Reference point — The N-numbers, and what each one joins
The core network
Where it sits
What it is
What it is. The other drawing of the same architecture: functions joined by named
point-to-point interfaces, each an N and a number. TS 23.501 §4.2.3 lists them. This is the
representation LTE used exclusively; 5G keeps it as the secondary view alongside the
service-based one.
The ones this folder needs, quoted from the specification:
| Point | Between | Carries |
|---|---|---|
| N1 | the device and the AMF | NAS — and the RAN only relays it |
| N2 | the (R)AN and the AMF | RAN control signalling — the RAN/core boundary, control side |
| N3 | the (R)AN and the UPF | user packets, in a GTP-U tunnel — the boundary, user side |
| N4 | the SMF and the UPF | the CP/UP split itself |
| N6 | the UPF and a data network | the exit to the internet, or to whatever the DNN named |
| N9 | two UPFs | one UPF to another, when the path needs more than one |
Control-plane pairs continue: N7 SMF–PCF, N8 UDM–AMF, N10 UDM–SMF, N11 AMF–SMF, N12 AMF–AUSF, N13 UDM–AUSF, N14 AMF–AMF, N15 PCF–AMF, N22 AMF–NSSF.
Why both drawings survive, and which one to use when. The reference-point view is the right one whenever the question is "what physically crosses this line?" — and there are exactly two lines that matter to the RAN half of these notes, N2 and N3. It is also the right view for the interfaces that are not HTTP: N3 is GTP-U, N4 is PFCP.
The service view is the right one whenever the question is "who can call whom?" — because under the SBA the honest answer is anyone who discovers them, which a fixed set of lines cannot express.
N1 is the interesting one. It is drawn as an interface between the device and the AMF, but no such link exists physically: NAS travels inside RRC over the radio, then inside N2 to the AMF. N1 is a logical reference point, and drawing it that way is what makes it obvious that the RAN is required to carry NAS without reading it.
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: