3.  FSO is a line-of-sight technology which uses LASERS and Photo detectors to provide optical connections between two points— without the fiber. FSO can transmit data, voice or video at speeds capable of reaching 2.5 Gbps.Products capable of speeds upto10 Gbps are expected to hit the markets within one year. FSO units consist of an optical transceiver with a laser (transmitter) and a Photo detector (receiver) to provide full duplex (bi-directional) capability. FSO systems use invisible infrared laser light wavelengths in the 750nm to 1550nm range.
4.  Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks polished their shields to send signals during battle. In the modern era, semaphores and wireless solar telegraphs called heliographs were developed, using coded signals to communicate with their recipients. The invention of lasers in the 1960s revolutionized free space optics. Military organizations were particularly interested and boosted their development. However the technology lost market momentum when the installation of optical fiber networks for civilian uses was at its peak.
5. Birth of Laser FSO Communications In the mid-1960s NASA initiated experiments to utilize the laser as a means of communication between the ground and space. Gemini-7
6.  Free-space point-to-point optical links can be implemented using infrared laser light, although low- data-rate communication over short distances is possible using LEDs. Free Space Optics are additionally used for communications between spacecraft. Maximum range for terrestrial links is in the order of 2 to 3 km,but the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. In outer space, the communication range of free-space optical communication is currently in the order of several thousand kilometers,but has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders.
7.  LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds. LAN-to-LAN connections in a city, a metropolitan area network To cross a public road or other barriers which the sender and receiver do not own. Speedy service delivery of high-bandwidth access to optical fiber networks. Converged Voice-Data-Connection. Temporary network installation (for events or other purposes). Reestablish high-speed connection quickly (disaster recovery) As an alternative or upgrade add-on to existing wireless technologies. As a safety add-on for important fiber connections (redundancy) For communications between spacecraft, including elements of a satellite constellation.
8.  No licensing required. Wireless communicate from A to B. High bit rate (10 Mbps to 2.5 Gbps). Low error bit rates . Set up a link in a few days. The lasers used are eye safe. Installation cost is very low. Highly secure transmission possible. High data rates,upto 2.5 Gbps at present and 10 Gbps in the near future. Very secure due to the high directionality and narrowness of the beam.
9. THE FOLLWING FACTORS CAN CAUSE HIGH ERROR BIT- RATES.. Fog: The major challenge to FSO communications is fog. The primary way to counter fog when deploying FSO is through a network design that shortens FSO link distances and adds network redundancies. FSO installations in foggy cities such as San Francisco have successfully achieved carrier- class reliability. Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power density (attenuation) of the FSO
10. Physical obstructions: Flying birds can temporarily block a single beam,but this tends to cause only short interruptions, andtransmissions are easily and automatically resumed.Building sway/ seismic activity: The movement of buildings can upset receiverand transmitter alignment.Safety: To those unfamiliar with FSO, safety is often aconcern because the technology uses lasers fortransmission.Scintillation: Heated air rising from the earth or man-madedevices such as heating ducts creates temperaturevariations among different air pockets. This can causefluctuations in signal amplitude which leads to imagefluctuations at the FSO receiver end.
11. Free-space optical communication (FSO) is an optical communication technology thatuses light propagating in free space to transmit data for telecommunications or computernetworking. "Free space" means air, outer space, vacuum, or something similar. Thiscontrasts with using solids such as optical fiber cable or an optical transmission line. Thetechnology is useful where the physical connections are impractical due to high costs orother considerations