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User Plane Protocol Stacks in LTE and 5G Networks

User Plane Protocol Stacks in LTE and 5G Networks
User Plane Protocol Stacks in LTE and 5G Networks

Introduction

In LTE and 5G networks, the user plane protocol stack is critical for transferring application data between end users and application servers. This article outlines the key components of the user plane protocol stack and their roles in different network configurations.


Overview of User Plane Protocol Stack

The user plane protocol stack manages the flow of application data from the end user to the application server. Figure 51 depicts this stack for a TCP/IP application and a standalone base station connected to the 5G Core Network. This scenario assumes a PDU session configured as 'IPv4', 'IPv6', or 'IPv4v6'. Other configurations may involve PDU sessions set to 'Ethernet' or 'Unstructured'.

Unstructured packets lack headers necessary for routing to the application server, making this method suitable for applications generating many small packets. In these cases, the anchor User Plane Function (UPF) creates a tunnel across the N6 interface to the application server, using UDP/IP headers for N6 transfer but not across the air interface or core network.


Application Data Flow

In the illustrated example (Figure 51), application data is transmitted between the User Equipment (UE) and the application server through a gNode B and a pair of UPFs. The UPF connecting to the application server is called the Anchor UPF, which supports the Data Network Name (DNN), indicating the exit point towards the Data Network (DN) hosting the application server. The PDU session terminates at the DNN within the Anchor UPF.

The application layer, present at both the UE and application server, is the highest layer in the user plane protocol stack. It uses lower layers to transfer data, often employing TCP or UDP. Typical applications include:

  • HTTP over TCP: For web browsing.

  • FTP over TCP: For file transfers.

  • SMTP over TCP: For email.

  • Telnet over TCP: For remote access.

  • RTP over UDP: For streaming audio and video.

  • VoIP using RTP over UDP: For voice calls.


Transmission Protocols In 5G

Transmission Control Protocol (TCP) provides reliable data transfer between the UE and application server by handling retransmissions for lost packets. It includes a flow control mechanism and headers, typically 20 bytes, ensuring in-order packet delivery.

User Datagram Protocol (UDP) offers a connectionless data transfer service. It does not support retransmissions or sequence numbers, allowing out-of-order packet delivery. Its headers are lightweight, making it suitable for applications that manage out-of-order packets independently.


Protocol Layers and Interfaces

The IP layer at the UE, anchor UPF, and application server routes user plane data. The IP address is allocated by the Session Management Function (SMF) during connection establishment. The IP layer supports both IPv4 and IPv6, with header sizes of 20 and 40 bytes, respectively.

The Service Data Adaptation Protocol (SDAP), as per 3GPP TS 37.324, maps QoS flows of a PDU session to Data Radio Bearers (DRB). It operates as the top layer of the Radio Access Network (RAN) protocol stack.

The Packet Data Convergence Protocol (PDCP), specified in 3GPP TS 38.323, provides header compression, ciphering, and integrity protection using the Robust Header Compression (RoHC) protocol. PDCP also ensures in-sequence packet delivery.

The Radio Link Control (RLC), as per 3GPP TS 38.322, operates between the UE and base station in two modes: Unacknowledged Mode (UM) for segmenting higher-layer packets and Acknowledged Mode (AM) for reliable data transfer with retransmission.

The Medium Access Control (MAC) layer, specified in 3GPP TS 38.321, functions between the UE and base station, multiplexing logical channel data and managing Transport Blocks. It supports the Hybrid Automatic Repeat Request (HARQ) protocol for fast retransmissions and includes various MAC Control Elements for layer 2 signaling tasks.


Physical Layer and Interfaces

Layer 1 (L1), detailed in 3GPP TS 38.211, 38.212, 38.213, and 38.214, handles physical layer processing and transmission over the air-interface. It includes error detection, channel coding for redundancy, interleaving, and modulation (CP-OFDM in downlink and CP-OFDM or DFT-S-FDMA in uplink).

The NG-U interface between the base station and UPF uses the PDU Session User Plane protocol (3GPP TS 38.415) to add headers for QoS flow identification. The GPRS Tunneling Protocol User Plane (GTP-U), specified in 3GPP TS 29.281, enables data transfer across the network using UDP over IP.

The N9 interface, connecting UPFs, uses the PDU Session User Plane protocol over GTP-U tunnels. These tunnels persist during UE transitions to CM-IDLE/RRC IDLE, unlike NG-U interface tunnels, which are released in these states.

Additional user plane protocol stacks for the Xn and F1 interfaces manage data transfer between neighboring base stations and between base station components (CU and DU), respectively.


Protocol Stack in Non-Standalone Configurations

In non-standalone setups, where a 5G base station connects to the 4G Core Network, the user plane protocol stack changes slightly. As shown in Figure 52, the SDAP layer is omitted since QoS flows do not exist in the 4G core. Instead, EPS Bearers are used, and the UPFs are replaced by the Serving Gateway and PDN Gateway, maintaining similar GTP-U tunnels for data transport.


Conclusion

The user plane protocol stack is vital for efficient application data transfer in LTE and 5G networks. Understanding each layer's role—from the application layer down to the physical layer—facilitates effective data management and optimization in various network scenarios.


References

  • 3GPP TS 38.300

  • 3GPP TS 38.414

  • 3GPP TS 38.415

  • 3GPP TS 29.281

  • 3GPP TR 29.891

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