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5GArchitecture-TheStack-Part2.md

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Below is an ASCII-based diagram illustrating the 5G protocol stacks side-by-side for User Plane and Control Plane, showing how they align between the UE and gNodeB. This version aims to be clearer and more structured than a single combined diagram.

User-Plane Protocol Stack

          +---------------------------------------------------+
          |                   Applications                    |
          |                  (User / Data)                    |
          +---------------------------------------------------+
          |                   IP / Transport                  |
          +-------------------+--------------+----------------+
UE (UP)   |        SDAP      |              |    gNB (UP)    |
          +-------------------+              +----------------+
          |        PDCP      | <----------> |     PDCP       |
          +-------------------+              +----------------+
          |        RLC       | <----------> |     RLC        |
          +-------------------+              +----------------+
          |        MAC       | <----------> |     MAC        |
          +-------------------+              +----------------+
          |        PHY       | <----------> |     PHY        |
          +-------------------+--------------+----------------+
  • SDAP (Service Data Adaptation Protocol): Maps 5G QoS flows to Data Radio Bearers, enforcing QoS rules.
  • PDCP (Packet Data Convergence Protocol): Handles ciphering, integrity protection (if configured), header compression, and data reordering.
  • RLC (Radio Link Control): Provides segmentation/reassembly and ARQ (acknowledged or unacknowledged modes).
  • MAC (Medium Access Control): Multiplexes/demultiplexes data between RLC and PHY, scheduling, and HARQ.
  • PHY (Physical): Actual radio transmission and reception using OFDM, channel coding, MIMO, etc.

Control-Plane Protocol Stack

          +---------------------------------------------------+
          |                       NAS                          |
          |    (Registration, Authentication, Security,       |
          |     PDU Session Management, Mobility, etc.)       |
          +---------------------------------------------------+
          |                       RRC                          |
          +-------------------+--------------+----------------+
UE (CP)   |        PDCP      |              |    gNB (CP)    |
          +-------------------+              +----------------+
          |        RLC       | <----------> |     RLC        |
          +-------------------+              +----------------+
          |        MAC       | <----------> |     MAC        |
          +-------------------+              +----------------+
          |        PHY       | <----------> |     PHY        |
          +-------------------+--------------+----------------+
  • NAS (Non-Access Stratum): Runs between UE and the 5G Core (AMF/SMF). Handles registration, authentication, and session management. Although shown in the diagram, NAS actually terminates in the core, not in the gNB.
  • RRC (Radio Resource Control): Manages radio resources (connection establishment/release, configuration, mobility, etc.).
  • PDCP/RLC/MAC/PHY: The same layers described above but dedicated to carrying control messages instead of user data.

Key Points:

  1. Separation of User Plane vs. Control Plane

    • User-plane stack carries end-user data (IP traffic).
    • Control-plane stack carries signaling for mobility, connection management, and session control.

  2. NAS Termination

    • Although depicted in the control-plane diagram, NAS does not terminate in the gNB. It goes transparently through the RRC layer to the 5G Core (AMF/SMF).

  3. SDAP is New in 5G

    • It is responsible for applying QoS flows (QFI) at the radio level, bridging the 5G Core QoS model and the radio bearers.

  4. PDCP, RLC, MAC, PHY

    • Exist in both UE and gNodeB with similar names but opposite roles in sending/receiving data. They handle everything from security and segmentation to scheduling and physical signal transmission.

This layered approach ensures flexibility, security, and end-to-end QoS in 5G networks, adapting to different types of services (eMBB, URLLC, mMTC) with different requirements.