Appendix of 3GPP Reference Points
The list contains reference points and service‐based interfaces applicable to the 5G System.
| Cx | Reference point between CSCF and HSS/UDM based on Diameter. |
| E1 | Reference point between CU‐CP and CU‐UP (Central Unit) to control user plane resources. E1 is based on E1AP (E1 Application Part). |
| F1 | Reference point between RAN DU (Distributed Unit) and RAN CU (Central Unit). It consists of F1‐C and F1‐U to exchange control and user plane traffic respectively. F1 is based on F1AP (F1 Application Part). |
| N1 | Reference point between UE and AMF to exchange NAS (Non‐Access Stratum) messages. |
| N2 | Reference point between 5G‐RAN and AMF based on NGAP (Next Generation Application Part). |
| N3 | Reference point between 5G RAN and UPF based on GTPv1‐U (GPRS Tunneling Protocol User Plane). |
| N4 | Reference point between SMF and UPF to manage data sessions at the user plane. N4 is based on PFCP (Packet Forwarding Control Protocol). |
| N5 | Reference point between PCF and AF. |
| N6 | Reference point between UPF and packet data networks. It can, e.g. transport IP and Ethernet packets. |
| N7 | Reference point between SMF and PCF. |
| N8 | Reference point between AMF and UDM. |
| N9 | Reference point between two UPF to transmit user plane data. It is based on GTPv1‐U. |
| N10 | Reference point between SMF and UDM. |
| N11 | Reference point between AMF and SMF. |
| N12 | Reference point between AMF and AUSF. |
| N13 | Reference point between AUSF and UDM. |
| N14 | Reference point between two AMF. |
| N15 | Reference point between AMF and PCF. |
| N16 | Reference point between visited SMF (vSMF) and home SMF (hSMF). |
| N17 | Reference point between AMF and 5G‐EIR. |
| N18 | Reference point between NF and UDSF. |
| N22 | Reference point between AMF and NSSF. |
| N23 | Reference point between PCF and NWDAF. |
| N24 | Reference point between visited PCF (vPCF) and home PCF (hPCF). |
| N27 | Reference point between visited NRF (vNRF) and home NRF (hNRF). |
| N31 | Reference point between visited NSSF (vNSSF) and home NSSF (hNSSF). |
| N32 | Reference point between V‐SEPP (visited SEPP) and H‐SEPP (home SEPP) in roaming cases. |
| N33 | Reference point between NEF and AF. |
| N40 | Reference point between SMF and CHF. |
| N50 | Reference point between AMF and CBCF. |
| N5g‐eir | Service based interface to offer 5G‐EIR services. |
| Naf | Service based interface to offer AF services. |
| Namf | Service based interface to offer AMF services. |
| Nausf | Service based interface to offer AUSF services. |
| Nnef | Service based interface to offer NEF services. |
| Nnrf | Service based interface to offer NRF services. |
| Nnssf | Service based interface to offer NSSF services. |
| Npcf | Service based interface to offer PCF services. |
| Nsmf | Service based interface to offer SMF services. |
| Nudm | Service based interface to offer UDM services. |
| Nudr | Service based interface to offer UDR services. |
| Nudsf | Service based interface to offer UDSF services. |
| Nnwdaf | Service based interface to offer NWDAF services. |
| Nwu | Reference point between UE and N3IWF for establishing secure tunnels. Nwu is based on IPsec/IKEv2. |
| Rx | The Rx reference point provides application layer information to the PCF, e.g. to establish new multimedia sessions. Rx is based on Diameter. |
| S1‐MME | Reference point between eNB and MME based on S1AP (S1 Application Protocol). |
| S1‐U | Reference point between eNB and S‐GW based on GTPv1‐U. |
| S5 | This reference point is used to tunnel user plane packets and manage the user plane tunnels between S‐GW and P‐GW. S5 is based on GTPv2‐C or alternatively on PMIPv6. |
| S6a | Reference point between MME and HSS to enable transfer of subscription and authentication data. It is based on Diameter. |
| S10 | Reference point between MMEs for MME relocation and information transfer, based on GTPv2‐C. |
| S11 | Reference point between MME and S‐GW, used to manage new or existing sessions, to relocate S‐GW during handover, establish direct or indirect forwarding tunnels and trigger paging. S11 is based on GTPv2‐C. |
| SBc | Reference point between CBC and MME for warning message delivery and control functions. The used protocol on this interface is the SBc Application Protocol (SBc‐AP). |
| SGi | This is the reference point between the P‐GW and a packet data network. Protocols on this interface are e.g. IPv4, IPv6, RADIUS, DIAMETER and DHCP. |
| X2 | Reference point between master eNB and secondary gNB in dual connectivity mode used for floor control and data forwarding. |
| Xn | Reference point between gNB and gNB, gNB and ng‐eNB. It consists of Xn‐C and Xn‐U. |
| Y1 | Reference point between UE and non‐3GPP access. Y1 is access technology dependent and not defined by 3GPP. |
| Y2 | Reference point between untrusted non‐3GPP access and N3IWF for the transport of NWu traffic. |
Following reference points are realized as service‐based interfaces: N5, N7, N8, N10, N11, N12, N13, N14, N15, N16, N17, N18, N22, N24, N27, N31, N33, N40 and N50.
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