Section: 5.6.2 Paging procedure 5.6.2.1 General
Content: The paging procedure is performed only in 3GPP access and used by the network to request the establishment of a NAS signalling connection to the UE. The paging procedure is also used by the network to request the UE to re-establish the user-plane resources of PDU sessions for downlink user data transport. Another purpose of the paging procedure is to request the UE to re-establish the user-plane resources of PDU session(s) associated with non-3GPP access over 3GPP access. Additionally, the network can use the paging procedure to initiate the mobile terminating SMS. For the UE using eDRX, the network initiates the paging procedure when NAS signalling messages or user data is pending to be sent to the UE within the paging time window and requests the lower layers to include the eDRX cycle length and paging time window length in the paging message. If NAS signalling messages or user data is pending to be sent to the UE outside the paging time window and the eDRX value that the network provides to the UE in the Negotiated extended DRX parameters IE during the last registration procedure indicates: a) the eDRX cycle length duration of the E-UTRA cell connected to 5GCN, is higher than 5.12 seconds; or b) the eDRX cycle length duration of the NR cell connected to 5GCN, is higher than 10.24 seconds, the network initiates the paging procedure at T time ahead of the beginning of the next paging time window. NOTE: T time is a short time period based on implementation. The operator can take possible imperfections in the synchronization between the 5GCN and the UE into account when choosing T time. If the network detects that the pending user data to be sent to the UE is related to the voice service as specified in 3GPP TS 23.502[ Procedures for the 5G System (5GS) ]  [9] and the network decides to initiate the paging procedure based on the stored paging restriction information, if any, the AMF should request the lower layer to include the Voice Service Indication in the paging message when the UE and the network support the paging indication for voice services.