Network function — A named part of the core, defined by the services it offers
The core network
Where it sits
What it is
What it is. The unit the 5G core is built from: a named function that offers one or more services to other functions, listed with its functionality in TS 23.501 §6.2. The specification defines more than thirty of them; six carry almost all of the traffic of an ordinary connection, and the rest are specialised.
The six that matter for a first pass:
| Function | Clause | The one sentence |
|---|---|---|
| AMF | §6.2.1 | The device's single contact point — registration, reachability, mobility |
| SMF | §6.2.2 | Owns a PDU session: addresses, UPF selection, steering |
| UPF | §6.2.3 | Forwards the packets; the only function on the data path |
| PCF | §6.2.4 | The operator's rules, as policy |
| UDM | §6.2.7 | The subscription, the credentials, and SUCI de-concealment |
| AUSF | §6.2.8 | Authentication |
Add NRF (§6.2.6) for discovery, NEF (§6.2.5) for exposure outward and NSSF (§6.2.14) for slice selection and you have the set this folder uses.
A network function is not a server. TS 23.501 §6.2.1 is careful about this: "Some or all of the AMF functionalities may be supported in a single instance of an AMF." The specification describes a function, and says nothing about how many processes implement it, on how many machines, or in how many places. An operator may run one AMF instance or a hundred; may split the function across a region; may run instances that support only part of the list.
This is the difference from a node. An MME was a box you bought. An AMF is a set of behaviours something must exhibit, addressed by the services it registers.
The functionality lists are worth reading literally. They are terse and they are exhaustive, and each line is a claim about a seam. The AMF list contains "Provide transport for SM messages between UE and SMF" and "Transparent proxy for routing SM messages" — two lines that between them say the AMF carries session-management signalling without reading it, which is the whole reason 5GSM is a separate protocol from 5GMM.
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: