Non-3GPP access, allows the UEs to connect to the 5G core network through access technologies that are not exist in 3GPP standards.

Examples for this are UEs accessing the 5G core network, through Wifi, or through Satellite communication.

There are special devices that the network operator is using, to allow the UEs to access the 5G core network, through non-3GPP access technologies. Which are the N3IWF, and the TNGF.

N3IWF refers to "Non-3GPP Interworking Function", and it is a bridge between the untrusted non-3GPP access network, and the 5G core network.

While TNGF refers to "Trusted Non-3GPP Access Network", and it is a bridge between the trusted non-3GPP access network, and the 5G core network.