primary functions include:

  • Mobility Management: This involves procedures like Attach, Detach, Tracking Area Update (TAU), and Service Request. These procedures ensure that the UE can move between cells, tracking areas, and different network nodes without interruption.
  • Session Management: NAS handles procedures for the setup, modification, and termination of EPS (Evolved Packet System) bearers, which are the logical connections between the UE and the core network.

2. NAS Protocol Architecture:

  • NAS Protocol Stack: The NAS layer comprises several protocols like EMM (EPS Mobility Management) and ESM (EPS Session Management). These protocols define the procedures and messages exchanged between the UE and the MME (Mobility Management Entity) or other relevant entities.
  • EMM Protocol: EMM deals with mobility-related procedures. It manages the states of the UE (like "detached," "attached," etc.) and handles security procedures like authentication and security mode procedures. Messages like Attach Request, Attach Accept, Security Mode Command, etc., are part of the EMM protocol.
  • ESM Protocol: ESM is responsible for handling EPS bearer-related procedures. It establishes, modifies, and releases EPS bearers based on the UE's data transfer requirements. Messages like Activate Default EPS Bearer Context Request, Activate Dedicated EPS Bearer Context Request, etc., are defined within ESM.

3. Security in NAS:

  • Authentication and Key Agreement (AKA): The NAS layer ensures the security of the communication by implementing mutual authentication between the UE and the network. This involves generating session keys that are used to encrypt and decrypt the user data and signaling messages.
  • Security Modes: NAS supports various security modes, such as integrity protection and ciphering, to safeguard user data and signaling messages against threats like eavesdropping and tampering.

4. NAS Procedures:

  • Attach Procedure: When the UE wants to connect to the network, it initiates the Attach procedure. This involves sending an Attach Request to the MME, followed by authentication, security mode setup, and finally, the Attach Accept message from the MME.
  • Detach Procedure: When the UE wants to disconnect from the network, it initiates the Detach procedure. This involves sending a Detach Request to the MME to release resources and update the UE's status.
  • Bearer Establishment and Modification: NAS handles the establishment, modification, and release of EPS bearers based on the UE's requirements for data transfer.

5. NAS States and Mobility:

  • UE States: NAS manages different states of the UE, such as "detached," "attached," and "registered," based on its interaction with the network.
  • Mobility Handling: NAS ensures seamless mobility of the UE across different cells, tracking areas, and network nodes by managing procedures like Tracking Area Updates (TAU) and Handovers.

The NAS layer in LTE is a vital component that handles mobility management, session management, security, and other essential functions to facilitate the establishment and maintenance of connections between the UE and the core network. Through various protocols and procedures, NAS ensures efficient and secure communication in the LTE network architecture.