AMF — Access and Mobility Management Function — the device's one contact in the core

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The core network

Where it sits

Sits atLevel 2 of the hierarchy · The core network · explained
Learn firstNetwork function · Reference point — 3 concepts in the full chain, see the paths
Unlocks5G-GUTI · NAS
Primary clauseTS 23.501 §6.2.1
Used inref-core 19 · ref-ue-state 16 · ref-ue-identity 9 · ref-session 6 · ref-ue-mobility 5 · index 3
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. The Access and Mobility Management Function — the device's single point of contact in the core. TS 23.501 §6.2.1. If a device signals to the core at all, it signals to an AMF; there is exactly one serving it per access at any moment, and the 5G-GUTI it holds names that AMF.

The functionality list, abridged to the load-bearing lines:

Termination of RAN CP interface (N2) · Termination of NAS (N1), NAS ciphering and integrity protection · Registration management · Connection management · Reachability management · Mobility management · Access authentication and authorisation · Provide transport for SM messages between UE and SMF · Transparent proxy for routing SM messages · Security Anchor Functionality (SEAF, per TS 33.501) · Provide transport for SMS messages · EPS Bearer ID allocation for interworking · Support for Network Slice-Specific Authentication and Authorization.

The two lines to read twice. "Provide transport for SM messages between UE and SMF" and "Transparent proxy for routing SM messages". Session-management signalling (5GSM) is generated by the device and consumed by the SMF, and it travels through the AMF, which forwards it without interpreting it.

That is a deliberate architectural refusal. The AMF is the only function talking to the device, so it would have been easy — and it was the LTE MME's habit — to let it understand everything. Keeping it transparent to 5GSM is what allows the session half of the core to evolve without touching the mobility half, and it is why TS 24.501 defines the two protocols with separate state machines rather than one.

It is also the security anchor. "Security Anchor Functionality (SEAF)" means the AMF holds the anchor key from which the access-stratum keys are derived — so authentication terminates at the AUSF but the key hierarchy is rooted at the AMF, which is where it has to be, since that is where the device's signalling connection lives.

What it does not do. It does not touch user packets — nothing on the AMF's list mentions forwarding — and it does not decide QoS. Both belong to the SMF and the UPF.

Read on

This concept is read as part of one argument in ref-core, alongside the rest of its group.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand AMF (level 2) you first need 3 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 5G Core
1 Network function · Reference point
Immediately before AMF: Network function, Reference point.
Keep going — where this sits on the route
◀PAPRstep 12 · level 2
Step 13 of 118level 2 · The core network
SMFstep 14 · level 2▶
Level 2 → 3 · this unlocks
5G-GUTI level 4 · NAS level 8
↳ all 9 concepts at level 3 · level 2 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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