, but that it has some limitations that you need to understand before deploying and relying on it. Although it would never deliver the degree of accuracy and certainty that the FTS approach delivers, in some cases, operators might only have limited options and APTS will certainly help. 11.4 mid-haul and front-haul Timing The goal in the preceding sections is to outline the possible trade-offs in designing a timing solution for the back-haul-delivering the +1.1-us phase alignment at the handoff point to the radio equip- ment and/or +1.5-us phase alignment at the radio antenna. But as described in Chapter 10, with the evolution of the RAN, there are additional (relative) timing requirements between components of that RAN. For that reason, the engineer needs to consider the extension of the back-haul timing design into the mid-haul and front-haul networks and take these additional requirements into account. The first consideration is the topology and type of RAN being deployed. As outlined in Chapter 10, there are two basic topologies of 4G/5G RAN, the centralized RAN (CRAN) and the distributed RAN (DRAN), defined by the location of the RAN components. Chapter 10 also covered the fact that there are basically three components to the modern 5G net- work: the Central Unit (CU), the Distributed Unit (DU), and the Radio Unit (RU). Figure 11-12 (taken from Chapter 10) illustrates the combination of topologies that these units support with front- haul (FH), mid-haul (MH), and back-haul (BH) networks. Backhaul Mobile D-RAN gateways Backhaul Fronthaul Mobile C-RAN gateways Midhaul Backhaul Mobile C-RAN gateways Fronthaul Midhaul Backhaul Mobile C-RAN gateways Figure 11-12 5G RAN Topologies The first two topologies apply to 4G and 5G networks (where the Baseband Unit [BBU] is a com- bined DU/CU from 5G), and the second two topologies apply to 5G. See the sections "5G RAN" and "Radio Access Network Evolution" in Chapter 10 for more details on 4G and 5G RAN evolution. There are three main methods related to time distribution (