Registration — How a device becomes known to the core
The device — identity, state, mobility
Where it sits
What it is
What it is. The procedure by which a device becomes known to the core: authenticated, authorised, located, and given a temporary identity. TS 24.501 §5.5.1 for the protocol, TS 23.502 §4.2.2.2 for the system procedure. Nothing else can happen until it has succeeded — no PDU session, no data, no reachability.
The chain, from radio silence to registered:
- Cell selection — find a suitable cell of a PLMN the device may use, and camp on it.
- Initial access — SSB, PBCH, system information, PRACH, and an RRC connection.
- Registration Request — a NAS message carrying a SUCI or a 5G-GUTI, the Requested NSSAI, and the device's capabilities.
- AMF selection — the RAN picks an AMF; the NSSF may decide a different one must serve the requested slices, and the registration is re-routed.
- Authentication — 5G AKA via the AUSF and UDM, which de-conceals the SUCI.
- Subscription retrieval — the AMF fetches subscription data from the UDM and registers itself there as the serving AMF.
- Registration Accept — carrying the Allowed NSSAI, a TAI list, and normally a fresh 5G-GUTI.
Step 6 is the one that is easy to skip past and hard to do without. The UDM is not only asked who is this subscriber — it is told which AMF is now serving them (TS 23.501 §6.2.7: "UE's Serving NF Registration Management"). That reverse index is how anything arriving from outside finds the device later. Registration is as much about the network learning where to find you as about you being admitted.
Three reasons to do it again, per TS 23.501 §5.3.2.1: periodically, "in order to remain reachable"; upon mobility, when the device leaves its TAI list; and "to update its capabilities or re-negotiate protocol parameters".
Periodic registration is a liveness check with a real cost. Every registered device must check in on a timer, whether or not it has anything to say, or the AMF cannot distinguish a device that left from one that is quietly camped. Set the timer short and a network of millions of idle devices generates continuous signalling for nothing; set it long and the AMF holds stale contexts. It is the same trade as the TAI list size, on the time axis rather than the space one.
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: