24-01-2010, 10:28 PM
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INTRODUCTION
When we talk about optical communication, most people think about optical-fiber. But optical communication is also possible without optical-fiber. We know that light travels through air for a lot less money. This makes possible the optical communication without optical-fiber. Optical communication without fiber is known as Free Space Optics. It is used due to economic advantages. Since the introduction of internet the backbone traffic is increasing at the rate greater than 100%, hence the owner of the backbone infrastructure (which is entirely based on fiber optics) are eagerly embracing technologies that add of the capacity of the fiber optics without adding mountains of optical cables.
FSO is not a new idea. 30-years back optical-fiber cables are used for high-speed communication. In those days FSO are used for high-speed connectivity over short distances. Todayâ„¢s FSO can carry full-duplex data at gigabit-per-second rates over metropolitan distances.
What is Free Space Optics (FSO)
Free Space Optics (FSO) is a line-of-sight technology that uses lasers to provide optical bandwidth connections. Currently, Free Space Optics are capable of up to 2.5 Gbps of data, voice and video communications through the air, allowing optical connectivity without requiring fiber-optic cable or securing spectrum licenses. Free Space Optics require light, which can be focused by using either light emitting diodes (LEDs) or lasers (light amplification by stimulated emission of radiation). The use of lasers is a simple concept similar to optical transmissions using fiber-optic cables; the only difference is the medium. Light travels through air faster than it does through glass, so it is fair to classify Free Space Optics as optical communications at the speed of light.
Free Space Optics (FSO) technology is relatively simple. It's based on connectivity between FSO units, each consisting of an optical transceiver with a laser transmitter and a receiver to provide full duplex (bi-directional) capability. Each FSO unit uses a high-power optical source (i.e. laser), plus a lens that transmits light through the atmosphere to another lens receiving the information. The receiving lens connects to a
high-sensitivity receiver via optical fiber. FSO technology requires no spectrum licensing. FSO is easily upgradeable, and its open interfaces support equipment from a variety of vendors, which helps service providers protect their investment in embedded telecommunications infrastructures.
HOW FREE SPACE OPTICS (FSO) WORKS
Free Space Optics (FSO) transmits invisible, eye-safe light beams from one "telescope" to another using low power infrared lasers in the teraHertz spectrum. The beams of light in Free Space Optics (FSO) systems are transmitted by laser light focused on highly sensitive photon detector receivers. These receivers are telescopic lenses able to collect the photon stream and transmit digital data containing a mix of Internet messages, video images, radio signals or computer files. Commercially available systems offer capacities in the range of 100 Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.
Free Space Optics (FSO) 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 (FSO) communication is possible
FSO: WIRELESS, AT THE SPEED OF LIGHT
Unlike radio and microwave systems, Free Space Optics (FSO) is an optical technology and no spectrum licensing or frequency coordination with other users is required, interference from or to other systems or equipment is not a concern, and the point-to-point laser signal is extremely difficult to intercept, and therefore secure. Data rates comparable to optical fiber transmission can be carried by Free Space Optics (FSO) systems with very low error rates, while the extremely narrow laser beam widths ensure that there is almost no practical limit to the number of separate Free Space Optics (FSO) links that can be installed in a given location.
Light Beam Used for FSO System
Generally equipment works at one of the two wavelengths: 850 nm or 1550 nm. Laser for 850 nm are much less expensive (around $30 versus more than $1000) and are favored for applications over moderate distances. One question arises that why we use 1550 nm wavelength. The
main reason revolves 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. 1550 nm beams operate at higher power than 850 nm, by about two orders of magnitude. That power can boost page link lengths by a factor of at least five while maintaining adequate strength for proper page link operation. So for high data rates, long distances, poor propagation conditions (like fog), or combinations of those conditions, 1550 nm can become quite attractive.
Why FSO Now
Substantial investments by carriers to augment the capacity of their core fiber backbones have facilitated dramatic improvements in both price and performance, and they have also increased the capacity of these large backbone networks. However, to generate the communications traffic and revenue needed to fully utilize and pay for these backbone upgrades, higher bandwidth connections must reach the end customers. This requires substantial bandwidth upgrades at the network edge. Essentially, to fully leverage their backbone investments, service providers will also need to expand and extend the reach of their metropolitan optical network to the
edge. FSO presents an opportunity that allows carriers to achieve that goal for one-fifth the cost when compared to fiber (if even available) and at a fraction of the time.
Increased competition: Regulation changes and significant investments by various funds have increased the competitive climate in these metro networks. Each of the existing or new entrants is racing to gain an advantage over their competition. FSO is one of the evolutionary technologies that allows a carrier to acquire and retain new customers quickly and cost-effectively, thereby gaining an entry point over competition. Metro optical networks are expected to see $57.3 billion invested by 2005.
International growth: Due to the growing number of Internet-based applications, most countries are experiencing tremendous growth in bandwidth needs. In growing economies like Latin America and Chinaâ€where the ability to have high-bandwidth connectivity outweighs standards for reliabilityâ€the lack of infrastructure and rising bandwidth demands offers a unique opportunity for FSO.
Changing traffic patterns and protocol standards: Multiple traffic types characterize metro networks. Where voice was once the dominant traffic type, data has emerged as the winner. Moreover, these networks are also a mixture of multiple protocols ranging from Ethernet, SONET, IP, ESCON, FICON, etc. As a Layer One technology, FSO is protocol agnostic.
Wireless world: With the rapid adoption and slow deployment of wireless technologies such as LMDS and MMDS in response to high bandwidth communication needs in the metro area, many service providers still find themselves short of bandwidth to satisfy their needs. To better understand this growing need for FSO, it is important to understand the key drivers for FSO.
