5.7.3.4 Packet Delay Budget

The Packet Delay Budget (PDB) defines an upper bound for the time that a packet may be delayed between the UE and the N6 termination point at the UPF. The PDB applies to the DL packet received by the UPF over the N6 interface and to the UL packet sent by the UE. For a certain 5QI the value of the PDB is the same in UL and DL. In the case of 3GPP access, the PDB is used to support the configuration of scheduling and link layer functions (e.g. the setting of scheduling priority weights and HARQ target operating points). For GBR QoS Flows using the Delay-critical resource type, a packet delayed more than PDB is counted as lost if the data burst is not exceeding the MDBV within the period of PDB and the QoS Flow is not exceeding the GFBR. For GBR QoS Flows with GBR resource type not exceeding GFBR, 98 percent of the packets shall not experience a delay exceeding the 5QI's PDB.

The 5G Access Network Packet Delay Budget (5G-AN PDB) is determined by subtracting a static value for the Core Network Packet Delay Budget (CN PDB), which represents the delay between any N6 termination point at the UPF (for any UPF that may possibly be selected for the PDU Session) and the 5G-AN from a given PDB.

NOTE 1: For a standardized 5QI, the static value for the CN PDB is specified in the QoS characteristics Table 5.7.4-1.

NOTE 2: For a non-standardized 5QI, the static value for the CN PDB is homogeneously configured in the network.

For GBR QoS Flows using the Delay-critical resource type, in order to obtain a more accurate delay budget PDB available for the NG-RAN, a dynamic value for the CN PDB, which represents the delay between the UPF terminating N6 for the QoS Flow and the 5G-AN, can be used. If used for a QoS Flow, the NG-RAN shall apply the dynamic value for the CN PDB instead of the static value for the CN PDB (which is only related to the 5QI). Different dynamic value for CN PDB may be configured per uplink and downlink direction.

NOTE 3: The configuration of transport network on CN tunnel can be different per UL and DL, which can be different value for CN PDB per UL and DL.

NOTE 4: It is expected that the UPF deployment ensures that the dynamic value for the CN PDB is not larger than the static value for the CN PDB. This avoids that the functionality that is based on the 5G-AN PDB (e.g. MDBV, NG-RAN scheduler) has to handle an unexpected value.

The dynamic value for the CN PDB of a Delay-critical GBR 5QI may be configured in the network in two ways:

• Configured in each NG-RAN node, based on a variety of inputs such as different IP address(es) or TEID range of UPF terminating the N3 tunnel and based on different combinations of PSA UPF to NG-RAN under consideration of any potential I-UPF, etc;
• Configured in the SMF, based on different combinations of PSA UPF to NG-RAN under consideration of any potential I-UPF. The dynamic value for the CN PDB for a particular QoS Flow shall be signalled to NG-RAN (during PDU Session Establishment, PDU Session Modification, Xn/N2 handover and the Service Request procedures) when the QoS Flow is established or the dynamic value for the CN PDB of a QoS Flow changes, e.g. when an I-UPF is inserted by the SMF.

If the NG-RAN node is configured locally with a dynamic value for the CN PDB for a Delay-critical GBR 5QI and receives a different value via N2 signalling for a QoS Flow with the same 5QI, local configuration in RAN node determines which value takes precedence.

Services using a GBR QoS Flow and sending at a rate smaller than or equal to the GFBR can in general assume that congestion related packet drops will not occur.

NOTE 5: Exceptions (e.g. transient link outages) can always occur in a radio access system which may then lead to congestion related packet drops. Packets surviving congestion related packet dropping may still be subject to non-congestion related packet losses (see PER below).

Services using Non-GBR QoS Flows should be prepared to experience congestion-related packet drops and delays. In uncongested scenarios, 98 percent of the packets should not experience a delay exceeding the 5QI's PDB.

The PDB for Non-GBR and GBR resource types denotes a "soft upper bound" in the sense that an "expired" packet, e.g. a link layer SDU that has exceeded the PDB, does not need to be discarded and is not added to the PER. However, for a Delay-critical GBR resource type, packets delayed more than the PDB are added to the PER and can be discarded or delivered depending on local decision.