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UL NAS Transport

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The User Equipment (UE) performs an uplink (UL) NAS (Non-Access Stratum) Transport when it needs to send NAS messages to the core network. NAS messages are used for various control plane procedures, including registration, security management, and mobility management. Here's a detailed technical explanation of the UL NAS Transport procedure:

1. NAS Message Generation:

  • The UL NAS Transport procedure begins with the generation of NAS messages within the UE. These messages can be related to network registration, authentication, mobility, or other control plane functions.
  • NAS messages are generated by the UE's NAS layer, typically in response to user actions or network-initiated procedures.

2. NAS Message Encapsulation:

  • Once generated, NAS messages are encapsulated into protocol data units (PDUs) for transmission over the radio interface.
  • The NAS messages are formatted and organized according to the NAS protocol, which is part of the control plane in mobile communication systems like LTE and 5G.

3. Radio Resource Allocation:

  • Before transmitting the NAS message, the UE requests and is granted UL radio resources from the network. This resource allocation is necessary to ensure that the NAS message can be sent without interference and collision with other transmissions.

4. MAC Layer Processing:

  • The NAS message is handed over to the MAC (Medium Access Control) layer, which is responsible for managing access to the shared radio channel.
  • The MAC layer prepares the NAS message for transmission by adding header information and organizing it into transport blocks suitable for transmission over the air interface.

5. RLC Segmentation and Concatenation:

  • In some mobile communication systems, such as LTE, the RLC (Radio Link Control) layer may further segment the NAS message into smaller RLC PDUs.
  • This segmentation helps optimize the use of radio resources by allowing for efficient transmission of smaller packets.

6. PDCP Processing:

  • The RLC PDUs or NAS messages are then passed to the PDCP (Packet Data Convergence Protocol) layer, which adds the necessary header and control information for UL transmission.
  • PDCP is responsible for ensuring data integrity and confidentiality by applying encryption and integrity protection if required.

7. UL Transmission:

  • The prepared NAS messages, now encapsulated, segmented, and processed by lower layers, are transmitted over the UL radio interface to the network's base station (eNodeB in LTE or gNodeB in 5G).
  • The UE uses the allocated radio resources to send the NAS messages to the network.

8. Base Station Reception:

  • The base station receives the UL NAS Transport messages from the UE and processes them at the radio access network (RAN) level.
  • It may perform initial error checking and radio resource management functions.

9. NAS Message Decapsulation:

  • The received NAS messages are then passed to the core network's NAS layer for further processing.
  • The encapsulation added by the UE's lower layers is removed, and the original NAS message is extracted.

10. NAS Protocol Processing: - The NAS messages are processed by the network's NAS layer, which interprets the messages and initiates the appropriate control plane procedures. - These procedures can include registration, authentication, handover requests, security management, or other network-related functions.

11. Procedure Completion: - The UL NAS Transport procedure is considered complete when the NAS messages are successfully received, processed, and result in the initiation of the intended control plane procedures in the network.

The UL NAS Transport procedure is a fundamental part of mobile communication networks, enabling UEs to communicate with the core network for various control and signaling purposes. It ensures that NAS messages are reliably transmitted from the UE to the network, facilitating essential network management and user-related procedures.