 The connection between the 5GC and external networks or servers generally utilizes the IP suite, which is defined by standards produced by the Internet Engineering Task Force (IETF). It may additionally entail Ethernet protocols, defined by the Institute of Electrical and Electronics Engineers (IEEE) and other organizations. The connection between the gNBs and the core is called the NG interface and gNBs can also be interconnected by interfaces named Xn (see Section 3.3). Transport of data over either the Xn or NG interface is conventionally known as backhaul, in line with the termi- nology of previous generations of mobile communications. As described in Chapter 4, gNB may be further split into a number of logical network nodes, thus resulting in more interfaces for the transport network to carry. 13 For the pur- poses of this chapter we consider a 5G base station that can be decomposed into an RU, a DU, and a CU, in which case, we define the F2 interface (which is equivalent to the O-RAN fronthaul interface described in Section 4.5) between the remote RU and the rest of the gNB, and call the transport segment between these two units fronthaul. If the CU is physi- cally detached from the DU, the two units are connected via an interface known as F1 (see Section 4.2), and the transport segment over this interface is sometimes termed midhaul. 12 It is generally assumed in 3GPP that such details (e.g. integration between a transport network and a 3GPP radio network) are addressed by each operator in a proprietary manner. 13 5GC is also split into logical network nodes (see Section 3.2), but these are not of interest here, as 5GC often runs in a data center where fiber is readily available. 6 Enabling Technologies ((HH)) RU - Radio Unit DU - Decentralized Unit CU - Centralized Unit Mobile Functional Decomposition Internet xHaul Transport Content/Apps Aggregation/Backbone Network Figure 6.6.3 Decomposition of the 5G base station and the resulting xHaul interfaces. As shown in Figure 6.6.3, transport in a 5G system ma