etworks or operate multiple cells, one for each mobile operator. Neutral host is popular in scenarios where a business benefits from ensuring its customers have a good mobile signal; for example, a hotel or shopping center. 4.6.5 Key Interfaces for Small Cells The diversity and variety of small cell architectures and deployments have resulted in an ecosystem dominated by relatively small companies. The size of these companies reflects the size of small cell deployments, in that there are many small cell network deployment opportunities (in particular, indoor opportunities), but that each of these opportunities is relatively small; for example, a hotel chain, a single factory, office locations of one company. These small cell vendors will procure the constituent parts of the small cell from several companies - in particular, there is a small cell ecosystem of protocol stack vendors (L3/L2), an ecosystem of PHY vendors (L1), and an ecosystem of RF vendors. This has resulted in the small cell industry adopting APIs to supporting integration of these different components. This chapter focuses on one of these APIs, nFAPI, which defines the MAC/PHY split (split option 6) identified by 3GPP (3GPP TR38.801) but not standardized. It is important to note that nFAPI is a wrapper around the FAPI interface described next. 4.6.5.1 FAPI The FAPI interface, which originally meant Femtocell Application Programming Interface, is now simply called FAPI and defines the interface between MAC (L2) and PHY (L1), as the term femtocell was replaced by small cell. FAPI is specified by the Small Cell Forum and the original interface defined the interac- tion between a 3G MAC and PHY (SCF 048). Subsequently a version was defined between a 4G MAC and PHY (SCF 082), and recently the 5G version has been completed (SCF 222). For each wireless technology FAPI defines a set of control messages for configuring a PHY, a set of messages for exchanging data between MAC and PHY, and indicates the timing constraints of these messages. The architecture 