CM state — Whether a NAS signalling connection exists at this moment
The device — identity, state, mobility
Where it sits
What it is
What it is. Connection Management state — whether a NAS signalling connection exists between the device and the AMF at this moment. TS 23.501 §5.3.3.2. Two states:
| State | Means |
|---|---|
| CM-IDLE | "no NAS signalling connection established with the AMF over N1" — and "no AN signalling connection, N2 connection and N3 connections" |
| CM-CONNECTED | a NAS signalling connection exists |
What a device in CM-IDLE does, per §5.3.3.2.2: it "performs cell selection/cell reselection according to TS 38.304 and PLMN selection according to TS 23.122" — that is, cell selection, reselection and PLMN selection are what idle mode is. And if it is also RM-REGISTERED it shall "respond to paging by performing a Service Request procedure" and "perform a Service Request procedure when the UE has uplink signalling or user data to be sent".
How the transition happens. "The transmission of an Initial NAS message (Registration Request, Service Request or Deregistration Request) initiates the transition from CM-IDLE to CM-CONNECTED state", and the device "shall enter CM-CONNECTED state whenever an AN signalling connection is established" — over 3GPP access, that means entering RRC_CONNECTED.
CM lines up with the RRC states below it — with one deliberate mismatch.
| CM state | RRC state |
|---|---|
| CM-IDLE | RRC_IDLE |
| CM-CONNECTED | RRC_CONNECTED |
| CM-CONNECTED | RRC_INACTIVE |
RRC_INACTIVE is the interesting row, and §5.3.3.2.5 gives it a clause of its own. The device has no active radio connection — it sleeps, reselects cells and behaves in almost every respect like an idle device — while the N2 and N3 connections stay up and the core still considers it connected.
Why that state was invented. Going from idle to connected costs a full random-access exchange, an RRC setup, a service request and a security handshake — tens of milliseconds and a burst of signalling. For traffic that is frequent but tiny — a messaging keepalive, a sensor reading — that cost dominates everything. RRC_INACTIVE lets the device sleep like an idle one and resume rather than re-establish, keeping the core-side connections in place the whole time. It is the state that makes bursty low-rate traffic affordable.
The 5G-S-TMSI is what makes the transition cheap. §5.3.3.2.2: the device "provides 5G-S-TMSI as part of AN parameters during AN signalling connection establishment" — so the base station knows which AMF holds the context from the very first message.
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: