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UE in CM-IDLE mode will request service request procedure to establish a secure connection to AMF.
UE in CM-IDLE and in CM-CONNECTED mode will initiate service request procedure to activate user plane connection to establish a PDU session.
-> The UE in CM-IDLE state initiates the Service Request procedure in order to send uplink signalling messages, user data, to request emergency services fallback, or as a response to a network paging request.
-> After receiving the Service Request message, the AMF may perform authentication. After the establishment of the signalling connection to an AMF, the UE or network may send signalling messages, e.g. PDU Session establishment from UE to the SMF, via the AMF.
1. Service Request NAS(UE -> AN)
-> UE will send Service Request message towards the AMF encapsulated in RRC message to RAN.
-> If the Service Request is triggered by the UE for user data, the UE identifies, using the List Of PDU Sessions To Be Activated
-> If the service request triggered by UE is for signalling only, the UE will not identify any list of PDU session to be activated.
-> If the service request is triggered in response paging or NAS notification for non 3gpp access, then service request message will identify the list of PDU sessions associated.
2. N2 Message Service Request (AN -> AMF)
-> If the AMF can’t handle the service request it will reject it.
-> The N2 parameters include the 5G-S-TMSI, Selected PLMN ID, Location information and Establishment cause, UE Context Request.
-> According to 5G-S-TMSI AMF will be selected.
-> 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.
3. Authentication Security
-> The 5G-AN uses the Security Context to protect the messages exchanged with the UE.
4. Nsmf_PDUSession_UpdateSMContext(AMF -> SMF [Conditional] )
Below are the list of reasons when Nsmf_PDUSession_UpdateSMContext request is invoked:
-> If the UE identifies List Of PDU Sessions To Be Activated.
-> If this procedure is triggered by the SMF in response to paging or NAS notification.
-> The AMF determines the PDU Session(s) for which the UP connection(s) shall be activated and sends an Nsmf_PDUSession_UpdateSMContext Request to SMF(s) associated with the PDU Session(s) with Operation Type set to “UP activate” to indicate establishment of User Plane resources for the PDU Session(s).
5a. SMF initiated SM Policy Association Modification (SMF -> PCF [Conditional])
-> the SMF perform an SMF initiated SM Policy Association Modification procedure
5b. UPF Selection
-> based on the location info received from the AMF, the SMF checks the UPF Selection Criteria and performs one of below:
-> accepts the activation of UP connection and continue using the current UPF(s);
-> accepts the activation of UP connection and selects a new intermediate UPF;
6a. [Conditional] N4 Session Modification Request SMF –> UPF (PSA)
-> If different CN Tunnel Info need be used, the SMF sends N4 Session Modification Request message to UPF (PSA) and requests CN Tunnel Info providing the target Network Instance.
6b. [Conditional] N4 Session Modification Response UPF(PSA) –> SMF
-> The UPF provides CN Tunnel Info to the SMF. The UPF (PSA) associate the CN Tunnel Info with UL Packet detection rules provided by the SMF.
6c. [Conditional] N4 Session Establishment Request SMF –> New UPF (middle)
-> If the SMF selects a new UPF to act as intermediate UPF for the PDU Session, or if the SMF selects to insert an intermediate UPF for a PDU Session which did not have an intermediate UPF, an N4 Session Establishment Request message is sent to the new UPF, providing Packet detection, Data forwarding, enforcement and reporting rules to be installed on the intermediate UPF.
6d. N4 Session Establishment Response New UPF (middle) –> SMF
-> The new intermediate UPF sends an N4 Session Establishment Response message to the SMF.
7a. [Conditional] N4 Session Modification Request SMF –> UPF (PSA)
-> If the SMF selects a new UPF to act as intermediate UPF for the PDU Session, the SMF sends N4 Session Modification Request message to PDU Session Anchor UPF, providing DL Tunnel Info from new intermediate UPF.
7b. N4 Session Modification Response message UPF(PSA) -> SMF
-> If requested by SMF, the UPF (PSA) sends DL CN tunnel info for the old (intermediate) UPF to the SMF.
8a. [Conditional] N4 Session Modification Request SMF –> old intermediate UPF
-> If the service request is triggered by the network, and the SMF removes the old (intermediate) UPF, the SMF sends the N4 Session Modification Request message to the old (intermediate) UPF, providing the DL Tunnel Info for the buffered DL data.
