# From Mirrors to Free-Space Optical Communication--Historical Aspects in Data Transmission
## 6 FSO Advantages and Disadvantages  
Free-Space Optical Communication is the answer to the worldwide demand for broadband communications offering flexible network connections [95]. FSO systems are more secure than RF or other wireless-based transmission systems since it is impossible to detect laser beams with spectrum analyzers or RF meters. It is difficult or even impossible to intercept the data transmission, as laser beams are narrow and invisible to the naked eye. Moreover, the information can be transmitted over an encrypted connection, which contributes significantly to increasing the security level [96]. FSO bridges the gaps in existing high-data-rate communication networks. Its ease of installation and high throughput with relatively low investment outlays make it a rapidly deployable mobile wireless communication infrastructure [78]. Freedom of licensing (no licenses are required) means that there are no administrative difficulties related to their installation [97]. Another FSO advantage includes a very low bit error rate, while extremely narrow laser beams ensure that there is almost no practical limit to the number of separate FSO links that can be installed in a given location, which is in direct contrast to omnidirectional RF links. Moreover, due to the narrow beam and easy supervision of interference in the beam, FSO links ensure high resistance to eavesdropping. The transmitted data are inaccessible to unauthorized system recipients, and the active methods of notification about any attempt to join the link, supported by appropriate transmission protocols, ensure a relatively high degree of transmitteddata confidentiality [98]. FSO is also, by definition, a low-latency technology [99]. In comparison to fiber optics, there is about a 40 percent reduction in latency, as the speed of light in air is about 40 percent higher than in a fiber cable. Moreover, the direct Line of Sight path between two endpoints is shorter than any curved path taken by a fiber-optic cable. FSO links are comparable to radio ones in terms of the transmission medium; however, they exceed them in terms of bandwidth, operating costs, and installation speed. They provide ranges of up to several kilometers [100]. Hence, they can be used in crises and military communication systems. It should also be emphasized that the return on investment costs is very fast in comparison to the cost of building a fiber-optic line connecting buildings that are separated by, e.g., 1000 m.  
As was mentioned in the \"Laser wavelengths used in the FSO links\" section, the main phenomena that disrupt FSO communication are rain, which absorbs radiation, and smoke/haze/fog-induced Mie scattering. Disruptions are also caused by Rayleigh scattering and photon absorption, snow, scintillations (fluctuations in the refractive index), and industrial pollution (smog and particulate matter in 2.5- and 10-micrometer dimensions (PM2.5/PM10)) [20]. High concentrations of carbon dioxide in the air can also cause the absorption of transmitted signals, which is particularly significant for near-infrared links and less so for SWIR band links [95]. Additionally, even heavy rain and snow enable data transmission, even though they weaken the signal reaching the receiver and lead to an increase in the bit error rate. However, the most dangerous weather phenomenon that can break the connection is fog [82]. One of the factors that may make the employment of the FSO system impractical occurs when the transceiver heads are oriented east-west, due to the possibility of transmission interference caused by direct solar radiation, especially during sunset/sunrise [101; 102]. This applies in particular to 850-nm systems. With a different link setting and the use of nontransparent covers, sunlight does not have a significant effect on the operation of the system.