5GSM — Session management, carried inside NAS but not read by the AMF
The device — identity, state, mobility
Where it sits
What it is
What it is. The 5GS Session Management sublayer — the NAS protocol that establishes, modifies and releases PDU sessions. TS 24.501 §6.1.3. It travels between the device and the SMF, through the AMF, and the AMF does not read it.
The transparency, from both sides of the specification:
TS 23.501 §6.2.1 gives the AMF: "Provide transport for SM messages between UE and SMF" and "Transparent proxy for routing SM messages".
TS 23.501 §6.2.2 gives the SMF: "Termination of SM parts of NAS messages".
One protocol, carried by a function that is forbidden to interpret it, terminating at a function the device never addresses directly.
Why it is worth a separate state machine. 5GMM is about the device: is it registered, is it reachable, where is it. 5GSM is about a session: does it exist, what is its address, which flows does it carry. A device has one 5GMM state and one 5GSM state machine per PDU session — so the two cannot be one thing, because they do not have the same cardinality.
And separating them lets the two halves of the core evolve independently. Because the AMF is a transparent proxy for 5GSM, a new session-management capability requires changes at the device and the SMF and nothing at the AMF. Had the LTE habit been kept — one node understanding everything — every session feature would have needed the mobility function updated too.
The cost is a routing problem the specification has to solve. If the AMF cannot read the message, how does it know which SMF to send it to? By the PDU Session ID carried in the 5GMM envelope around the 5GSM payload, and the binding the AMF holds from when the session was established. The outer layer carries just enough routing information for the inner layer to stay opaque — the same trick as a GTP-U header.
Where a session actually comes from. The device sends a PDU Session Establishment Request as a 5GSM message inside a 5GMM envelope; the AMF selects an SMF (querying the NRF with the S-NSSAI and DNN) and forwards it; the SMF does the work. See PDU session.
Read on
This concept is read as part of one argument in ref-ue-state, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: