5G Core — The core network behind the RAN, and the one decision that reshaped it
The core network
Where it sits
What it is
What it is. The network behind the radio: everything that authenticates a subscription, decides what a device is allowed to do, keeps track of where it is, and moves its packets to and from the internet. TS 23.501 is its architecture specification and TS 23.502 its procedures. The RAN notes in this folder stop at the base station; the 5GC is what the base station is connected to.
The one decision worth knowing first. Every generation up to and including LTE described the core as a set of nodes joined by named interfaces — an MME here, an S-GW there, an S11 between them, each interface with its own protocol and its own message set. 5G kept that drawing (see reference points) but made it the secondary one. The primary description in TS 23.501 §4.2 is a set of network functions that offer services to each other over a common interface — the service-based architecture.
Why that is more than a redrawing. In the node model, "the MME asks the HSS for subscription data" is a protocol: a specific message on a specific interface, and adding a new consumer of subscription data means specifying a new interface. In the service model it is an HTTP request to a named service, and a new consumer needs no new specification at all — it discovers the producer through the NRF and calls the same service everything else calls.
That is the property everything else in 5G leans on. Network slicing is only affordable because standing up another logical core means standing up more instances of functions that already find each other by discovery, not cabling another set of interfaces.
What the core does not do. It does not schedule. It does not know what a resource block is. The whole of this folder's layer 1 — numerology, coding, MIMO — is invisible to the core, which sees only a device it can reach and packets marked with a QFI. The one place the two halves genuinely touch is the QoS profile the SMF hands to the RAN, and the ten bits of 5G-TMSI that decide when a device is paged.
Where the boundary physically is. Two reference points and no more: N2 carries signalling between the base station and the AMF, and N3 carries user packets between the base station and the UPF. Everything in the RAN half of these notes happens below those two lines.
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:
5G Core needs nothing first — it is one of the places to start reading.