AMF — Access and Mobility Management Function¶

Learning objectives¶

After reading this page, you will be able to:

• Explain in one sentence the role the AMF plays in the 5GC (registration, connection, and mobility management).
• Diagram which interfaces (N1/N2 and others) the AMF terminates and which NFs it works with.
• Explain how the AMF behaves during the Registration procedure.
• Explain the difference from the 4G (EPC) MME (session management is split off to the SMF).

Prerequisites¶

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

• Basics of SBA (Service Based Architecture), NFs, and SBI → Curriculum
• The concept of interfaces → Interface dictionary
• The positioning of NAS and NGAP → Protocol dictionary

What you will learn in this chapter¶

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

Why — why it is needed¶

When you turn on a smartphone, it first declares to the network, "I am here. Let me use you." Something is needed that accepts this declaration, arranges the identity check (authentication), and continuously keeps track of "which area this device is currently in, and whether it is connected or idle" — a reception and guidance desk. That is the AMF.

Without the AMF, the network cannot manage "who" is "where" and "in what state." It would not know where to deliver an incoming call (paging), and it could not follow a device as it moves. In other words, the AMF is what makes the very existence of a device on the network possible.

Analogy: The AMF is the main reception desk of a large office building. It handles entry procedures (registration) for visitors (UEs), issues employee badges (5G-GUTI), delegates identity verification to the security room (AUSF/UDM), and hands off guidance to the actual workrooms (SMF/UPF) to other departments. If reception stops, no one can enter the building.

Overview¶

The AMF (Access and Mobility Management Function) is the entry point for control signaling between the UE and the network. It terminates N2(NGAP) from the base station (gNB) on the radio side and N1(NAS) from the UE, and handles registration management, connection management (CM), reachability management, and mobility management. It delegates authentication itself to the AUSF and session management to the SMF, functioning as a command tower that coordinates and bridges them.

Basic Concept — explanation for beginners¶

Think of the AMF as a telephone-exchange operator. When you (the UE) call the telephone office saying "I want to subscribe," the operator (AMF) responds. The operator confirms your identity with a specialized department (AUSF/UDM), and once confirmed, creates a record (the UE context) saying "this is your number." Afterward, even as you move, the operator tracks which regional office you are in so that incoming calls can be connected. However, actually laying the call circuit is a separate role (SMF/UPF); the operator only hands things off — a division of labor.

Network Function details¶

About the SEAF (Security Anchor Function): The SEAF is the function that serves as the security anchor, and it is co-located with the AMF. The SEAF is the entity that receives the key KSEAF derived by the AUSF upon successful authentication, and from it the SEAF derives KAMF and subsequent keys. In other words, by embedding the SEAF, the AMF serves as the origin of the 5G security key hierarchy (basis: TS 33.501 §6).

Architecture¶

The AMF sits at the center of the 5GC control plane and is the hub connecting the UE and gNB with each control NF.

flowchart LR
  UE(("UE"))
  gNB["gNB (base station)"]
  AMF["AMF"]
  AUSF["AUSF"]
  UDM["UDM"]
  SMF["SMF"]
  PCF["PCF"]

  UE -- "N1 (NAS)" --> AMF
  gNB -- "N2 (NGAP)" --> AMF
  UE -.radio.- gNB
  AMF -- "N12 (Nausf)" --> AUSF
  AMF -- "N8 (Nudm)" --> UDM
  AMF -- "N11 (Nsmf)" --> SMF
  AMF -- "N15 (Npcf)" --> PCF

How to read the diagram: Both the NAS from the UE and the NGAP from the gNB are terminated at the AMF. From there the AMF invokes the services of each NF on the right (Nausf/Nudm/Nsmf/Npcf) to proceed with authentication, data retrieval, session management, and policy control.

Interface¶

The main interfaces terminated by the AMF and their roles.

See N1 / N2 for details.

Appearance in procedures¶

In Registration (initial registration), the AMF becomes both the starting point and command tower of the entire procedure. It receives the Registration Request (NAS) from the UE via the gNB, requests authentication from the AUSF (N12), retrieves subscriber information from the UDM and registers the UE context (N8), queries the PCF for policy if needed (N15), and finally returns the Registration Accept to the UE.

See Registration for the detailed flow.

Comparison with EPC¶

• In 4G: The MME (Mobility Management Entity) handled authentication, registration, and mobility management, plus session management (bearer control).
• In 5G: Of the MME's roles, access/mobility management is taken over by the AMF, while session management is split off to the SMF. This allows "connectivity management" and "session management" to scale independently. The authentication functions have also been reorganized into the AUSF/UDM.

Release differences¶

• Rel-15: Defines the AMF's basic functions (registration, connection, and mobility management, N1/N2 termination).
• Rel-16: Functional extensions (e.g., strengthened eNPN/NPN support, etc.). Specific section numbers are to be confirmed.
• Rel-17: Further extensions. Specific items and section numbers are to be confirmed.

3GPP Specification¶

• 3GPP TS 23.501 §6.2.1 — AMF functional definition (role of the Network Function)
• 3GPP TS 29.518 — Namf service (Communication / EventExposure / MT / Location). Individual section numbers are to be confirmed
• 3GPP TS 33.501 — 5G security/authentication procedures (the AMF embeds the SEAF and handles KSEAF). Individual section numbers are to be confirmed
• 3GPP TS 23.502 — AMF behavior in procedures (Registration, etc.). Individual section numbers are to be confirmed

Summary¶

• The AMF is the control-plane command tower that terminates N1(NAS) and N2(NGAP).
• Its roles are registration management, connection management (CM), reachability management, and mobility management, plus termination of NAS security.
• It delegates authentication to the AUSF and session management to the SMF, dedicating itself to the bridging role.
• The core of its retained information is the UE context (5G-GUTI, 5GMM state, security context, allowed NSSAI).
• It corresponds to the EPC MME, but session management has been split off to the SMF.

Next Step¶

• AUSF — the party the AMF requests authentication from
• UDM — the party the AMF retrieves subscriber data from
• Registration — the procedure in which the AMF plays the lead role
• NF dictionary / Interface dictionary — for checking terminology