UDM — Unified Data Management¶

Difficulty: Intermediate ／ Estimated study time: 15 min ／ Related NFs: AMF, SMF, AUSF, UDR ／ Related interfaces: N8, N10, N13, Nudr

Learning objectives¶

After reading this page, you will be able to:

Explain in one sentence the role the UDM plays in the 5GC (the front end of subscriber data management).
Explain the division of labor between the UDM and UDR (front end vs. data storage).
Diagram how the UDM appears within the Registration procedure.
Explain the correspondence with the 4G (EPC) HSS (data storage split off to the UDR, authentication to the AUSF).

Prerequisites¶

It will go more smoothly if you understand the following first:

Basics of SBA (Service Based Architecture) and NFs → Curriculum
The concept of subscriber identifiers such as SUPI → Message dictionary
The roles of the AMF/AUSF (the sides that use the UDM) → AMF / AUSF

What you will learn in this chapter¶

We enter from Why (what goes wrong without the UDM), then organize What (its role, retained information, and APIs) with tables and Mermaid diagrams, and move to How (its appearance in Registration, comparison with EPC, and reference specs).

Why — why it is needed¶

Unless the network knows the subscriber information — "who this subscriber is, on what plan, and which services they can use" — it can perform neither authentication nor service provisioning. Something is needed that coordinates access to this subscriber information and provides it to each NF — a service window. That is the UDM.

Without the UDM, the AUSF (authentication) cannot retrieve authentication information, and the AMF/SMF cannot reference the subscriber's profile. In other words, neither registration nor session establishment can succeed. The UDM is the front end (the service window) for subscriber data, and the actual data itself is stored in the UDR (the data warehouse), which the UDM retrieves and provides.

Analogy: The UDM is the company's HR service window. When each department (AMF/SMF/AUSF) asks "I want this employee's information," the HR window retrieves the relevant data from the roster storeroom (UDR) and hands it over. Separating the window from the storeroom lets you both add more windows and manage the storeroom centrally.

Overview¶

The UDM (Unified Data Management) is the front end for subscriber data management. It handles authentication information generation (handled by the ARPF; described later), subscriber information provisioning (SDM), and UE context registration management (UECM). The substance of the persistent data is stored in the UDR (Unified Data Repository), which the UDM logically manages and provides. Many NFs — the AUSF, AMF, SMF, and others — use the UDM.

Note that the ARPF (Authentication credential Repository and Processing Function) logically belongs to the UDM and is the function that holds and generates authentication information such as the subscriber's permanent key (depending on the implementation, it may be placed near the UDR = implementation-dependent). This corresponds to the AuC (Authentication Centre) within the 4G HSS.

Basic Concept — explanation for beginners¶

The UDM can be likened to a library counter (librarian). When a user (AUSF/AMF/SMF) requests "I want this material," the librarian (UDM) locates the book (subscriber data) in the stacks (UDR) and hands it over. The librarian does not pile the books up on their own desk but fetches them from the stacks when needed. This cleanly separates the "search-and-provide role (UDM)" from the "storage role (UDR)."

Network Function details¶

Item	Content
Role	Front end of subscriber data management. Authentication information generation (handled by the ARPF, which logically belongs to the UDM), subscriber information provisioning (SDM), and UE context registration management (UECM). Actual data is stored in the UDR
Retained information	Logically manages subscriber data (persistent data is stored in the UDR). Subscription and authentication data tied to the SUPI
Main APIs used (calls to other NFs)	Nudr (DataRepository) — retrieves data from the UDR
APIs provided (exposed by itself)	Nudm (UECM, SDM, UEAU, EE, etc.) — 3GPP TS 29.503
Related interfaces	N8 (AMF), N10 (SMF), N13 (AUSF), toward the UDR (Nudr)
Impact on failure	Authentication information and subscriber information cannot be retrieved, making registration and sessions impossible

Architecture¶

The UDM is used by multiple NFs as the service window for subscriber data, and retrieves data from the UDR behind it.

flowchart LR
  AMF["AMF"]
  AUSF["AUSF"]
  SMF["SMF"]
  UDM["UDM"]
  UDR[("UDR (data storage)")]

  AMF -- "N8 (Nudm)" --> UDM
  AUSF -- "N13 (Nudm)" --> UDM
  SMF -- "N10 (Nudm)" --> UDM
  UDM -- "Nudr" --> UDR

How to read the diagram: The AMF, AUSF, and SMF each use the UDM's service (Nudm) over N8/N13/N10. The UDM itself holds no persistent data and retrieves the actual data from the UDR over Nudr to provide to each NF. The UDM is the "window" and the UDR is the "warehouse" — a division of labor.

Interface¶

The main interfaces terminated by the UDM and their roles.

Interface	Endpoints	Role
N8	AMF ⇔ UDM	Subscriber data retrieval and UE context registration (UECM/SDM) (Nudm)
N10	SMF ⇔ UDM	Subscriber data for session management and UECM (Nudm)
N13	AUSF ⇔ UDM	Authentication vector generation/retrieval (UEAU) (Nudm)
Nudr	UDM ⇔ UDR	Read/write of persistent subscriber data

Appearance in procedures¶

In Registration (initial registration), the UDM appears in two stages. First, in the authentication stage, the AUSF retrieves an authentication vector from the UDM (N13). Next, the AMF retrieves subscriber information (SDM) from the UDM (N8) and performs UE context registration (UECM). The UDM responds to these requests by retrieving the actual data from the UDR (Nudr).

See Registration for the detailed flow.

Comparison with EPC¶

In 4G: The HSS (Home Subscriber Server) handled subscriber data storage, authentication vector generation, and subscriber profile provisioning as one unit.
In 5G: The HSS's roles were split into the UDM (front end) + UDR (data storage) + AUSF (authentication server). By separating data storage, provisioning, and authentication, it achieves scalability and clarity of responsibilities.

4G (EPC)	5G (5GC)
HSS (subscriber data provisioning portion)	UDM
HSS (data storage portion)	UDR
AuC (Authentication Centre / authentication vector generation) within the HSS	ARPF (logically belongs to the UDM) + AUSF (server side)

Release differences¶

Rel-15: Defines the UDM's basic functions (SDM, UECM, UEAU, EE, and the structure separated from the UDR).
Rel-16: Presence of functional extensions and section numbers are to be confirmed.
Rel-17: Presence of functional extensions and section numbers are to be confirmed.

3GPP Specification¶

3GPP TS 23.501 §6.2.7 — UDM functional definition (role of the Network Function)
3GPP TS 29.503 — Nudm service (UECM / SDM / UEAU / EE). Individual section numbers are to be confirmed
3GPP TS 33.501 — provisions related to authentication information generation (ARPF, etc.). Individual section numbers are to be confirmed

Summary¶

The UDM is the front-end (service window) NF for subscriber data management.
It provides authentication information generation (ARPF), subscriber information (SDM), and UE context registration (UECM).
The substance of persistent data is stored in the UDR, which the UDM retrieves and provides over Nudr.
Its terminating interfaces are N8 (AMF), N10 (SMF), N13 (AUSF), and Nudr (UDR).
It corresponds to the 4G HSS, structured with data storage split off to the UDR and authentication to the AUSF.

Next Step¶

AUSF — the party that retrieves authentication vectors from the UDM
AMF — the party that retrieves subscriber information from the UDM
Registration — the procedure in which the UDM appears for authentication and subscriber information retrieval
NF dictionary / Interface dictionary — for checking terminology