5G NR QoS Parameters

5G, Interviews, New Radio, NR, Tech Fundas

3GPP specification 23.501 defined the following 5G QoS Parameters as part of QoS Implementation.

  • 5G QoS Identifier (5GQI)
  • Allocation and Retention Priority (ARP)
  • Reflective QoS Attribute (RQA)
  • Notification Control
  • Flow Bit Rates
  • Aggregate Bit Rates
  • Default values
  • Maximum Packet Loss Rate

5G QoS Identifier (5GQI)

A 5QI is a scalar that is used as a reference to 5G QoS characteristics  i.e. access node-specific parameters that control QoS forwarding treatment for the QoS Flow (e.g. scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc.).

  • Standardized 5QI values have one-to-one mapping to a standardized combination of 5G QoS characteristics .
  • The 5G QoS characteristics for pre-configured 5QI values are pre-configured in the Access Node (gNB).
  • Standardized or pre-configured 5G QoS characteristics, are indicated through the 5QI value, and are not signalled on any interface, unless certain 5G QoS characteristics are modified .
  • The 5G QoS characteristics for QoS Flows with dynamically assigned 5QI are signalled as part of the QoS profile.

Allocation and Retention Priority (ARP)

The QoS parameter ARP contains information about the priority level, the pre-emption capability and the pre-emption vulnerability. The ARP priority level defines the relative importance of a resource request to allows in deciding whether a new QoS Flow may be accepted or needs to be rejected in the case of resource limitations (typically used for admission control of GBR traffic). It may also be used to decide which existing QoS Flow to pre-empt during resource limitations. ARP has following characteristics:

  • The range of the ARP priority level is 1 to 15 with 1 as the highest level of priority
  • The ARP priority levels 1-8 should only be assigned to resources for services that are authorized to receive prioritized treatment within an operator domain (i.e. that are authorized by the serving network)
  • The ARP priority levels 9-15 may be assigned to resources that are authorized by the home network and thus applicable when a UE is roaming
  • The ARP pre-emption capability defines whether a service data flow may get resources that were already assigned to another service data flow with a lower ARP priority level
  • The ARP pre-emption capability and the ARP pre-emption vulnerability shall be either set to ‘enabled’ or ‘disabled’
  • The ARP pre-emption vulnerability defines whether a service data flow may lose the resources assigned to it in order to admit a service data flow with higher ARP priority level
  • The ARP pre-emption vulnerability of the QoS Flow which the default QoS rule is associated with should be set appropriately to minimize the risk of unnecessary release of this QoS Flow

Reflective QoS Attribute (RQA)

The  RQA is an optional parameter which indicates that certain traffic (not necessarily all) carried on this QoS Flow is subject to Reflective QoS. Only when the RQA is signalled for a QoS Flow, the RAN/AN enables the transfer of the RQI for AN resource corresponding to this QoS Flow. The RQA may be signalled to NG-RAN via the N2 reference point at UE context establishment in NG-RAN and at QoS Flow establishment or modification.

Notification Control

The QoS Parameter Notification control indicates whether notifications are requested from the NG-RAN when the GFBR can no longer (or can again) be guaranteed for a QoS Flow during the lifetime of the QoS Flow. Notification control may be used for a GBR QoS Flow if the application traffic is able to adapt to the change in the QoS (e.g. if the AF is capable to trigger rate adaptation).

  • The SMF shall only enable Notification control when the QoS Notification Control parameter is set in the PCC rule (received from the PCF) that is bound to the QoS Flow.
  • The Notification control parameter is signalled to the NG-RAN as part of the QoS profile.
  • If, for a given GBR QoS Flow, Notification control is enabled and the NG-RAN determines that the GFBR can no longer be guaranteed, NG-RAN shall send a notification towards SMF and keep the QoS Flow (i.e. while the NG-RAN is not delivering the requested GFBR for this QoS Flow), unless specific conditions at the NG-RAN require the release of the NG-RAN resources for this GBR QoS Flow, e.g. due to Radio link failure or RAN internal congestion. The NG-RAN should try to guarantee the GFBR again.
  • Upon receiving a notification from the NG-RAN that the GFBR can no longer be guaranteed, the SMF may forward the notification to the PCF, 5GC may initiate N2 signalling to modify or remove the QoS Flow
  • When the NG-RAN determines that the GFBR can be guaranteed again for a QoS Flow (for which a notification that the GFBR can no longer be guaranteed has been sent), the NG-RAN shall send a notification, informing the SMF that the GFBR can be guaranteed again and the SMF may forward the notification to the PCF,  The NG-RAN shall send a subsequent notification that the GFBR can no longer be guaranteed whenever necessary.
  • During a handover, the Source NG-RAN does not explicitly inform the Target NG-RAN about those QoS Flows for which a notification that the GFBR can no longer be guaranteed has been sent. The Target NG-RAN performs admission control rejecting any QoS Flows for which resources cannot be permanently allocated. The accepted QoS Flows are included in the N2 Path Switch Request message from the NG-RAN to the AMF.

Flow Bit Rates

For GBR QoS Flows only, the additional QoS parameters are Guaranteed Flow Bit Rate (GFBR) and Maximum Flow Bit Rate (MFBR) applicable for Uplink  and Downlink.

  • The GFBR denotes the bit rate that is guaranteed to be provided by the network to the QoS Flow over the Averaging Time Window
  • The MFBR limits the bit rate to the highest bit rate that is expected by the QoS Flow (e.g. excess traffic may get discarded or delayed by a rate shaping or policing function at the UE, RAN, UPF)

Bit rates above the GFBR value and up to the MFBR value, may be provided with relative priority determined by the Priority Level of the QoS Flows. GFBR and MFBR are signalled to the (R)AN in the QoS Profile and signalled to the UE as QoS Flow level QoS parameter for each individual QoS Flow.

Aggregate Bit Rates

Each PDU Session of a UE is associated with the following aggregate rate limit QoS parameter:

  • per Session Aggregate Maximum Bit Rate (Session-AMBR). The subscribed Session-AMBR is a subscription parameter which is retrieved by the SMF from UDM. SMF may use the subscribed Session-AMBR or modify it based on local policy or use the authorized Session-AMBR received from PCF to get the Session-AMBR, which is signalled to the appropriate UPF entity/ies to the UE and to the (R)AN (to enable the calculation of the UE-AMBR). The Session-AMBR limits the aggregate bit rate that can be expected to be provided across all Non-GBR QoS Flows for a specific PDU Session. The Session-AMBR is measured over an AMBR averaging window which is a standardized value. The Session-AMBR is not applicable to GBR QoS Flows.

Each UE is associated with the following aggregate rate limit QoS parameter:

  • per UE Aggregate Maximum Bit Rate (UE-AMBR). The UE-AMBR limits the aggregate bit rate that can be expected to be provided across all Non-GBR QoS Flows of a UE. Each (R)AN shall set its UE-AMBR to the sum of the Session-AMBR of all PDU Sessions with active user plane to this (R)AN up to the value of the subscribed UE-AMBR. The subscribed UE-AMBR is a subscription parameter which is retrieved from UDM and provided to the (R)AN by the AMF. The UE-AMBR is measured over an AMBR averaging window which is a standardized value. The UE-AMBR is not applicable to GBR QoS Flows.

Default values

For each PDU Session Setup, the SMF retrieves the subscribed default values for the 5QI and the ARP priority level and optionally, the 5QI Priority Level, from the UDM. The subscribed default 5QI value shall be a Non-GBR 5QI from the standardized value range.

  • The SMF may change the subscribed values for the default 5QI and the ARP priority level and if received, the 5QI Priority Level, based on local configuration or interaction with the PCF to set QoS parameters for the QoS Flow which the default QoS rule is associated with
  • The SMF shall set the ARP pre-emption capability and the ARP pre-emption vulnerability of the QoS Flow which the default QoS rule is associated with based on local configuration or interaction with the PCF
  • The SMF shall apply the same values for the ARP priority level, the ARP pre-emption capability and the ARP pre-emption vulnerability for all QoS Flows of the PDU Session unless a different ARP setting is required for a QoS Flow
  • If dynamic PCC is not deployed, the SMF can have a DNN based configuration to enable the establishment of a GBR QoS Flow as the QoS Flow that is associated with the default QoS rule. This configuration contains a standardized GBR 5QI as well as GFBR and MFBR for UL and DL.

Maximum Packet Loss Rate

The Maximum Packet Loss Rate (UL, DL) indicates the maximum rate for lost packets of the QoS flow that can be tolerated in the uplink and downlink direction. . This is provided to the QoS flow if it is compliant to the GFBR.

Reference: 3GPP TS 23.501; System architecture for the 5G System (5GS)