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ABSTRACT
FREE SPACE OPTICS
Mention optical communication and most people think of fiber optics. But light travels through air for a lot less money. So it is hardly a surprise that clever entrepreneurs and technologists are borrowing many of the devices and techniques developed for fiber-optic systems and applying them to what some call fiber-free optical communication.
FSO uses lasers, or light pulses, to send packetized data in the terahertz (THz) spectrum range. Air. not fiber, is the transport medium. This means that urban businesses needing fast data and Internet access have a significantly lower-cost option.
An FSO system for local loop access comprises several laser terminals, each one residing at a network node to create a single, point-to-point link; optical mesh architecture; or a star topology, which is usually point-to-multipoint. These laser terminals, or nodes, are installed on top of customers' rooftops or inside a window to complete the last-mile connection. Signals are beamed to and from hubs or central nodes throughout a city or urban area. Each node requires a Line-Of-Sight (LOS) view of the hub.
Known within the industry as free-space optics (FSO), this form of delivering communications services has compelling economic advantages. Despite its potential, FSO still has many hurdles to overcome before it will be deployed widely.
This Seminar consists of an outline of the technology behind FSO, the different architectures followed, some of its advantages and the hurdles it has to overcome before its wide implementation. This technology, with patches, will undoubtedly become one of the major leaps in communication industry.

l.INTRODUCTION
Mention optical communication and most people think of fiber optics. But light travels through air for a lot less money. So it is hardly a surprise that clever entrepreneurs and technologists are borrowing many of the devices and techniques developed for fiber-optic systems and applying them to what some call fiber-free optical communication. Although it only recently, and rather suddenly, sprang into public awareness, free-space optics is not a new idea. It has roots that go back over 30 years”to the era before fiber-optic cable became the preferred transport medium for high-speed communication. In those days, the notion that FSO systems could provide high-speed connectivity over short distances seemed futuristic, to say the least. But research done at that time has made possible today's free-space optical systems, which can carry full-duplex (simultaneous bidirectional) data at gigabit-per-second rates over metropolitan distances of a few city blocks to a few kilometers.
FSO first appeared in the 60's, for military applications. At the end of 80's, it appeared as a commercial option but technological restrictions prevented it from success. Low reach transmission, low capacity, severe alignment problems as well as vulnerability to weather interferences were the major drawbacks at that time. The optical communication without wire, however, evolved! Today, FSO systems guarantee 2.5 Gb/s taxes with carrier class availability. Metropolitan, access and LAN networks are reaping the benefits.
The use of free space optics is particularly interesting when we perceive that the majority of customers does not possess access to fibers as well as fiber installation is expensive and demands long time. Moreover, right-of-way costs, difficulties in obataining government licenses for new fiber installation etc. are further problems that has turned FSO into the option of choice for short reach application.
FSO uses lasers, or light pulses, to send packetized data in the terahertz (THz) spectrum range. Air, ot fiber, is the transport medium. This means that urban businesses needing fast data and Internet access have a significantly lower-cost option.
FREE SPACE OPTICS 2.WHAT IS FSO?
FSO technology is implemented using a laser device .These laser devices or terminals can be mounted on rooftops .Corners of bindings or even inside offices behind windows. FSOdevices look like security video cameras.
Low-power infrared beams, which do not harm the eyes, are the means by which free-space optics technology transmits data through the air between transceivers, or page link heads, mounted on rooftops or behind windows. It works over distances of several hundred meters to a few kilometers, depending upon atmospheric conditions.Commercially available free-space optics equipment provides data rates much higher than digital subscriber lines or coaxial cables can ever hope to offer. And systems even faster than the present range of 10 Mb/s to 1.25 Gb/s have been announced, though not yet delivered.
Generally the equipment works at one of two wavelengths: 850 nm or 1550 nm. Lasers for 850 nm are much less expensive (around $30 versus more than $1000) and are therefore favored for applications over moderate distances. But a 1550 nm lasers are also used. The main reasons revolve around power, distance, and eye safety. Infrared radiation at 1550 nm tends not to reach the retina of the eye, being mostly absorbed by the cornea. Regulations accordingly allow these longer-wavelength beams to operate at higher power than the 850-nm beams, by about two orders of magnitude. That power increase can boost page link lengths by a factor of at least five while maintaining adequate signal strength for proper page link operation. Alternatively, it can boost data rate considerably over the same length of link. So for high data rates, long distances, poor propagation conditions (like fog), or combinations of those conditions, 1550 nm can become quite attractive.
3.HOW FREE SPACE OPTICS WORKS
Free space optics transmits invisible, eye-safe light beams from one telescope to another using low power infrared laser in the terahertz spectrum. The beam of light in Free space optics systems are transmitted by laser light focused on highly sensitive photon detector receivers. These receivers are telescopic lens able to collect the photon stream and transmit digital data containing a mix of internet messages, video images, radio signals or computer files.

A Free Space Optical system is a point-to-point. Infra red, wireless laser transmission designed for the interconnection of two points which have a direct line of sight. The systems operate by taking a standard data or telecommunications signal, converting it into a digital format and transmitting it through free space. The carrier used for the transmission of this signal is Infra red light and is generated by either high power LED or low power laser diode(s).

Free space optics systems can function over distances of several kilometers.As long as there is a clear line of sight between the source and the destination, and enough transmitter power, free space optics communication is possible,
Forward link

The FSO remains simple: a narrow beam of light is launched at a transmission station, transmitted through the atmosphere, and subsequently received at the receive station. In wireless optical system it uses the infrared of visual range frequencies to transmit data
Housing
TransmiRer
Rovers© link
Housing ^Receiver
Range 100m-5km
FSO transmitter and receiver
Free Space Optics (FSO) is a telecommunication technology that uses light propagating in free space to transmit data between two points. The technology is useful where the physical connection of the transmit and receive locations is difficult, for example in cities where the laying of fibre optic cables is expensive. Free Space Optics is also used to communicate between space-craft, since outside of the atmosphere there is little to distort the signal.

4.HOW FREE SPACE OPTICS CAN HELP YOU
FSO's freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting Free Space Optics (FSO) equipment to operate in a very favorable environment. The only essential requirement for Free Space Optics (FSO) or optical wireless transmission is line of sight between the two end of the link.
For Metro Area Network (MAN) providers the last mile or even feet can be the most daunting. Free Space Optics (FSO) networks can close this gap and allow new customers access to high-speed MAN's. Providers also can take advantage of the reduced risk of installing an Free Space Optics (FSO) network which can later be redeployed.

5. WHY FSO?
The increasing demand for high bandwidth in metro networks is relentless, and service providers' pursuit of a range of applications, including metro network extension, enterprise LAN-to-LAN connectivity, wireless backhaul and LMDS supplement has created an imbalance. This imbalance is often referred to as the "last mile bottleneck." Service providers are faced with the need to turn up services quickly and cost-effectively at a time when capital expenditures are constrained. But the last mile bottleneck is only part of a larger problem. Similar issues exist in other parts of the metro networks. "Connectivity bottleneck" better addresses the core dilemma. As any network planner will tell you, the connectivity bottleneck is everywhere in metro networks.
From a technology standpoint, there are several options to address this "connectivity bottleneck," but most don't make economic sense.
The first, most obvious choice is fiber-optic cable. Without a doubt, fiber is the most reliable means of providing optical communications. But the digging, delays and associated costs to lay fiber often make it economically prohibitive. Moreover, once fiber is deployed, it becomes a "sunk" cost and cannot be re-deployed if a customer relocates or switches to a competing service provider, making it extremely difficult to recover the investment in a reasonable timeframe.
Another option is radio frequency (RF) technology. RF is a mature technology that offers longer ranges distances than FSO, but RF-based networks require immense capital investments to acquire spectrum license. Yet, RF technologies cannot scale to optical capacities of 2.5 gigabits. The current RF bandwidth ceiling is 622 megabits. When compared to FSO, RF does not make economic sense for service providers looking to extend optical networks.

