5GMM — The device's mobility-management state machine
The device — identity, state, mobility
Where it sits
What it is
What it is. The 5GS Mobility Management sublayer state machine in the device — the one the AMF reads. TS 24.501 §5.1.3.2. Six main states, and the specification's own one-line definitions are the clearest available:
| State | TS 24.501's words |
|---|---|
| 5GMM-NULL | "5GS services are disabled in the UE. No 5GS mobility management function shall be performed" |
| 5GMM-DEREGISTERED | "no 5GMM context has been established and the UE location is unknown to the network and hence it is unreachable" |
| 5GMM-REGISTERED-INITIATED | registration started, "waiting for a response from the network" |
| 5GMM-REGISTERED | "a 5GMM context has been established. Additionally, one or more PDU session(s) may be established" |
| 5GMM-DEREGISTERED-INITIATED | deregistration requested, waiting for the answer |
| 5GMM-SERVICE-REQUEST-INITIATED | service request sent, waiting for the answer |
Three of the six exist only because a message can be in flight. REGISTERED-INITIATED, DEREGISTERED-INITIATED and SERVICE-REQUEST-INITIATED are all "waiting for a response" states. That is not padding: over a radio link a request may be lost, answered late, or answered with a rejection, and the device must know what it is entitled to do meanwhile. Half of the state machine is about uncertainty, not about status.
The substates are where the reality is. 5GMM-DEREGISTERED alone has nine, and reading their names is a tour of everything that can be wrong:
NORMAL-SERVICE · LIMITED-SERVICE · ATTEMPTING-REGISTRATION · PLMN-SEARCH · NO-SUPI · NO-CELL-AVAILABLE · eCALL-INACTIVE · INITIAL-REGISTRATION-NEEDED
"No service" on a phone is not one condition. NO-CELL-AVAILABLE means there is no radio;
PLMN-SEARCH means there is radio and the device is looking for a network it may use
(PLMN selection); NO-SUPI means there is no usable
subscription; LIMITED-SERVICE means a cell was found but it will only carry
emergency calls. The device distinguishes all of these and the user interface collapses them into one
symbol.
5GMM is not the whole story. It is one of two state machines: this one tracks the device's view, RM state tracks the registration status shared with the AMF, and CM state tracks whether a signalling connection exists right now.
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: