5G System Terminal (UE) Paging Strategy

1.5G paging is based on operator configuration, and 5GS supports AMF and NG-RAN in applying different paging strategies for different types of traffic; specifically:

• When the UE is in CM-IDLE state, the AMF performs paging and determines the paging strategy based on information such as local configuration, the NF that triggered the paging, and the information available in the request that triggered the paging.
• If NWDAF is deployed, the AMF can also use analytical data (i.e., statistical or predictive data - see TS 23.288 [86]) provided by NWDAF regarding UE mobility.
• When the UE is in CM-CONNECTED state and the RRC connection is in RRC_INACTIVE state, the NG-RAN performs paging and determines the paging strategy based on information such as local configuration and information received from the AMF (as described in TS 23.501 Section 5.4.6.3) and SMF (as described in TS 23.501 Section 5.4.3.2).

2.SMF Service Triggered Paging: For network-triggered service requests from the SMF, the SMF determines the 5QI and ARP based on the following information:

• Downlink data packets (if the SMF performs buffering) or downlink data reports received from the UPF (if the UPF performs buffering). The SMF includes the 5QI and ARP corresponding to the QoS flow of the received downlink PDU in the request sent to the AMF.
• If the UE is in CM-IDLE state, the AMF can use, for example, the 5QI and ARP to derive different paging strategies, as described in Section 4.2.3.3 of TS 23.502 [3].

----The AMF uses the 5QI to determine the appropriate paging strategy.

3.Paging Strategy Area: This is an optional function that allows the AMF to apply different paging strategies to different traffic or service types provided within the same PDU session, based on operator configuration. In the R18 version specifications, this function is only applicable to IP type PDU sessions, where, when 5GS supports Paging Policy Differentiation (PPD) functionality, the DSCP value (TOS in IPv4/TC in IPv6) is set by the application to indicate which paging policy the 5GS should apply to a specific IP packet, as described in TS23.228 [15]. The P-CSCF can support paging policy differentiation by marking packets related to specific IMS services (such as session voice defined in IMS multimedia telephony services) to be sent to the UE.

----This PPD function can be used to determine the paging cause indication for voice services, as described in Section 5.38.3 of TS23501. Operators should be able to configure the SMF to apply the paging policy differentiation function only to certain HPLMNs, DNNs, and 5QIs. For HR roaming, this configuration is done in the SMF in the VPLMN.

4.Roaming Paging: Paging Policy Differentiation (PPD) support in HR roaming requires inter-operator agreements, including the DSCP values associated with this function, where:

• For network-triggered service requests and cases where the UPF caches downlink data packets, the UPF should include the TOS (IPv4)/TC (IPv6) value in the downlink data packet IP header and the corresponding QoS flow indication in the downlink data report sent to the SMF. When PPD is applicable, the SMF determines the Paging Policy Indicator (PPI) based on the DSCP value received from the UPF.
• For network-triggered service requests and cases where the SMF buffers downlink data packets, when PPD is applicable, the SMF determines the PPI based on the TOS (IPv4)/TC (IPv6) value in the received downlink data packet IP header and identifies the corresponding QoS flow from the QFI of the received downlink data packet. The SMF includes the PPI, ARP, and 5QI of the corresponding QoS flow in the N11 message sent to the AMF.
• If the UE is in CM-IDLE state, the AMF uses this information to generate a paging policy and sends a paging message to the NGRAN via N2.

• Network configuration needs to ensure that the information used as a trigger for paging policy indication does not change during the 5GS period; the network configuration needs to ensure that the specific DSCP in the TOS (IPv4)/TC (IPv6) values ​​used as triggers for paging policy indication is correctly managed to avoid unintended use of certain paging policies; where: for a UE in RRC_INACTIVE state, the NG-RAN can enforce a specific paging policy in the case of NG-RAN paging, based on the 5QI, ARP, and PPI associated with the incoming downlink PDU. To achieve this, the SMF instructs the UPF to detect the DSCP in the TOS (IPv4)/TC (IPv6) value in the downlink PDU IP header (by using a downlink PDR containing the DSCP for that traffic) and transmit the corresponding PPI in the CN tunnel header (by using a QER containing the PPI value). The NG-RAN can then utilize the PPI in the CN tunnel header of the received downlink PDU to apply the corresponding paging policy for paging when the UE is in RRC_INACTIVE state.

----In the case of home-routed roaming, the V-SMF is responsible for controlling the UPF settings for the PPI. In the case of a PDU session established with the I-SMF, the I-SMF is responsible for controlling the UPF settings for the PPI.

5.Paging Priority: This is a function that allows the AMF to include an indication in the paging message sent to the NG-RAN, indicating that the UE needs to be paged with priority. Whether the AMF includes paging priority in the paging message depends on the ARP value of the IP packet received from the SMF and waiting to be delivered in the UPF. If:

• The ARP value is associated with a specific priority service (e.g., MPS, MCS), the AMF will include paging priority in the paging message. When the NG-RAN receives a paging message containing paging priority, it will prioritize the processing of that paging.
• When the AMF is waiting for a response from the UE to a paging message that does not include priority, if it receives another message from the SMF whose ARP value is associated with a specific priority service (e.g., MPS, MCS), the AMF will send another paging message containing paging priority to the (RAN).
• For subsequent messages, the AMF may decide, based on local policy, whether to send a paging message with a higher paging priority.

For UEs in the RRC Inactive state, the NG-RAN determines the paging priority based on the ARP associated with the QoS flow, configured according to the operator's policy, and core network-assisted RAN paging information from the AMF (as described in Section 5.4.6.3).