8b. N4 Session Modification Response old intermediate UPF –> SMF
-> The old (intermediate) UPF sends N4 Session Modification Response message to SMF.
9. [Conditional] buffered downlink data forwarding old UPF –> new UPF
-> If the I-UPF is changed and forwarding tunnel was established to the new I-UPF, the old (intermediate) UPF forwards its buffered data to the new (intermediate) UPF acting as N3 terminating point.
10. [Conditional] buffered downlink data forwarding old UPF –> new UPF(PSA)
-> If the old I-UPF is removed and no new I-UPF is assigned for the PDU Session and forwarding tunnel was established to the UPF (PSA), the old (intermediate) UPF forwards its buffered data to the UPF (PSA) acting as N3 Terminating Point
11. [Conditional] Nsmf_PDUSession_UpdateSMContext Response SMF –> AMF
-> For a PDU Session that the SMF has determined to accept the activation of UP connection in step 5a or 5b, the SMF generates only N2 SM information and sends Nsmf_PDUSession_UpdateSMContext Response to the AMF to establish the User Plane(s).
12. N2 Request AMF->(R)AN
-> If the UE triggered the Service Request while in CM-CONNECTED state, only N2 SM information received from SMF and MM NAS Service Accept are included in the N2 Request.
13. RRC Connection reconfiguration R(AN) –> UE
-> The NG-RAN performs RRC Connection Reconfiguration with the UE depending on the QoS Information for all the QoS Flows of the PDU Sessions whose UP connections are activated and Data Radio Bearers.
14. [Conditional] N2 Request ACK (R)AN -> AM
-> The message may include N2 SM information(s), e.g. AN Tunnel Info. NG-RAN may respond N2 SM information with separate N2 message (e.g. N2 tunnel setup response) if AMF sends separate N2 message in step 11.
15. [Conditional] Nsmf_PDUSession_UpdateSMContext Request AMF->SMF
-> Nsmf_PDUSession_UpdateSMContext Request (N2 SM information, RAT Type, Access Type) per PDU Session to the SMF. The AMF determines Access Type and RAT Type
16. [Conditional] SMF Initialized SM Policy Modification SMF->PCF
-> If dynamic PCC is deployed, SMF may initiate notification about new location information to the PCF
17a. [Conditional] N4 Session Modification Request SMF-> new I-UPF
-> If the SMF selected a new UPF to act as intermediate UPF for the PDU Session in step 5b, the SMF initiates a N4 Session Modification procedure to the new I-UPF and provides AN Tunnel Info.
17b. [Conditional] N4 Session Modification Response UPF->SMF
-> N4 Session Modification Response.
18a. [Conditional] N4 Session Modification Request SMF->UPF(PSA)
-> If a User Plane is to be setup or modified and after the modification there is no I-UPF, the SMF initiates a N4 Session Modification procedure to UPF (PSA) and provides AN Tunnel Info.
18b. [Conditional] N4 Session Modification Response UPF->SMF
19. [Conditional] Nsmf_PDUSession_UpdateSMContext Response SMF->AMF
20a. [Conditional] N4 Session Modification Request SMF-> new I-UPF
-> If forwarding tunnel has been established to the new I-UPF and if the timer SMF set for forwarding tunnel at step 8a has expired, SMF sends N4 Session modification request to new (intermediate) UPF acting as N3 terminating point to release the forwarding tunnel.
20b. [Conditional] N4 Session modification response new I-UPF->SMF
21a. [Conditional] N4 Session Modification Request SMF->UPF(PSA)
-> If forwarding tunnel has been established to the UPF (PSA) and if the timer SMF set for forwarding tunnel at step 7b has expired, SMF sends N4 Session modification request to UPF (PSA) acting as N3 Terminating Point to release the forwarding tunnel.
21b. [Conditional] N4 Session Modification Response UPF(PSA)->SMF
-> UPF (PSA) acting as N3 Terminating Point sends N4 Session Modification Response to SMF.
22a. [Conditional] N4 Session Modification Request or N4 Session Release Request SMF->旧UPF
-> If the SMF decided to continue using the old UPF in step 5b, the SMF sends an N4 Session Modification Request, providing AN Tunnel Info.
22b. N4 Session Modification Response / N4 Session Release Response I-UPF->SMF
-> The old UPF acknowledges with an N4 Session Modification Response or N4 Session Release Response message to confirm the modification or release of resources.
Source: TS 38 502