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5GArchitecture-TheStack-Part1.md

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Below is an overview of the 5G NR (New Radio) protocol stack as defined by 3GPP, showing both the user plane and control plane stacks across the UE (User Equipment) and the gNodeB (gNB). We’ll discuss what each layer does and how these layers map between UE and gNB.

High-Level Diagram

A simplified view of the protocol stacks on the User Equipment (UE) side and the gNodeB side can be represented as follows:

                  +-----------------------------------+
                  |           Application(s)          |
                  +-----------------------------------+
UE User Plane      |      IP / Transport (TCP/UDP)     |        gNB User Plane
                  +--------------------+---------------+
                  |     SDAP (5G QoS) |               |
                  +--------------------+               |
                  |        PDCP       |               |
                  +--------------------+    PDCP       |
                  |         RLC       | <---------->   |        RLC
                  +--------------------+               |
                  |         MAC       |               |        MAC
                  +--------------------+               |
                  |         PHY       |               |        PHY
                  +--------------------+---------------+

                  +-----------------------------------+
                  |             NAS                    |   (terminates in Core)
                  +-----------------------------------+
UE Control Plane   |             RRC                   |        gNB Control Plane
                  +--------------------+---------------+
                  |        PDCP       |               |
                  +--------------------+    PDCP       |
                  |         RLC       | <---------->   |        RLC
                  +--------------------+               |
                  |         MAC       |               |        MAC
                  +--------------------+               |
                  |         PHY       |               |        PHY
                  +--------------------+---------------+

Layers and Their Responsibilities

1. PHY (Physical) Layer

  • Where: Both UE and gNB.
  • What it does:
    • Handles the actual radio transmission and reception of signals over the air.
    • In 5G NR, it supports advanced features such as Massive MIMO, beamforming, and mmWave frequencies.
  • Key Functions:
    • Modulation/demodulation of signals (e.g., OFDM).
    • Channel coding/decoding (e.g., LDPC for data, Polar codes for control).
    • Time/frequency synchronization.
    • HARQ (Hybrid ARQ) at the physical layer interface with MAC.

2. MAC (Medium Access Control) Layer

  • Where: Both UE and gNB.
  • What it does:
    • Scheduling of uplink (UL) and downlink (DL) data transmissions.
    • Implements HARQ (in combination with PHY).
    • Multiplexes/demultiplexes data from different logical channels (e.g., RLC channels) onto transport channels (PHY).
  • Key Functions:
    • Dynamic resource allocation.
    • Error correction via HARQ.
    • Priority handling (mapping of logical channels).

3. RLC (Radio Link Control) Layer

  • Where: Both UE and gNB.
  • What it does:
    • Provides ARQ (Automatic Repeat reQuest) functionality for reliable data delivery (in acknowledged mode).
    • Segmentation and reassembly of PDCP PDUs to fit MAC-layer transport constraints.
    • Three modes:
      1. Transparent Mode (TM) – minimal overhead, mainly for broadcast info.
      2. Unacknowledged Mode (UM) – no retransmission, used for real-time traffic where delays must be minimized.
      3. Acknowledged Mode (AM) – uses ARQ for error-free delivery.
  • Key Functions:
    • Error correction via ARQ (AM mode).
    • In-sequence delivery and duplication detection (AM mode).
    • Segmentation and reassembly.

4. PDCP (Packet Data Convergence Protocol) Layer

  • Where: Both UE and gNB.
  • What it does:
    • Handles header compression (ROHC) to reduce overhead over the air interface.
    • Ciphering and integrity protection to secure user and control plane data.
    • In-sequence delivery and reordering of packets (particularly when handovers occur).
  • Key Functions:
    • User-plane: Data encryption, integrity protection (as configured), header compression.
    • Control-plane: RRC message security and integrity.
    • Supports split/aggregation of data in case of multi-RAT or multi-carrier (e.g., Carrier Aggregation or Dual Connectivity).

5. SDAP (Service Data Adaptation Protocol)

  • Where: User Plane only, both UE and gNB.
  • What it does:
    • Maps 5G QoS flows (defined in the 5G Core) to the appropriate data radio bearer on the air interface.
    • Enforces QoS policies (priority, traffic handling) as defined by the 5G core network.
  • Key Functions:
    • Maintains QoS flow identifiers (QFI).
    • Ensures QoS consistency end-to-end.

6. RRC (Radio Resource Control)

  • Where: UE and gNB (control plane).
  • What it does:
    • Controls radio resources (connection establishment, configuration, maintenance, and release).
    • Broadcasts system information.
    • Performs mobility functions (handover, cell reselection).
  • Key Functions:
    • Connection management (RRC Connected, RRC Idle, RRC Inactive states).
    • Measurement configuration and reporting.
    • Security mode control (key setup, algorithms selection).

7. NAS (Non-Access Stratum)

  • Where: Runs between the UE and the 5G Core’s AMF/SMF (not actually terminated in the gNB).
  • What it does:
    • Responsible for registration, authentication, security control, session management, and mobility management at the core network level.
    • In 5G, NAS messages are simply encapsulated in RRC messages for transport across the air interface.
  • Key Functions:
    • UE registration with the 5G core (AMF).
    • PDU session establishment (SMF).
    • Authentication and security procedures.

Putting It All Together

  1. UE Side

    • The application traffic (e.g., web, video) hits the IP layer and is then directed through SDAP (for QoS flow mapping) before going to PDCP.
    • PDCP performs ciphering, integrity protection, and possible header compression.
    • RLC segments or reassembles the data for lower layers.
    • MAC schedules data transmission opportunities (in uplink) and receives them in downlink.
    • Finally, the PHY layer sends the data over the air interface as electromagnetic signals.

  2. gNB Side

    • Receives those electromagnetic signals at the PHY layer.
    • MAC reassembles data from transport channels and forwards it to the corresponding RLC instance.
    • PDCP decrypts (or checks integrity), reorders packets as needed, and forwards them upward.
    • SDAP ensures the data is associated with correct QoS flows toward the 5G core (via NG-U interface).
    • On the control plane, RRC at the gNB communicates with RRC on the UE to set up and maintain the connection. NAS messages are forwarded between UE and the 5G core.

Overall, each layer adds functionality to make the link more reliable, secure, and QoS-aware, ensuring end-to-end performance for both user and control traffic in a 5G system.

Key Takeaways

  • SDAP is new in 5G, tying together 5G Core QoS flows with the radio QoS.
  • PDCP is crucial for encryption, integrity, and header compression.
  • RLC and MAC provide error correction and multiplexing.
  • RRC and NAS handle control and signaling (including mobility, connections, and core network procedures).
  • Everything below RRC (PDCP/RLC/MAC/PHY) is considered the Access Stratum (AS), while NAS is the Non-Access Stratum (NAS).

This layered approach in 5G ensures flexibility, security, and robust Quality of Service for diverse applications ranging from mobile broadband to mission-critical IoT.