the high-level split, which is described in the present section. Additional information about the 3GPP study of the gNB split architecture can be found in 3GPP TR 38.801, 1 section 11.1. 4.2.3 Functional Description As mentioned above, 3GPP defined two logical gNB architectures: a monolithic architecture with a gNB defined as a single logical network node encompassing all the functionality and a split architecture with a gNB-CU connected to one or multiple gNB-DUs. Explicit stan- dardization of two such options is somewhat unusual (and strictly speaking unnecessary), as normally 3GPP aims at defining only a single solution. As already explained above, nor- mally any modifications to the defined single solution are up to the implementation; in the case of network architecture, this means that each standardized logical network node can be implemented in a different number of physical network nodes. The split and the no-split architectures are virtually indistinguishable from the point of view of a UE, a 5GC, or another gNB. Split deployment may affect latency or performance, but functionality-wise it is equivalent to the monolithic deployment. In the split case, all the network interfaces (i.e. NG and Xn) are terminated at the gNB-CU, the air interface to a UE is terminated in the gNB-DU, and a gNB-CU and gNB-DU(s) are connected via the F1 interface, as illustrated in Figure 4.2.1. In this case, all the "external" interfaces, that is NG, Xn and Uu, are exactly the same as in the case where they terminate at a monolithic gNB. For brevity, Figure 4.2.1 does not show the OAM system, which manages a gNB-CU and a gNB-DU separately. OAM aspects are explained in more detail in Section 6.5. NG-RAN gNB-CU gNB-DU gNB-DU Figure 4.2.1 Overall NG-RAN architecture. (Source: Reproduced by permission of © 3GPP). 1 3GPP Technical Report TR) 38.801, as all 3GPP TRs, is not normative and should only be used as a source of information about 3GPP study progress. 4 NG-RAN Architecture gNB-DU gNB-CU Figure 4.2.2 gNB-CU/gNB-DU protocol st