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The Service Request procedure is used by a UE in CM-IDLE state or the 5G Core (5GC) to request the establishment of a secure connection to an AMF.
The Service Request procedure is also used both when the UE is in CM-IDLE and in CMCONNECTED to activate a User Plane connection for an established PDU Session.
(CM-IDLE and CM-Connected are two Connection Management modes of UE. They are described in a separate blog.)
There are two variants of the Service Request procedure:
1. UE-triggered Service Request
As the name indicates, this type of Service Request is initiated by the UE. The UE in CM-IDLE state could initiate this type of Service Request procedure to send uplink (UL) signalling messages, UL user data, to request emergency services fallback, or as a response to a network paging request (4.2.3, TS 23.502).
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.
For UE in CM-Connected mode, the Service Request procedure is used to request activation of a U-Plane connection for PDU Sessions and to respond to a NAS Notification message from the AMF. When a U-Plane connection for a PDU Session is activated, the AS (Access Stratum) layer in the UE indicates it to the NAS layer.
(See the Fig. 4.2.3.2-1 of TS 23.502 for further details)
2. Network-triggered Service Request.
This type of Service Request procedure is initiated by the network to send control signals to UE (e.g. N1 signalling to UE, Mobile-terminated SMS, U-Plane connection activation for PDU Session(s) to deliver mobile terminating data). In case some data has arrived to a UPF, this procedure is activated to send Paging and/or establish PDU session for Downlink data transmission. This procedure is also useful when the UE is in CM-IDLE state and the 5GC wants to send a NAS (N1) message to the UE. (Read 4.2.3.3 of TS 23.502, and read the accompanying figure to the right.)
Note 4a: 4a is activated when UE is in CM-Connected state (i.e. N1, N2, and N3 established). The UE would initiate the PDU Session Establishment or PDU Session Modification procedure. Once PDU Session established, Downlink Data transfer occurs.
Note 4b: Paging from AMF-gNB-UE is done when UE is in CM-IDLE state (i.e. no c-plane signaling activated between UE and AMF+UPF). After which the UE would do the (6) and initiate the PDU Session Establishment procedure. Once PDU Session established, Downlink Data transfer occurs.
Reference
Reference
Read 4.2.3 of TS 23.502
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