oning support. NG-U connects an NG-RAN network node to a UPF. It supports non-guaranteed delivery of PDU session user-plane PDUs between the NG-RAN node and the UPF. 8 Note that the NG-C interface is referred to as N2 reference point in some stage-2 specifications (3GPP TS 23.501). 9 Note that the NG-U interface is referred to as N3 reference point in some stage-2 specifications (3GPP TS 23.501). 3 5G System Overview 3.3.3.1 NG-C Interface As mentioned above, the NG-C interface (3GPP TS 38.413) connects a NG-RAN to an AMF. A gNB may be connected to multiple AMFs in the SO called NG-Flex configuration, where a gNB is connected to all AMFs within an AMF region. A gNB selects an AMF for a UE based on configuration, service requirements, slicing information, and other parameters. NG-Flex allows AMF load balancing across an AMF pool. NG Application Protocol (NG-AP) (3GPP TS 38.413) is used on the NG-C interface on top of SCTP, which is conceptually similar to the S1 Application Protocol (S1-AP) (3GPPT 36.413) used in LTE. One new distinct feature of NG-C, not present in LTE, is the capability of both NG-RAN and AMF to use multiple SCTP associations through multiple SCTP endpoints on both ends. This functionality facilitates the deployment of 5GC and NG-RAN in virtualized environ- ments, where new computational resources can be added or removed "on the fly." As a computational (or network) resource may have a separate IP address, this new functionality allows adding these resources in a seamless manner, which has no impact on a UE. Furthermore, in contrast to LTE, the NG-C interface also supports non-3GPP access technologies, e.g. WLAN (IEEE 802.11). When non-3GPP access is used, a new network node referred to as N3IWF terminates the NG interfaces toward 5GC, making the usage of non-3GPP access largely transparent for the 5GC. Of course, not all 3GPP functionalities are supported by all access technologies. For example, there is no notion of paging in WLAN. Therefore, 3GPP have specified (3GPP TS 29.413) which NG-AP pr