ding node." These terms have been introduced to make it possible to generalize the protocol defini- tions SO that they can be used on multiple network interfaces (e.g. F1 which is explained in Section 4.2) between different network nodes (e.g. between gNBs or between gNB-CU and gNB-DU), and for different purposes (e.g. dual connectivity, which is explained in Section 4.3). In the Xn-AP specification, the "corresponding node" is referred to as the "as- sisting node." Xn-U PDUs may also be sent without payload, if delivery of the NR-U message is needed and there are no data to transfer. 3.3.5 Additional NG-RAN Features In the present section we explain a few select NG-RAN features, which are either new in 5G or different compared with LTE. 3.3.5.1 RAN Sharing Similar to E-UTRAN in LTE, NG-RAN may be shared by multiple operators. In Release-15, only the 5G Multi-Operator Core Network (5G MOCN) network sharing architecture is supported, which is illustrated in Figure 3.3.9. In MOCN, a NG-RAN can be shared between multiple operators each having their own 5GC. To support this functionality, NG-RAN can broadcast multiple PLMNs per cell. 3.3 NG Radio Access Network Operator A Operator B Operator C NG RAN Radio Access Network Operator X Figure 3.3.9 RAN sharing. (Source: Reproduced by permission of © 3GPP). In contrast to LTE, two slightly different architecture options for NG-RAN sharing have been defined. In the first option, which is similar to what is commonly used in LTE, all NG-RAN network interfaces (e.g. Xn and F1) are shared between all PLMNs (e.g. all core networks) that NG-RAN is connected to. In the second option, the notion of an interface instance is used on the NG-RAN network interfaces. The interface instance is identified by the Interface Instance Identification IE carried in all Xn-AP and F1-AP messages. For each interface instance a separate setup procedure is performed, however they all can use the same transport (i.e. the same SCTP association). The latter option is assumed to be beneficial for the ca