 and scattering. Absorption takes place due to gasses (water vapor and oxygen) while scattering due to liquid water (precipitation, clouds, fog). 7.3.11.7 Ducting Atmospheric ducting is a mode of propagation of electromagnetic radiation, usually in the lower layers of Earth's atmosphere, where the waves are bent by atmospheric refraction. Ducting occurs in the following situations: In valleys when the sun is setting. On clear nights when ground cools down by radiation but high level air stays warm. When there is freezing rain. Transmission Networks Transmission Transmission Transmission Transmission MSS/MGW Transmission OC-48 MSS/MGW SONET/ SDH Ring MSPP Backbone Network Transmission Transmission Transmission Transmission Transmission Transmission Transmission Figure 7.8 Metro transport network 7.3.11.8 Capacity/Throughput Today's microwave radio links can support up to STM-4 (622 Mbps) and with gigabit ethernet radios it can go up to 1.25 Gbps. Millimeter-wave and E-Band (70-100 GHz) backhaul solutions are well suited for broadband applications in enterprises and urban areas. 7.3.11.9 Propagation Models It is difficult to calculate the exact field at arbitrary points in various topologies. Instead propagation models are used which are based on statistical measurements and propagation theory. Longley Rice, the Okumura model and the Hata model are typically used for radio links. In addition factors like fading, latency, quality of service, security, and SO on, also need to be considered. 7.4 Metro Regional and Aggregation Transport Networks This section looks into transport technologies that are used to connect the cellular and mobile broadband transmission hub locations to each other and (Figure 7.8) to: BSCs/RNCs within a city and BSCs/RNCs to MSCs and data networking components (SGSN/GGSN, PDSN) within a regional area. Today's Metro Regional Networks consist of multiple BSCs/RNCs that are connected to both circuit and packet switched components via Optical Technologies like SONET rings and DWDM (Dense Wave Divi