7.2. Reference Point Representation
The figure above shows the same NFs as the service-based representation and connects them with the reference points that TS 23.501 clause 4.2.7 defines between them. TS 23.501 figure 4.2.3-2 leaves out the NRF, the NEF, the UDR, the UDSF, and the NWDAF for clarity (NOTE 2, NOTE 4, and NOTE 5 of TS 23.501 clause 4.2.3) and does not show the CHF and the 5G-EIR, and TS 23.501 figure 4.2.3-5 shows the NEF separately; this figure combines these NFs into one view.
The figure is arranged around the AMF and the SMF, which interact with more NFs than any other NF. The centre column holds the NFs that both the AMF and the SMF use, namely the UDM, the NEF, the PCF, the CHF, and the NSACF. The AMF reaches each of these NFs over a reference point that rises from the top of the AMF and turns right at the height of the target NF, and the SMF reaches the same NFs over the mirrored reference points on the right, so that each row of the centre column reads as one pair of reference points, for example N8 and N10 for the UDM. The NFs that the AMF uses and the SMF does not use sit to the left: the AUSF, the NSSAAF, and the 5G-EIR to the left of the AMF, and the NSSF, together with the NWDAF that serves the NSSF and the PCF, between the AMF and the centre column. The NFs that only connect to the centre column, namely the UDR and the AF, sit to its right. The bottom row holds the UE and the user-plane path from the NG-RAN through the UPF to the DN.
The NRF, the SCP, and the UDSF are grouped in a frame because every NF can interact with them. The NRF and the SCP have no reference point to a specific NF in a non-roaming Public Land Mobile Network (PLMN), apart from N116 between the SCP and the NWDAF, and N18 connects any NF to the UDSF, so the figure does not draw these reference points.
Next to each reference point, the figure gives the 3GPP technical specification (TS) that defines the protocol on that reference point and the name of the protocol. For a reference point between two control-plane NFs, the TS is the stage 3 specification of the SBI that realizes the reference point, and the protocol is HTTP/2. Where a reference point crosses another reference point without a connection, the figure draws a small arc.