d associated with macro cell towers, as typical in 1G and 2G networks. The internal architecture of the all-in-one base station was highly proprietary and not vis- ible to the network operator. Given its all-in-one nature, there was no possibility to mix and match components from different vendors. Note that the very concept of the cell site (and the handover of a voice call between cell sites) was only established in 1G, deployed in the 1982, with the Advanced Mobile Phone System in the United States. 3G/4G/5G macro cell Starting from 3G standard support in the 2000s and migrating to 4G/LTE in the 2010s, large vendors such as Ericsson, Huawei, and Nokia started to disaggregate the base station by splitting the radio head from the baseband through a semistandardized interface: Common Public Radio Interface (CPRI) in the case of Ericsson and Huawei or Open Base Station Architecture Initiative (OBSAI) in the case of Nokia (Figs 1.11 and 1.12). Even through CPRI and OBSAI are standardized interfaces, the implementation was made highly proprietary through use of optional features. As a result, the disaggregation achieved the goal of All-in-one Base Station Low Physical High Physical PDCP/ E1/T1 DFE/RF MAC/RLC Layer Layer Transport FIGURE 1-10 All-in-one base station. Base Station eCPRI Low Physical High Physical PDCP/ Ethernet DFE/RF MAC/RLC OBSAI Layer Layer Transport FIGURE 1-11 Macro base station architecture. 16 Open Radio Access Network (O-RAN) Systems Architecture and Design Base Station Line Card Network IF Card Low Physical High Physical PDCP/ Ethernet DFE/RF MAC/RLC OBSAI Layer Layer Transport Base Station Line Card Ethernet, sRIO, Low Physical High Physical DFE/RF MAC/RLC OBSAI Layer Layer Base Station Line Card Low Physical High Physical DFE/RF MAC/RLC OBSAI Layer Layer FIGURE 1-12 Multisector base station architecture. removing high-cost (RF power coax) cabling but did not allow for vendor mix and match between baseband and radio head. Note that both CPRI and OBSAI standards use proprietary serialization/