 protocols to support these capabilities. 78 Chapter 1 Logical connection for synchronization (eREC<eRE) Logical connection for C&M data eREC eRE) connection for (eREC<>eRE) Transport network SAPCM User-plane User-plane SAPUL SAPCM SAPU. SAPCM eCPRI Standard eCPRI Standard specific protocls specific protocls Transport network layer Transport network layer eCPRI radio equipment controller eCPRI radio equipment Figure 1.30 eCPRI system and interface definition [76]. As shown in Fig. 1.30, in eCPRI, the radio base station is divided into two building blocks: eCPRI radio equipment control (eREC) and eCPRI radio equipment (eRE), which are physi- cally separated and are connected via a transport network. The eREC implements part of the physical layer functions and higher layer functions of the air interface, whereas the eRE con- tains the remaining part of the physical layer functions and the analog RF functions. User- plane data, control and management, and synchronization signals (i.e., synchronization data used for frame and timing alignment) are packetized, multiplexed, and transferred over the transport network which connects eREC(s) and eRE(s). The eCPRI does not rely on specific transport network and data-link-layer protocols, thus any type of network can be used for eCPRI provided that eCPRI requirements are fulfilled (see Fig. 1.31). Fig. 1.32 shows high-level protocol stack and physical layer processing of an LTE eNB or NR gNB. The eCPRI specification defines five functional splits identified as A to E splits. An additional set of intra-PHY functional divisions identified as I, II, and IU are also defined. It is understood that the CPRI specification supports only functional split E. The physical layer processing stages shown in Fig. 1.32 are consistent with those of the NR. The eCPRI specification focuses on three different reference splits, two splits in the down- link and one split in the uplink. Any combination of the different DL/UL splits is also possi- ble. Other functional splits within the physical l