(R)AN to AMF: N2 Message (N2 parameters, Service Request.
Details of this step are described in TS 38.413 . If the AMF can't handle the Service Request it will reject it.
When NG-RAN is used, the N2 parameters include the 5G-S-TMSI, Selected PLMN ID, Location information and Establishment cause, UE Context Request.
If the UE is in CM-IDLE state, the NG-RAN obtains the 5G-S-TMSI in RRC procedure. NG-RAN selects the AMF according to 5G-S-TMSI. The Location Information relates to the cell in which the UE is camping.
Based on the PDU Session status, the AMF may initiate PDU Session Release procedure in the network for the PDU Sessions whose PDU Session ID(s) were indicated by the UE as not available.
When the Establishment cause is associated with priority services (e.g. MPS, MCS), the AMF includes a Message Priority header to indicate priority information. Other NFs relay the priority information by including the Message Priority header in service-based interfaces, as specified in TS 29.500 .