UDM — Unified Data Management — where the subscription lives
The core network
Where it sits
What it is
What it is. Unified Data Management — where a subscription lives. TS 23.501 §6.2.7. It is the authority on who a subscriber is, what they are allowed, and the credentials that prove it.
The functionality list, abridged:
Generation of 3GPP AKA authentication credentials · User identification handling — storage and management of the SUPI for each subscriber · Support of de-concealment of the privacy-protected subscription identifier (SUCI) · Access authorization based on subscription data, e.g. roaming restrictions · UE's serving NF registration management — storing the serving AMF, and the serving SMF per PDU session · support for service and session continuity · subscription management · SMS management.
It is the only function that can undo a SUCI. The de-concealment line is the SIDF — Subscription Identifier De-concealing Function, TS 33.501 §6.12.5 — and it is a UDM function because the private key that reverses the concealment must never leave the home network. That single placement decision is what makes SUCI worth anything: a visited network relays a concealed identifier it cannot itself read.
"UE's serving NF registration management" is the reverse index. The UDM does not only answer "who is this subscriber?"; it also records which AMF is currently serving them. That is how the network finds a device when something arrives from outside — and why the UDM is on the path of almost every procedure even though it is nowhere near the data.
UDM and UDR. TS 23.501 §6.2.7 is explicit that the UDM may keep its data in a UDR, in which case "a UDM implements the application logic and does not require an internal user data storage" — and several different UDMs may then serve the same subscriber in different transactions. Logic and storage are deliberately separable, which is the same instinct as the CP/UP split applied to subscriber data: make the part that holds state few, and the part that runs logic many.
It is always in the home network. NOTE 2 of the clause: the UDM is located in the HPLMN of the subscribers it serves. Roaming works by the visited network asking the home network, never by copying the subscription.
Read on
This concept is read as part of one argument in ref-core, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: