04-02-2010, 10:18 AM
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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. FSO success can be measured by its market numbers: forecasts predict it will reach a USS 2.5 billion market by 2006.
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 applications.
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.
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; an 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.
WHAT IS FSO?
FSO technology is implemented using a laser device .These laser devices or terminals can be mounted on rooftops ,Corners of buidings 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.
As the differences in laser prices suggest, such systems are quite a bit more expensive than 850-nm links. An 850-nm transceiver can cost as little as $5000 (for a 10-100-Mb/s unit spanning a few hundred meters), while a 1550-nm unit can go for $50 000 (for gigabit-per-second setups encompassing a kilometer or two).
Air fibre, a major FSO vendor, says it can get a page link up and running within two to three days at one-third to one-tenth the cost of fiber (about $20,000 per building). FSO is not only cost-effective and easy to deploy but also fast.The technology is not for everyone. A major reason companies might not adopt FSO is its confinement to urban areas. FSO deployments must be located relatively close to big hubs, which means only customers in major cities will be eligible-at least initially. Businesses in more remote locations are out of luck, unless a provider sets up hubs in their area, wh ich seems like a distant reality right now.
When fiber was compared with free-space optics, deployment costs for service to the three buildings worked out to $396 500 versus $59 000, respectively. The fiber cost was calculated on a need for 1220 meters: 530 meters of trunk fiber from the CLEC's central office to its hub in the office park plus an average of 230 meters of feeder fiber for each of the runs from the hub to a target building, all at $325 per meter. Free-space optics is calculated as $18 000 for free-space optics equipment per building and $5000 for installation. Supposing a 15 percent annual revenue increase for future sales and customer acquisition, the internal rate of return for fiber over five years is 22 percent versus 196 percent for free-space optics.
WHY FREE SPACE OPTICS?
Ultra high bandwidth :
The laser systems operate in the terahertz frequency spectrum and usually operate in the 194 THz or 375 THz range. Their performance is comparable to the best fibre optic system available, giving speeds between 622 Mbps and 1.25 Gbps. This technology uses devices and techniques developed for fibre optic systems.
RAPID DEPLOYMENT TIME:
Installing a FSO system can be done in a matter of days even faster if the gear cn be placed in offices behind windows instead of on rooftops. A fibre based competitor has to seek municipal approval to dig up a street to lay its cable. Unlike most of the lower frequency portion of the electromagnetic spectrum, the part above 300 GHz is unlicensed worldwide. So no extra time is needed to obtain right-of-way permits or trench up the streets or to obtain FCC frequency licenses.
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 10Gbits/sec at a distance of 2 kilometers (km) to 4km. Access claims it can deliver 10Gbits/ sec. Terabeam can provide up to 2Gbits/sec now, while AirFiber and Lightpointe have promised Gigabit Ethernet capabilities sometime in 2001..
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-MULTIPOINT 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 10Gbits/sec-at 1km to 2km.
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.
With FSO, there's also no capital overhang. FSO carriers can avoid heavy buildouts by deploying laser terminals after customers have signed on. No heavy capital investments for buildout are required. Low risk investment is another advantage of FSO.
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.
High transmission capacity is an advantage of this technology.
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.
FSO communications systems deal with scintillation by sending the same information from several separate laser transmitters. These are mounted in the same housing, or page link head, but separated from one another by distances of about 200 mm. It is unlikely that in traveling to the receiver, all the parallel beams will encounter the same pocket of turbulence since the scintillation pockets are usually quite small. Most probably, at least one of the beams will arrive at the target node with adequate strength to be properly received. This approach is called spatial diversity, because it exploits multiple regions of space.
Dealing with fog, more formally known as Mie scattering, is largely a matter of boosting the transmitted power, although spatial diversity also helps to some extent. In areas with frequent heavy fogs, it is often necessary to choose 1550-nm lasers because of the higher power permitted at that wavelength. Also, there seems to be some evidence that Mie scattering is slightly lower at 1550 nm than at 850 nm. However, this assumption has recently been challenged, with some studies implying that scattering is independent of the wavelength under heavy fog conditions.
One of the more common difficulties that arises when deploying free-space optics links on tall buildings or towers is sway due to wind or seismic activity. Both storms and earthquakes can cause buildings to move enough to affect beam aiming. The problem can be dealt with in two complementary ways: through beam divergence and active tracking.
With beam divergence, the transmitted beam is purposely allowed to diverge, or spread, so that by the time it arrives at the receiving page link head, it forms a fairly large optical cone. Depending on product design, the typical free-space optics light beam subtends an angle of 3-6 milliradians (10-20 minutes of arc) and will have a diameter of 3-6 meters after traveling 1 km. If the receiver is initially positioned at the center of the beam, divergence alone can deal with many perturbations. This inexpensive approach to maintaining system alignment has been used quite successfully by FSO vendors like LightPointe for several years now.
If, however, the page link heads are mounted on the tops of extremely tall buildings or towers, an active tracking system may be called for. More sophisticated and costly than beam divergence, active tracking is based on movable mirrors that control the direction in which the beams are launched. A feedback mechanism continuously adjusts the mirrors so that the beams stay on target.
Beam wander arises when turbulent eddies bigger than the beam diameter cause slow, but large, displacements of the transmitted beam. It occurs not so much in cities as over deserts over long distances. When it does occur, however, the wandering beam can completely miss its target receiver. Like building sway, beam wander is readily handled by active tracking.
APPLICATIONS OF FSO
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.
CONCLUSION
Clearly, FSO is not the ideal choice for all communications applications. Equally clearly, it has important roles to play both as a primary access medium and as a backup technology. Key to its success will be a realistic analysis of historical weather patterns in combination with customers' needs for network availability. With proper planning, path blocks like window washers and rooftop maintenance workers can also be dealt with, and the technology will be able to realize its great potential.
[attachment=1718]
[attachment=1719]
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. FSO success can be measured by its market numbers: forecasts predict it will reach a USS 2.5 billion market by 2006.
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 applications.
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.
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; an 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.
WHAT IS FSO?
FSO technology is implemented using a laser device .These laser devices or terminals can be mounted on rooftops ,Corners of buidings 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.
As the differences in laser prices suggest, such systems are quite a bit more expensive than 850-nm links. An 850-nm transceiver can cost as little as $5000 (for a 10-100-Mb/s unit spanning a few hundred meters), while a 1550-nm unit can go for $50 000 (for gigabit-per-second setups encompassing a kilometer or two).
Air fibre, a major FSO vendor, says it can get a page link up and running within two to three days at one-third to one-tenth the cost of fiber (about $20,000 per building). FSO is not only cost-effective and easy to deploy but also fast.The technology is not for everyone. A major reason companies might not adopt FSO is its confinement to urban areas. FSO deployments must be located relatively close to big hubs, which means only customers in major cities will be eligible-at least initially. Businesses in more remote locations are out of luck, unless a provider sets up hubs in their area, wh ich seems like a distant reality right now.
When fiber was compared with free-space optics, deployment costs for service to the three buildings worked out to $396 500 versus $59 000, respectively. The fiber cost was calculated on a need for 1220 meters: 530 meters of trunk fiber from the CLEC's central office to its hub in the office park plus an average of 230 meters of feeder fiber for each of the runs from the hub to a target building, all at $325 per meter. Free-space optics is calculated as $18 000 for free-space optics equipment per building and $5000 for installation. Supposing a 15 percent annual revenue increase for future sales and customer acquisition, the internal rate of return for fiber over five years is 22 percent versus 196 percent for free-space optics.
WHY FREE SPACE OPTICS?
Ultra high bandwidth :
The laser systems operate in the terahertz frequency spectrum and usually operate in the 194 THz or 375 THz range. Their performance is comparable to the best fibre optic system available, giving speeds between 622 Mbps and 1.25 Gbps. This technology uses devices and techniques developed for fibre optic systems.
RAPID DEPLOYMENT TIME:
Installing a FSO system can be done in a matter of days even faster if the gear cn be placed in offices behind windows instead of on rooftops. A fibre based competitor has to seek municipal approval to dig up a street to lay its cable. Unlike most of the lower frequency portion of the electromagnetic spectrum, the part above 300 GHz is unlicensed worldwide. So no extra time is needed to obtain right-of-way permits or trench up the streets or to obtain FCC frequency licenses.
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 10Gbits/sec at a distance of 2 kilometers (km) to 4km. Access claims it can deliver 10Gbits/ sec. Terabeam can provide up to 2Gbits/sec now, while AirFiber and Lightpointe have promised Gigabit Ethernet capabilities sometime in 2001..
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-MULTIPOINT 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 10Gbits/sec-at 1km to 2km.
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.
With FSO, there's also no capital overhang. FSO carriers can avoid heavy buildouts by deploying laser terminals after customers have signed on. No heavy capital investments for buildout are required. Low risk investment is another advantage of FSO.
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.
High transmission capacity is an advantage of this technology.
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.
FSO communications systems deal with scintillation by sending the same information from several separate laser transmitters. These are mounted in the same housing, or page link head, but separated from one another by distances of about 200 mm. It is unlikely that in traveling to the receiver, all the parallel beams will encounter the same pocket of turbulence since the scintillation pockets are usually quite small. Most probably, at least one of the beams will arrive at the target node with adequate strength to be properly received. This approach is called spatial diversity, because it exploits multiple regions of space.
Dealing with fog, more formally known as Mie scattering, is largely a matter of boosting the transmitted power, although spatial diversity also helps to some extent. In areas with frequent heavy fogs, it is often necessary to choose 1550-nm lasers because of the higher power permitted at that wavelength. Also, there seems to be some evidence that Mie scattering is slightly lower at 1550 nm than at 850 nm. However, this assumption has recently been challenged, with some studies implying that scattering is independent of the wavelength under heavy fog conditions.
One of the more common difficulties that arises when deploying free-space optics links on tall buildings or towers is sway due to wind or seismic activity. Both storms and earthquakes can cause buildings to move enough to affect beam aiming. The problem can be dealt with in two complementary ways: through beam divergence and active tracking.
With beam divergence, the transmitted beam is purposely allowed to diverge, or spread, so that by the time it arrives at the receiving page link head, it forms a fairly large optical cone. Depending on product design, the typical free-space optics light beam subtends an angle of 3-6 milliradians (10-20 minutes of arc) and will have a diameter of 3-6 meters after traveling 1 km. If the receiver is initially positioned at the center of the beam, divergence alone can deal with many perturbations. This inexpensive approach to maintaining system alignment has been used quite successfully by FSO vendors like LightPointe for several years now.
If, however, the page link heads are mounted on the tops of extremely tall buildings or towers, an active tracking system may be called for. More sophisticated and costly than beam divergence, active tracking is based on movable mirrors that control the direction in which the beams are launched. A feedback mechanism continuously adjusts the mirrors so that the beams stay on target.
Beam wander arises when turbulent eddies bigger than the beam diameter cause slow, but large, displacements of the transmitted beam. It occurs not so much in cities as over deserts over long distances. When it does occur, however, the wandering beam can completely miss its target receiver. Like building sway, beam wander is readily handled by active tracking.
APPLICATIONS OF FSO
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.
CONCLUSION
Clearly, FSO is not the ideal choice for all communications applications. Equally clearly, it has important roles to play both as a primary access medium and as a backup technology. Key to its success will be a realistic analysis of historical weather patterns in combination with customers' needs for network availability. With proper planning, path blocks like window washers and rooftop maintenance workers can also be dealt with, and the technology will be able to realize its great potential.
