Overview
Quality of Service (QoS) in 5G networks is a crucial aspect that ensures efficient data transmission by categorizing traffic into different QoS Flows. Each QoS Flow is identified by a QoS Flow Identifier (QFI) and is handled based on its specific requirements. This categorization enables the network to prioritize traffic effectively, ensuring optimal performance for various applications.
Categories of QoS Flows
QoS Flows are divided into three primary categories:
Guaranteed Bit Rate (GBR) QoS Flow:
Characteristics: Provides a Guaranteed Flow Bit Rate (GFBR) to the end-user.
Applications: Suitable for time-sensitive applications such as voice and video calls, real-time gaming, and Vehicle-to-Everything (V2X) communications.
Example: A GBR QoS Flow may permit a 100 ms delay budget.
Non-Guaranteed Bit Rate (non-GBR) QoS Flow:
Characteristics: Does not provide a GFBR.
Applications: Ideal for non-time sensitive applications like web browsing, buffered streaming, instant messaging, and IMS signaling.
Example: Non-GBR QoS Flows are treated as 'best effort' relative to GBR Flows.
Delay Critical GBR QoS Flow:
Characteristics: Offers significantly lower latencies compared to GBR QoS Flows and has low Packet Error Rates for improved reliability.
Applications: Used in automation and intelligent transport systems where minimal delay is critical.
Example: A Delay Critical GBR QoS Flow may permit a 10 ms delay budget.
QoS Parameters and Characteristics
Each QoS Flow is associated with a set of parameters and characteristics, which define its behavior and treatment within the network. The key parameters include:
5G QoS Identifier (5QI): A pointer to a set of QoS characteristics, with standardized values for different types of flows.
Allocation and Retention Priority (ARP): Determines the priority of QoS Flows and their ability to pre-empt or be pre-empted by other flows.
Guaranteed Flow Bit Rate (GFBR) and Maximum Flow Bit Rate (MFBR): Define the minimum and maximum bit rates expected from GBR and Delay Critical GBR QoS Flows.
Notification Control: Indicates whether the Base Station should notify the SMF if the GFBR is not achieved.
Maximum Packet Loss Rate: Applicable to QoS Flows transferring voice media, defining the tolerated packet loss rate.
Session-AMBR and UE-AMBR: Define the maximum bit rate for non-GBR QoS Flows at the session and user equipment levels, respectively.
Reflective QoS Attribute (RQA): Indicates whether the UE should apply Reflective QoS, allowing the UE to learn QoS rules from downlink patterns.
Detailed Characteristics of QoS Flows
Guaranteed Flow Bit Rate (GFBR):
Application: Only applicable to GBR and Delay Critical GBR QoS Flows.
Function: Defines the minimum bit rate that can be expected from the QoS Flow when measured across the Averaging Window.
Uplink and Downlink: Can be specified independently for uplink and downlink.
Maximum Flow Bit Rate (MFBR):
Application: Only applicable to GBR and Delay Critical GBR QoS Flows.
Function: Defines the maximum bit rate expected from the QoS Flow when measured across the Averaging Window.
Uplink and Downlink: Can be specified independently for uplink and downlink.
Notification Control:
Function: Indicates whether the Base Station should notify the SMF if the QoS Flow fails to achieve its GFBR.
Behavior: If GFBR is not achieved, the Base Station continues attempting while notifying the SMF, which may reconfigure or release the QoS Flow.
Maximum Packet Loss Rate:
Application: Only applicable to GBR and Delay Critical GBR QoS Flows, and only to voice media in the release 15 version of the 3GPP specifications.
Function: Defines the maximum rate of lost packets that can be tolerated in uplink and downlink.
Session-AMBR:
Function: Defines the maximum permitted bit rate summed across all non-GBR QoS Flows belonging to a specific PDU Session.
Enforcement: Managed by the User Plane Functions (UPF) involved with the relevant PDU Session.
UE-AMBR:
Function: Defines the maximum permitted bit rate summed across all non-GBR QoS Flows belonging to a specific UE.
Enforcement: Managed by the serving Base Station.
Reflective QoS Attribute (RQA):
Function: Indicates whether packets in the QoS Flow require the UE to apply Reflective QoS, which involves learning uplink rules from downlink patterns.
Applicability: Used in PDU Sessions for IP or Ethernet packets (not applicable to Unstructured packets).
Prioritization and Scheduling
QoS Flows are prioritized based on their category and specified parameters. GBR QoS Flows are given priority over non-GBR QoS Flows until their GFBR is achieved. Once the GFBR is met, excess GBR packets can be deprioritized to manage network resources efficiently. Non-GBR QoS Flows are treated as 'best effort' and are scheduled after the needs of GBR Flows are met. Within each category, packets are further prioritized based on their individual characteristics such as delay budgets and error rates.
Packet Handling and Delay Management
The Packet Delay Budget (PDB) and Packet Error Rate (PER) are critical for managing the performance of QoS Flows:
Packet Delay Budget (PDB):
Function: Defines the maximum allowable delay for packet transmission between the UE and the network's UPF.
Performance Requirement: For GBR and non-GBR Flows, 98% of packets should meet this delay requirement. For Delay Critical GBR Flows, packets exceeding the delay budget contribute to the PER.
Packet Error Rate (PER):
Function: Measures the percentage of packets not successfully transferred, excluding those lost due to congestion.
Performance Requirement: For Delay Critical GBR Flows, delayed packets are counted as lost unless specific conditions are met.
Maximum Data Burst Volume and Averaging Window
Maximum Data Burst Volume:
Application: Relevant to Delay Critical GBR Flows.
Function: Specifies the maximum data amount the Base Station must handle within a time window, aiding in managing peak data transmission efficiently.
Averaging Window:
Function: Ensures that the GFBR and MFBR are met consistently over short, frequent intervals rather than through sporadic, large allocations, promoting steady performance.
Reflective QoS
Reflective QoS allows the UE to mirror the QoS settings observed in downlink transmissions for uplink packets, simplifying the QoS management for dynamic sessions and improving the overall network efficiency. Reflective QoS is particularly applicable to IP and Ethernet packet transfers and is optional for the UE.
Conclusion
Understanding and implementing QoS in 5G networks is essential for providing differentiated services that meet the diverse needs of modern applications. By categorizing traffic and defining specific parameters for each QoS Flow, 5G networks can ensure high performance, reliability, and user satisfaction. The detailed parameters and prioritization mechanisms enable networks to manage resources efficiently, providing a robust framework for various applications ranging from critical real-time communications to non-time sensitive data transfers.
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