The third alternative is wire- and copper-based technologies, (i.e. cable modem, Tls or DSL). Although copper infrastructure is available almost everywhere and the percentage of buildings connected to copper is much higher than fiber, it is still not a viable alternative for solving the connectivity bottleneck. The biggest hurdle is bandwidth scalability. Copper technologies may ease some short-term pain, but the bandwidth limitations of 2 megabits to 3 megabits make them a marginal solution, even on a good day.
The fourth-and often most viable-alternative is FSO. The technology is an optimal solution, given its optical base, bandwidth scalability, speed of deployment (hours versus weeks or months), re-deployment and portability, and cost-effectiveness (on average, one-fifth the cost of installing fiber-optic cable).
Only 5 percent of the buildings in the United States are connected to fiber-optic infrastructure (backbone), yet 75 percent are within one mile of fiber. As bandwidth demands increase and businesses turn to high-speed LANs, it becomes more frustrating to be connected to the outside world through lower-speed connections such as DSL, cable modems or Tls. Most of the recent trenching to lay fiber has been to improve the metro core (backbone), while the metro access and edge have completely been ignored. Studies show that disconnects occurs in the metro network core, primarily due to cost constraints and the deployment of such non-scalable, non-optical technologies such as LMDS. Metro optical networks have not yet delivered on their promise. High capacity at affordable prices still eludes the ultimate end-user.
6. FSO ARCHITECTURES
POINT-TO-POINT ARCHITECTURE
Point-to-point architecture is a dedicated connection that offers higher bandwidth but is less scalable .In a point-to-point configuration, FSO can support speeds between 155Mbits/sec and lOGbits/sec at a distance of 2 kilometers (km) to 4km. "Access" claims it can deliver lOGbits/ sec. "Terabeam" can provide up to 2Gbits/sec now, while "AirFiber" and "Lightpointe" have promised Gigabit Ethernet capabilities sometime in 2001..

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MESH ARCHITECTURE
Mesh architectures may offer redundancy and higher reliability with easy node addition but restrict distances more than the other options.

A meshed configuration can support 622Mbits/sec at a distance of 200 meters (m) to 450m. TeraBeam claims to have successfully tested 160Gbit/sec speeds in its lab, but such speeds in the real world are surely a year or two off.
POINT-.TO-MULT1POLNT ARCHITECTURE
Point-to-multipoint architecture offers cheaper connections and facilitates node addition but at the expense of lower bandwidth than the point-to-point option.

In a point-to-multipoint arrangement, FSO can support the same speeds as the point-to-point arrangement -155Mbits/sec to lOGbits/sec-at 1km to 2km.
7.ADVANTAGES OF FSO
Known within the industry as free-space optics (FSO), this form of delivering communications services has compelling economic advantages.
Free-space systems require less than a fifth the capital outlay of comparable ground-based fiber-optic technologies. Moreover, they can be up and running much more quickly. Installing an FSO system can be done in a matter of days”even faster if the gear can be placed in offices behind windows instead of on rooftops. Using FSO, a service provider can be generating revenue while a fiber-based competitor is still seeking municipal approval to dig up a street to lay its cable.Street trenching and digging are not only expensive, they cause traffic jams (which increase air pollution), displace trees, and sometimes destroy historical areas. For such reasons, some cities, such as Washington, D.C., are considering a moratorium on fiber trenching. Others, like San Francisco, are hoping to limit disruptions by encouraging competing carriers to lay fiber within the same trench at the same time.
FSO works in a completely unregulated frequency spectrum (THz), unlike LMDS or MMDS. Because there's little or no traffic currently in this range, the FCC hasn't required licenses above 600GHz. This means FSO isn't likely to interfere with other transmissions. Regulation could come about, however, when and if FSO carriers start to fill up the spectrum. License free frequency band is an advantage of FSO.
Cost is one of the major advantage of this technology. Airfiber has prepared a cost model based on deploying an FSO mesh in Boston. According to its analysis, deployment would cost about $20,000 per building, with an average page link length of 55 meters and a maximum length of 200 meters. The mesh would also provide full redundancy. A comparable fiber network would run between $50,000 to $200,000 per building.
Another plus is that an FSO network architecture needn't be changed when other nodes (buildings) are added; customer capacity can be easily increased by changing the node numbers and configurations.
8.DISADVANTAGES OF FSO
Despite its potential, FSO has many hurdles to overcome before it will be deployed widely.
FSO is an LOS technology, which means nodes must have an unobstructed path to the hub antenna. This, of course, means that interference of any kind can pose problems.
Inclement weather is the main threat. Although rain and snow can distort a signal, fog does the most damage to transmission. Fog is composed of extremely small moisture particles that act like prisms upon the light beam, scattering and breaking up the signal. Most vendors know they have to prove reliability in bad weather cities in order to gain carrier confidence, especially if those carriers want to carry voice. So these vendors try to distinguish themselves by running trials in foggy cities. TeraBeam, for example, ran trials in Seattle, figuring if it could make it there, it could make it anywhere.
The technology is affected badly by the environmental phenomena that vary widely from one meteorological area to another. Some of them are scattering, scintillations, beam spread and beam wander.
Scintillation is best defined as the temporal and spatial variations in light intensity caused by atmospheric turbulence. Such turbulence is caused by wind and temperature gradients that create pockets of air with rapidly varying densities and therefore fast-changing indices of optical refraction. These air pockets act like prisms and lenses with time-varying properties. Their action is readily observed in the twinkling of stars in the night sky and the shimmering of the horizon on a hot day.

