e by the DU initiating the START message sequence. A DU in running state instantiates a PHY in IDLE mode. FAPI P5 messages are used to query, configure, and start the PHY. Stopping a DU deletes any PHY which have been instantiated. The nFAPI state machine in Figure 4.6.8 additionally includes a Network Monitor Mode (NMM) state, where NMM is unique to small cells and is designed to support self- configuration of the small cell. A small cell has an NMM for each technology it wants to monitor, SO previously, 4G small cells had a 2G, 3G, and 4G NMM, while going forward 5G small cells will have 2G, 3G, 4G, and 5G NMM. The procedures in NMM can be summarized as: Measure the Received Signal Strength Indicator (RSSI) over a range of frequencies and channel bandwidths to identify potential neighbor cells. Search for neighbor cells by identifying their synchronization signals. Collect MIB information from a neighbor cell. Collect SIB information from a neighbor cell. The hierarchal nature of the four procedures ensures an efficient way to build up detailed knowledge of its surrounding environment. Finally, it is worth reiterating that nFAPI is a wrapper for FAPI, SO in order to transmit downlink and uplink data, the procedures in Figures 4.6.5 and 4.6.6 are still followed. 4.6.5.3 Management Plane Within a mobile network every cell requires a management plane to ensure that the operator can configure a cell and monitor relevant key performance indicators. Even though 3GPP have defined OAM specifications, historically these management planes have been proprietary. Small cell networks still require a management plane but there have been efforts to provide a standardized interface used across small cell vendors. 4.6 Small Cells In 4G this management plane for small cells was defined by the Broadband Forum (BBF TR-196). The motivation of involvement from the Broadband Forum was that the first small cells were residential cells, allowing the industry to develop a new data model to support the 4G-specific parameters, but also le