sion to this model, where dark fiber is deployed from the cell sites to PoPs where the baseband unit is located, and the aggregation network commences at the PoPs. Local Internet breakout (if deployed) further muddies the waters. Hub&Spokes Transport Aggregation Transport Aggregation Backbone Backbone Backhaul Backhaul, Pre-Aggregation Ring(s) Aggregation Transport Aggregation Transport Backbone Backbone Backhaul Pre-Aggregation Backhaul Pre-Aggregation Figure 6.6.5 Transport network topologies. 6.6 Transport Network Migration from 4G to 5G will further impact the classical model in several ways. First, the rates will drastically increase, both due to the increased user data rates, and to fron- thaul transport. While in principle rate does not affect topology, we shall see that in practice it does. More importantly, although at first 5G eMBB coverage may be collocated with 4G cells, to reap the benefits of 5G the number of cell sites will radically increase over time (if for no reason other than the limited range of mmWave propagation). The decomposi- tion of the gNB into RU, DU, and CU also means that there will be more distinct types of physical or virtual elements to be connected. The automation of network slices will fun- damentally alter the management plane, but will have limited effect on network topology. Additionally, the requirement for low-delay inter-cell site connectivity is greater, impact- ing the connectivity at some level. Finally, because of commoditization of pure transport services ("dumb pipes"), wholesale providers will need to move up the value chain. This means that they may attempt to additionally provide hosting of virtualized NFs (e.g. MEC) or even virtualized RAN components (e.g. vCU). A generic depiction of the RAN network segments and their interconnection to the 5G core is given in Figure 6.6.6. 6.6.5.2 Transport Protocols There are a small number of packet forwarding protocols (Carrier Ethernet, MPLS, IP), and a much larger number of their variants (MPLS-TE, MPLS-TP, segment routin