LAST MILE ACCESS:
FSO can be used in high-speed links that connect end-users with Internet service providers or other networks. It can also be used to bypass local-loop systems to provide businesses with high-speed connections.
ENTERPRISE CONNECTIVITY:
The ease with which FSO links can be installed makes them a natural for interconnecting local-area network segments that are housed in buildings separated by public streets or other right-of-way property
FIBER BACKUP:
FSO may also be deployed in redundant links to back up fiber in place of a second fiber link.
BACKHAUL:
FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
SERVICE ACCELERATION:
FSO can be also used to provide instant service to fiber-optic customers while their fiber infrastructure is being laid.

10.CONCLUSION
Free space optics (FSO) provides a low cost, rapidly deployable method of gaining access to the fiber optic backbone. FSO technology not only delivers fiber-quality connections, it provides the lowest cost transmission capacity in the broadband industry.
As a truly protocol-independent broadband conduit, FSO systems complement legacy network investments and work in harmony with any protocol, saving substantial up-front capital investments.
A FSO page link can be procured and installed for as little as one-tenth of the cost of laying fiber cable, and about half as much as comparable microwave/RF wireless systems. By transmitting data through the atmosphere, FSO systems dispense with the substantial costs of digging up sidewalks to install a fiber link. Unlike RF wireless technologies, FSO eliminates the need to obtain costly spectrum licenses or meet further regulatory requirements

ll.FUTUKE DEVELOPMENTS
The free space optical wireless or Free Space Optics (FSO) do not come without some cost. So FSO will reduce the costs.
Fog: The primary challenge to FSO-based communications is dense fog. Rain and snow have little effect on FSO technology, but fog is different. Fog is vapor composed of water droplets, which are only a few hundred microns in diameter but can modify light characteristics or completely hinder the passage of light through a combination of absorption, scattering, and reflection. The primary answer to counter fog when deploying FSO-based optical wireless products is through a network design that shortens FSO page link distances and adds network redundancies. FSO installations in extremely 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 beam and directly affects the availability of a system. Absorption occurs more readily at some wavelengths than others. However, the use of appropriate power, based on atmospheric conditions, and use of spatial diversity (multiple beams within an FSO-based unit) helps maintain the required level of network availability.
Currently researches are being conducted to overcome the disadvantages of FSO such as fog and absorption.

BIBLIOGRAPHY
freespaceoptics .org
networkmagazin.com
freespaceoptics.com
wikepidea.com
howstuff.com
CONTENTS
1.INTRODUCTION 1
2.WHAT IS FSO? 2
3 HOW FREE SPACE OPTICS WORKS? 3
4.HOW FSO CAN HELP YOU 5
5. WHY FSO? 6
6.FSO ARCHITECTURES 8
7 ADVANTAGES OF FSO 10
8.DISAD VANTAGES OF FSO 11
9.APPLICATION OF FSO 12
10.CONCLUSION 13
11 .FURTURE DEVELOPMENT 14
12.BIBLIOGRAPHY 15

FREE SPACE OPTICS