port. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.463 (2019). NG-RAN; E1 Application Protocol (E1AP). Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 28.541 (2019). Management and orchestration; 5G Network Resource Model (NRM); Stage 2 and stage 3. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification TS23.501. (2019). System Architecture for the 5G System. Available at: www.3gpp.org (accessed May 29, 2020). Jain. S., Kumar, A., Mandal, S. et al. (2013). B4: Experience with a globally-deployed software defined wan. Proceedings of the ACM SIGCOMM Conference, 2013, Hong Kong, pp. 3-14. 4.5 Lower-Layer Split Lower-Layer Split Jianli Sun Intel Corporation, USA As we mentioned previously, operators have many choices of different NG-RAN architec- tures to deploy; in particular, choices of mapping logical NG-RAN functions into different physical network nodes and further partition of RAN functions. A study conducted by 3GPP in Release-14 discussed some of the possible RAN split options, which are elaborated on in Section 4.1. In Section 4.2, we discussed the high-level split option specified in 3GPP, which divides the RAN into CU and DU. In that split, the CU hosting PDCP and RRC functions can be implemented in a cloud to realize the benefits of centralized RRM and resource pooling. The advantage of the high-level CU/DU split is that it can be deployed using a non-ideal fronthaul transport network, that is, it is particularly well-suited for the case when fiber transport is not available. This, however, comes at a cost as the high-level split does not pro- vide significant gains compared with the monolithic gNB. In the present section we discuss the characteristics of physical layer functions, and the challenges separating these functions into separate network nodes. Below we explain the details of the lower-layer split (LLS) of an NG-RAN and the rea- sons why that particular choice of functional split has been adopted by