Applications of FSO
The applications of free-space-optics are many. Some of them are as follows “
1:- Metro Network Extensions
Carriers can deploy FSO to extend existing metropolitan-area fiber rings, to connect new networks, and, in their core infrastructure, to complete Sonet rings.
2:- 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 business with high-speed connections.
3:- 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.
4:- Fiber Backup
FSO may also be deployed in redundant links to backup fiber in place of a second fiber link.
5:- Backhaul
FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
6:- Service Acceleration
FSO can be also used to provide instant service to fiber-optic customers while their fiber infrastructure is being laid.
FSO: Optical or Wireless
FSO is clearly an optical technology and not a wireless technology for two primary reasons. One, FSO enables optical transmission at speeds of up to 2.5 Gbps and in the future 10 Gbps using WDM. This is not possible using any fixed wireless/RF technology existing today. Two, FSO obviates the need to buy expensive spectrum (it requires no FCC or municipal license approvals), which distinguishes it clearly from fixed wireless technologies. Thus, FSO should not be classified as a wireless technology. Its similarity to conventional optical solutions will enable a seamless integration of access networks with optical core networks and help to realize the vision of an all-optical network.
Free-Space Optics (FSO) Security
The common perception of wireless is that it offers less security than wireline connections. In fact, Free Space Optics (FSO) is far more secure than RF or other wireless-based transmission technologies for several reasons:
Free Space Optics (FSO) laser beams cannot be detected with spectrum analyzers or RF meters
Free Space Optics (FSO) laser transmissions are optical and travel along a line of sight path that cannot be intercepted easily. It requires a matching Free Space Optics (FSO) transceiver carefully aligned to complete the transmission. Interception is very difficult and extremely unlikely.
The laser beams generated by Free Space Optics (FSO) systems are narrow and invisible, making them harder to find and even harder to intercept and crack
Data can be transmitted over an encrypted connection adding to the degree of security available in Free Space Optics (FSO) network transmissions
Challenges To Free-Space Optics
Fiber-optic cable and FSO share many similarities. However, there is a difference in how each technology transmits information. While fiber uses a relatively predictable medium that is subject to outside disturbances from wayward construction backhoes, gnawing rodents and even sharks when deployed under sea, FSO uses an open medium (the atmosphere) that is subject to its own potential outside disturbances. Networks with FSO must be designed to counter the atmosphere, which can affect an FSO system's capacity. FSO is also a line-of-sight technology and interconnecting points must be free from physical obstruction and able to "see" each other.
1:- Scintillation
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 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, 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.
2:- Mie-scattering
It is the scattering of beam due to fog. It is largely a matter of boosting the transmitted power. Spatial diversity also helps to deal with scattering. 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. But some studies shows that scattering is independent of the wavelength under heavy fog conditions. Other atmospheric disturbances, like snow and especially rain, are less of a problem for free-space optics than fog.
3:- Swaying Buildings
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 activities. Both storms and earthquakes can cause buildings to move enough to affect beam aiming.
The problem of swaying buildings can be dealt with in two ways.
Beam Divergence
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 kilometer. If the receiver is initially positioned at the center of the beam, divergence alone can deal with many perturbations.
Active Tracking
This method is used when the page link heads are mounted on the top of extremely tall buildings or towers.
Active tracking is based on movable mirrors that control the direction in which the beams are launched.
A feedback mechanism continuously adjust the mirrors so that the beams stay on target. It is more sophisticated and costly than beam divergence method.
6:- Physical Obstructions
Flying birds can temporarily block a single beam, but this tends to cause only short interruptions, and transmissions are easily and automatically resumed. LightPointe uses multi-beam systems (spatial diversity) to address this issue, as well as other atmospheric conditions, to provide for greater availability.
5:- Safety
To those unfamiliar with FSO, safety is often a concern because the technology uses lasers for transmission. This concern, however, is based on perception more than reality. The proper use and safety of lasers have been discussed since FSO devices first appeared in laboratories more than two decades ago. The two major concerns involve human exposure to laser beams (which present
much more danger to the eyes than any other part of the human body) and high voltages within the laser systems and their power supplies. Standards have been set for laser safety and performance and FSO systems comply with these standards.
Advantages Of Free-Space Optics
The FSO system requires less than one fifth of the capital outlay of comparable ground-based fiber-optic technologies. Optical-fibers are too costly. Connecting the buildings with optical-fiber cost US $100000 - $200000/km in metropolitan areas, 85 percent of the total figure tied to trenching and installation. To install fiber you have to dig the road. Street trenching and digging are not only expensive, they cause traffic jams (which increase air pollution), displace trees, and sometimes destroy historical areas. 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.
It is flexible, offers freedom, and is fast (speeds from 20 Mbps to 2.5 Gbps and beyond)
Demand for bandwidth is increasing and has been increasing exponentially for the past few years. Service providers have been struggling to keep up with such demand. Service providers must extend the reach of metro optical networks, and FSO offers service providers the opportunity to accomplish this objective.
The primary advantages of FSO are high throughput, solid security, and low cost.
Conclusion
The entire face of the Free-Space Optics community is about to change radically as driven by the need for high-speed local loop connectivity and the costs and difficulties of deploying fibers. FSO can be the ultimate solution for high-speed access. Instead of hybrid fiber-coax system, hybrid fiber-laser system may turn out to be the best way to deliver the high capacity last-mile access. FSO provide higher security, and throughput. FSO is capable to fulfill the increasing demand of bandwidth.
