620 ITU‐T's Technical Reports and Specifications A dedicated in‐building system usually consists of: Base station equipment, often located in a facilities’ room or other service area; Cables which run from the base station through the building risers connecting the base station equipment to antennas; and Small antennas located on the ceilings or walls in strategic locations. 7.3 Sharing and co‐location There is an increasing trend for mobile network operators to adopt a variety of infrastructure models. This is being driven mainly by commercial and efficiency considerations, rather than by regulatory mandates. Sharing can also permit the co‐location of SSC (for example, emergency communication networks) with the equipment of wireless network operators. Infrastructure sharing may be passive or active: 1. Passive sharing includes site sharing, where operators use the same physical components but have different site masts, antennas, cabinets and backhaul. A common example is shared rooftop installations. Practical challenges include the availability of space and property rights. A second type of passive sharing is mast sharing, where the antennas of different operators are placed on the same mast or antenna frame, but the radio transmission equipment remains separate. 2. In active sharing, operators may share the radio access network (RAN) or the core network. The RAN sharing case may create operational and architectural challenges. For additional core sharing, operators also share the core functionality, demanding more efforts and alignments from operators. Again there may be issues of compatibility between the technology platforms used by the operators. The different approaches to infrastructure sharing are illustrated in Figure 17. Source: Adapted from GSMA, Mobile Infrastructure Sharing, (2008). Figure 17 – Main types of infrastructure sharing