SMS stands for Short Message Service and is used both for end-user messages and between the network and the device without any human interaction.
There are two options for SMS when the mobile is attached to a 5G network without the need to fallback to a 4G network:
- • SMS over IP.
- • SMS over NAS (Non-Access Stratum).
SMS Over IP
SMS Over IP needs the IMS (IP Multimedia Subsystem) system to be connected to the 5GC (5G Core) as exactly in the case of VoLTE when the IMS system is connected to EPC (Evolved Packet Core). SMS over IP relies on the fact that SMS to and from the device are encapsulated inside SIP (Session Initiation Protocol) messages (IMS control signaling) carried transparently through the access network and the 5GC UPF function.
The SMS is then forwarded between CSCF and the SMS system via a gateway function that 3GPP calls IP-SM-GW or IP_ShortMessage_Gateway.
SMS over NAS
SMS over NAS which doesn’t rely on the IMS is appropriate mainly for devices (not smartphones) like sensors, or any industry devices but can also work with smartphones.
Like SMS over IP, also SMS relies on encapsulating SMS within control signaling for efficiency reasons. Here the NSA control messages between AMF (Access and Mobility Management Function) and the device are used to carry the SMS. Then, the AMF terminates the NAS signaling to and from the device and forwards the decapsulated SMS to and from the SMSF or SMS function in the 5GC.
Note: SMSF is an optional network function that is only deployed for enabling SMS over NAS.