RM state — Registered or not — what the core knows about where you are
The device — identity, state, mobility
Where it sits
What it is
What it is. Registration Management state — whether the core knows where you are. TS 23.501 §5.3.2.2. Two states, held by both the device and the AMF:
| State | Means |
|---|---|
| RM-DEREGISTERED | "the UE is not registered with the network. The UE context in AMF holds no valid location or routing information for the UE so the UE is not reachable by the AMF" |
| RM-REGISTERED | a context exists; the AMF can reach the device |
The transitions are simple and the specification states them as rules. In RM-DEREGISTERED the device shall "attempt to register with the selected PLMN using the Initial Registration procedure if it needs to receive service that requires registration", shall remain deregistered on a Registration Reject, and shall "enter RM-REGISTERED state upon receiving a Registration Accept".
Deregistered does not mean nothing is remembered. §5.3.2.2.2 adds: "However, some parts of UE context may still be stored in the UE and the AMF e.g. to avoid running an authentication procedure during every Registration procedure."
That single sentence is why turning a phone off and on again does not require a full 5G AKA run. The security context survives deregistration, so the next registration can be integrity-protected from its first message and reuse the existing keys. Reachability is forgotten; trust is not.
RM against CM — the distinction that matters. They are easy to confuse and they answer different questions:
RM asks: does the network know where you are? It changes on registration and deregistration — rarely, and on a scale of hours or days.
CM asks: is there a signalling connection right now? It changes every time the device sends or receives anything — constantly, on a scale of seconds.
A phone in your pocket is normally RM-REGISTERED and CM-IDLE: the network knows which tracking area it is in and could page it, but there is no connection, no N2 and no N3. That combination is the normal state of almost every device almost all of the time, and the entire idle-mode design exists to make it cheap.
Periodic registration is the liveness check. A registered device must re-register periodically "in order to remain reachable" (§5.3.2.1) — otherwise the AMF would hold contexts for devices that had left, been switched off, or gone flat.
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: