The distinguishing characteristic of 5G is its capacity to handle various traffic flows with distinct Quality of Service (QoS) requirements. This paper examines how 5G achieves this through tailored QoS mechanisms for different types of traffic.
The paper explores the foundational elements within the 5G architecture that facilitate QoS implementation. We discuss how QoS flows are created, managed, and enforced, alongside a detailed analysis of the essential QoS parameters within 5G networks.
This discussion delves into the sophisticated methods embedded in 5G, shedding light on the techniques that drive the creation of QoS flows.
Quality of Service (QoS) defines the performance level of a network as perceived by its users. Within 5G, QoS encompasses factors like latency, reliability, throughput, availability, and traffic prioritization. These factors are critical in ensuring a smooth user experience and accommodating a wide range of application requirements.
- Key Features of 5G QoS:
- Network Slicing:
A pivotal advancement in 5G QoS is network slicing. This technology allows operators to segment a single physical infrastructure into multiple virtual networks. Each slice can be configured with specific QoS characteristics to meet the needs of different applications or user groups. This leads to better resource management and more tailored service delivery.
- Enhanced Mobile Broadband (eMBB):
eMBB is one of the cornerstone use cases for 5G, delivering significantly higher download and upload speeds compared to 4G. With 5G QoS, eMBB supports high-bandwidth applications like 4K video streaming, virtual reality (VR), augmented reality (AR), and cloud gaming, ensuring smooth and uninterrupted multimedia experiences.
- Ultra-Reliable Low-Latency Communication (URLLC):
URLLC is designed for scenarios requiring ultra-low latency and high reliability. Use cases like autonomous vehicles, remote surgeries, and industrial automation depend on URLLC’s real-time, mission-critical capabilities. By leveraging 5G’s potential, URLLC ensures minimal delays and highly reliable connections, backed by strict QoS guarantees.
- Massive Machine-Type Communication (mMTC):
The mMTC model underpins the Internet of Things (IoT), supporting billions of low-power, low-data devices. 5G QoS optimizes resource allocation for these devices, allowing seamless operation in smart cities, smart homes, and industrial IoT environments.
- QoS Framework in 5G:
5G employs a comprehensive QoS framework, managing both control and user plane functions to optimize performance. The registration process and subsequent communications involve several entities, ensuring that the User Equipment (UE) is validated and connected securely to the network.
5G Control and User Plane Functions
- Control Plane:
In 5G QoS, the control plane is responsible for policy control and enforcement. Operators use policy control to set QoS policies based on application needs and user preferences, enabling dynamic allocation of resources.
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- UE Registration:
The UE connects to the network through the Access and Mobility Management Function (AMF), which handles registration and mobility. The AMF, along with the Authentication Server Function (AUSF), ensures secure communication.
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- AUSF – Authentication Server Function:
The AUSF collaborates with the User Data Management (UDM) function, retrieving authentication vectors and validating the UE during the registration process.
- User Plane:
The user plane manages data transmission, focusing on packet scheduling, traffic shaping, and prioritization. The User Plane Function (UPF) ensures that data flows meet their respective QoS requirements.
Interfaces:
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- N3: Connects the RAN (gNB) to the initial UPF.
- N9: Links intermediate UPFs with the session anchor UPF.
- N6: Bridges the UPF and the Data Network (DN).
- N4: Connects the SMF and UPF for coordinated control.
- PDU Session Establishment
PDU Session Establishment Flow
The Policy Control Function (PCF) defines QoS parameters for PDU sessions, determining the charging mechanisms for different flows. The UE establishes a PDU session with the UPF, allowing it to interact with the Data Network (DN) while adhering to predefined QoS policies.
- QoS Flows:
In 5G, QoS flows determine the traffic management approach, with categories like Guaranteed Bit Rate (GBR), Non-Guaranteed Bit Rate (Non-GBR), and Reflective flows enabling adaptive configurations.
- QoS Flow Characterization:
QoS flow characterization defines the exact service levels required for different data flows. The SMF configures these profiles and sends them to the RAN to manage the QoS of ongoing communications.
- QoS Profile:
The QoS profile outlines the key parameters for each flow, including the Allocation and Retention Priority (ARP) and the 5G QoS Identifier (5QI). These parameters define priority levels and service guarantees.
QoS Profile Parameters
- QoS Profile 5QI – Parameters:
Specific parameters like Resource Type, Priority Level, Packet Delay Budget, Maximum Data Burst Volume, and Packet Error Rate are critical in defining the expected performance for different flows.
Maximum Data Burst Volume (MDBV)
- QoS Profile – GFBR and MFBR:
For GBR flows, the Guaranteed Flow Bit Rate (GFBR) and Maximum Flow Bit Rate (MFBR) parameters ensure consistent service quality.
- QoS Profile – Notification Control:
This parameter controls whether notifications should be sent when QoS levels cannot be maintained, enabling dynamic adjustments based on network conditions.
- QoS Rules:
QoS rules determine how uplink traffic is associated with different flows, using identifiers like QFI and QRI to manage traffic flows efficiently.
- Packet Detection Rule (PDR):
The PDR defines how packets should be identified and processed within the user plane, ensuring consistent QoS enforcement.
- SDAP:
The SDAP layer is responsible for mapping QoS flows to data Radio Bearers (DRBs), maintaining uniform packet treatment across the network.
RB creation by SDAP
- PDCP and RLC:
The Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) layers play pivotal roles in the management of Data Radio Bearers (DRBs) within the 5G protocol stack. Their primary responsibilities revolve around ensuring efficient data transmission, handling errors, maintaining data integrity, and implementing security mechanisms across the network.
- PDCP Layer:
The PDCP layer operates between the upper layers and the RLC layer and is responsible for key functions that enhance the efficiency and security of data transmission in 5G networks. Below are the primary functions of the PDCP layer:
- Header Compression: In 5G, the data packets can contain headers that introduce significant overhead, especially when transmitting small packets, which is common in IoT scenarios. PDCP uses robust header compression techniques, such as Robust Header Compression (ROHC), to reduce this overhead. By compressing IP headers, ROHC allows for more efficient use of radio resources, ensuring higher data throughput and reduced latency.
- Security Functions: The PDCP layer is crucial for securing user data. It performs both encryption and integrity protection of the data that is transmitted over the air interface. Encryption ensures confidentiality by preventing unauthorized access to user data, while integrity protection detects and prevents tampering with data during transmission.
- In-Order Delivery and Duplicate Detection: The PDCP layer ensures that data packets are delivered in the correct order to the upper layers, even in the presence of packet loss or reordering in the lower layers. Additionally, the PDCP layer has mechanisms for detecting and eliminating duplicate packets, which is critical in scenarios involving retransmissions or handovers between cells.
- Reordering and Retransmission: In mobility scenarios where handovers occur, the PDCP layer handles the reordering of out-of-sequence packets and ensures they are delivered in the correct order. This function is crucial for maintaining data continuity and session integrity during handovers.
- Segmentation and Reassembly: The PDCP layer segments large data packets into smaller units that can be managed by the underlying RLC layer. It then reassembles these segments back into complete packets before passing them to the upper layers.
- RLC Layer:
The RLC layer sits between the PDCP and the MAC (Medium Access Control) layer. It plays a critical role in ensuring reliable data transmission over the unreliable and variable wireless medium. The RLC layer operates in three modes: Acknowledged Mode (AM), Unacknowledged Mode (UM), and Transparent Mode (TM), each serving different use cases and QoS requirements.
- Segmentation and Reassembly: Like the PDCP layer, the RLC layer performs segmentation of larger packets into smaller RLC Protocol Data Units (PDUs) that can be efficiently transmitted over the air interface. This is particularly useful when the transmission conditions require smaller packet sizes.
- Error Correction through ARQ: In Acknowledged Mode (AM), the RLC layer implements Automatic Repeat reQuest (ARQ) for error correction. ARQ ensures reliable delivery by retransmitting any lost or corrupted PDUs. This feature is essential for applications that require high reliability, such as voice and video communications.
- Buffering and Flow Control: The RLC layer manages the flow of data between the higher layers and the MAC layer by buffering PDUs and controlling the rate at which data is passed down to the MAC layer. This function is crucial for handling congestion and ensuring smooth data flow even when network conditions fluctuate.
- In-Order Delivery and Duplicate Elimination: Similar to PDCP, the RLC layer also ensures that PDUs are delivered in sequence and without duplication. In Unacknowledged Mode (UM), where retransmissions are not performed, the RLC layer still manages reordering and duplicate detection based on sequence numbers.
- Segmentation and Concatenation: The RLC layer can segment larger SDUs (Service Data Units) into smaller PDUs and concatenate smaller SDUs into a single PDU for efficient transmission. This flexibility allows the RLC layer to adapt to varying radio conditions and optimize the use of available bandwidth.
Downlink Layer 2 Structure
Challenges and Future Directions
While 5G QoS holds immense promise, challenges like ensuring end-to-end QoS, integrating with existing systems, and managing network slices require careful attention. Future enhancements, such as support for emerging use cases like holographic communication, will drive further innovation.
Conclusion
5G QoS is essential for realizing the full potential of next-generation networks. By enabling customized services, prioritizing traffic, and supporting diverse applications, 5G QoS paves the way for new opportunities across industries. Continuous advancements in QoS will be key to unlocking the transformative capabilities of 5G.
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References
[1] 3GPP 23.501–5G System Architecture
[2] 3GPP 38.300 — NR and NG-RAN Overall
[3] 3GPP 23.502 — Procedures for the 5G System
[4] 3GPP 38.415 — PDU Session User Plane Protocol
[5] 3GPP 29.244 — Interface Between UP and CP
[6] 3GPP 37.324 — SDAP
[7] 3GPP 38.321 — MAC
[8] 3GPP 38.322 — RLC
[9] 3GPP 38.323 — PDCP
