0/20/40 MHz 37 MHz 5/10/20 MHz # of RF Channels Max Reach Link Availability 99.999% 99.99% 99.999% Source: Asif © 2009 IEEE [8] Transmission Networks 10 km long link Slave TDM/IP Converter TDM/IP Converter Master Figure 7.6 Trial setup. Reproduced with permission from Asif 2009 IEEE [8] 7.3.3.3 Results The key results of the testing are as follows [8]: The products A and C that have multiple RF channels can set up robust links at smaller channel bandwidths (5 to 10 MHz). The product B that supports only one channel was not able to hold the radio link for more than a day. The BERT (Bit Error Rate Test) test results were satisfactory for all the products. The links were kept operational for a month for products A and C. Both links had a few minutes of downtime due to power outage and heavy rain. The results showed link availability of 99.995% and 99.99% for products A and C respectively. There was no degradation in the KPIs (Key Performance Indicators); only Abis fluctuation due to power outages and call drop rates due to heavy rainfall. 7.3.3.4 Benefits Primarily, these systems can be used in villages, remote areas, frequency relief and extending cellular coverage at city boundaries. To do a cost benefit analysis, the price of 5.7/5.8 GHz systems was compared with licensed 10.5 GHz microwave radios which are closest to ISM radios in terms of coverage (radial distance). [8] shows that ISM band radios provide 60-70% savings as compared to 10.5 GHz radios (if a repeater is not used by 10.5 GHz link to cover up to 25 km in a rural setting). If a repeater is used in a 10.5 GHz link which is not required by license-free band radio to cover a distance of 40 km then the savings can go as high as 89%. 7.3.4 Free Space Optics (FSO) In a Free Space Optics (FSO) system a narrow beam of light is launched at a transmission station, transmitted through the atmosphere, and subsequently received at a receiving station. FSO links are often referred to as Optical Wireless transmission systems given they use similar infrared (IR) wav