3GPP 23.501 v20.2.0 — the document's own text
5.16.2.2 SMS over NAS transport
Taught in 21. Emergency calls, messages, voice and location (The 5G system architecture, in depth).
5G System supports SMS over NAS via both 3GPP access and non-3GPP access.
During Registration procedure, a UE that wants to use SMS provides an "SMS supported" indication over NAS signalling indicating the UE's capability for SMS over NAS transport. "SMS supported" indication indicates whether UE can support SMS delivery over NAS. If the core network supports SMS functionality, the AMF includes "SMS allowed" indication to the UE and whether SMS delivery over NAS is accepted by the network.
SMS is transported via NAS transport message, which can carry SMS messages as payload.
To support the transmission of MO-SMS to Emergency Response Centre, the AMF may provide local emergency numbers with associated emergency service types of the serving PLMN to the UE during UE registration procedure (see details in TS 24.501 [47]), based on configuration. The UE uses the emergency number and associated service type to formulate the MO-SMS as specified in clause 4.13.3.3 of TS 23.502 [3].
NOTE: The transmission of MO-SMS to Emergency Response Centre is handled without priority treatment.