rability testing and issue resolution from a vendor to an operator, who may or may not be willing or capable of carrying this burden. As was concluded during the 5G study, the lower (in the protocol stack) the base station is split, the more its functionality is centralized, resulting in bigger performance gains (due to e.g. centralized RRM) and bigger resource sharing gains. Moreover, only sufficiently "low" splits (e.g. below MAC or intra-PHY) allow for centralized scheduling, which provides for substantial performance gains. However, lower split (especial intra-PHY) introduces much more stringent requirements to the fronthaul transport network, in terms of throughput and latency. In practice, "low" (i.e. intra-PHY) splits can only be deployed by operators which have a sufficiently large fiber transport network. Deploying a new fiber network just for the sake of using the low-level split architecture may not be economically feasible. Therefore, the 3GPP has decided to standardize the so-called "high-level split," in contrast to the "low-level split" described in Section 4.5. It is worth mentioning that this architecture, initially defined for 5G gNB, was later "backported" to 4G ng-eNB as well. The decision to focus on this particular flavor of the high-level split in 3GPP was driven by the following somewhat contradictory factors: 1. Maximization of centralization gains. 2. Relaxed requirements on the transport network. 3. Limited impact on latency (of UE procedures involving a UE). 4. Consistency (and potential reuse of) with other protocols and architectures already defined in 3GPP. For non-technical reasons, 3GPP could not reach consensus on standardizing the low- level split, even though numerous advantages of such split have been identified during the study. Eventually, the work on the low-level split was pushed to the O-RAN Alliance, which 4.2 High-Level gNB-CU/DU Split produced the low-level split specification, covering control, user, and management planes (see Section 4.5). 3GPP has only standardized 