nvironment for 5G RAN. These O-RAN front-haul specifications support both 5G NR and LTE RAN. They define control, user, and synchronization plane specifications as well as detailed signal formats and messages for multi-vendor RAN opera- tion over the management plane. Please refer to ORAN specifications for more details, especially the front-haul Control, User and Synchronization Plane Specification (currently release 5.0). The most important thing to understand is that because CPRI has changed into a packet-based im- plementation (either eCPRI, O-RAN, or something else), the formerly synchronous protocol between the BBU and the radio (or between the DU and RU) has disappeared and been replaced by a nondeterministic packet stream. The result is that the synchronization requirement is no longer be- ing "handled for you" by CPRI. The front-haul network must now carry the phase/time and fre- quency synchronization all the way down to the RU. In general, the requirement for synchronization applies only to the DU and the RU (they must be synchronized with each other-to a level that depends on the implementation) and not to the CU (beyond normal operational needs such as timestamps in log files). However, the synchronization requirements demanded by the advanced radio techniques (see the following section "5G New Ra- dio Synchronization Requirements") must be delivered all the way to the RU because the require- ments are specified as being delivered to the antenna (rather than the BBU when using CPRI). And for that, the network engineer needs to rely on the familiar tools of PTP and SyncE (see Chapter 11), not only in the back-haul network, but now also in the mid-haul and front-haul networks. Network Slicing The combination of flexible radio technology splits, the categorization of different use cases, and the CUPS architecture allows the 5G RAN to adopt what is known as network slicing. Network sli- cing allows an operator to offer different categories of services over the shared physical network in- frastructure. As