FREE SPACE OPTICS: Lasers
to Hit the Rooftops!
by Niraj K.Gupta
The exciting new world of free space optics
involving laser beams as the communication medium.
This may appear to
be a scene out of "Star Wars" or some other sci-fi magnum opus: laser
beams flashing across high-rise buildings in the metros, bombarding gigabits of
information that were so far considered the exclusive domain of optical fibers.
And, no more worries of disruptions by torrential rains like those faced with
good old microwaves.
Rapid growth in use of the Internet and multimedia
services is already creating congestion in telecom networks worldwide. It has
fueled demand for solutions that offer increased bandwidth, speed and
reliability, in particular, to meet the growing needs of corporates/businesses.
While high-speed, fiber-based infrastructure has solved the problem for long
distance networks, metropolitan or local loop environments continue to face
challenges to deliver the same level of capacity and efficiency as fiber to
carry data, voice and video in "the first mile" (some call it
"the last mile"). Although DSL and LMDS have provided some relief,
there are still some limitations, including the bandwidth, associated with
them.
Lasers Come to Help
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FSO Applications |
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· The First Mile (or the last mile) Access · Wireless (Optical) Backhaul, e.g. for 3G networks · Supplement Fiber/LMDS · Other Network Extensions · Enterprise Networks: LAN-to-LAN Connections |
Free Space Optics (FSO)—also called Optical
Wireless Communication—is emerging as the new ‘avatar’ to mitigate the
bottlenecks in the local loop or "the first mile". It uses laser
technology, which was originally developed in the 1960s. It involves
transmission of digital signals over the beams of light through the atmosphere
or the "Free Space". The beams of light are transmitted by highly
focused lasers on to sensitive photon detector receivers located on the other
end, within the line of sight (LoS). The antenna or receivers of these systems
are telescopic lenses, acting as collectors for the photon stream carrying the
signal, as well as processors of the digital stream itself.
Since the early 1980s, lasers have been
successfully used by both military and space agencies. For instance, in 1991,
BT Labs began holding trials of low power, "eye-safe", 155 Mbps and 1
Gbps optical wireless links in public spaces, within metropolitan areas of the
UK.
The main advantage of these systems—transmitting
information using infrared frequencies—is that these do not require government
licences/clearances, and are therefore cost-effective and ideally suited for
fast deployment. Hence, free-space optical networking technology offers an
effective and economically compelling way to address "the first mile"
challenge of connecting to fiber infrastructure in metropolitan areas. It
enables businesses to transmit and receive data transmission among buildings up
to around 4 km apart, at speeds much faster than conventional media including
high-speed leased lines.
These transceiver products are targeting to
achieve between 99 percent and 99.9 percent availability in free space over
their dynamic range (from 2 to 4 km). They are expected to offer not only
superior transmission performance but also address human eye safety issues, a concern
when laser based technology is being deployed.
The current generation of products support data rates ranging from few
Mbps to 1.25 Gbps using the short wavelength infrared spectral range in the 850
nm as well as 1550 nm atmospheric window—depending on the vendor. The
technology is also expected to be protocol-transparent in order to transport
commonly used telecommunication, data communication and multimedia protocols.
No Need to Dig up the Streets!
For "the first
mile" connectivity, currently popular solutions include DSLs which can
provide performance gains over traditional E1/T1 leased data connections
(2/1.5Mbps lines); however, its top speed of around 8 Mbps falls far short when
compared with fiber’s multi-Gbps speeds. Moreover, DSLs cannot be installed in
offices more than 18,000 feet away from the central switch.
Wireless solutions such as Local Multipoint
Distribution System (LMDS) are offering speeds comparable to FSO, but in most
places they require service providers to obtain licences for use of the radio
spectrum. And since available spectrum is limited, they are also subject to the
capacity limitations besides being vulnerable to rain, storms, etc.
In contrast to the above, FSO or optical wireless
use lasers to transmit beams on to the rooftops or through the windows of
nearby buildings—typically within one to a few kilometers or so. That is
normally sufficient to reach most customers who have data-intensive businesses,
because a large number of the commercial buildings in major cities are concentrated
within a miles’ reach of an optical fiber cable.
Clearing the Fog: Hybrid Systems
As FSO moves closer to reality as an alternative
to fiber in access networks, the biggest snag facing it is the effect of
"fog" which, depending on its density, may drastically cut/shorten
its reach. Though the effect may vary depending on the weather pattern of a
specific city, redundancy or resilience issue needs to be addressed as for any
other reliable telecom network.
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Free Space Optics or Optical Wireless |
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Infrared lasers offer a unique, cost-effective local loop solution: · Across highways, railroads, rivers or other physical barriers to cabling. · Across land not owned by the network operator. · Cabling prohibited by building: due to the owner, physical conditions, regulations. · Dense metropolitan areas where radio spectrum is already crowded. · Waiting for a licence is not possible. · Campus environment: trenching is impossible or prohibitively expensive. · Temporary connectivity required e.g. special events. |
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Key advantages over other wireless alternatives: · Infrared laser communications systems are safe, secure and easy to deploy for building-to-building network connectivity. · Much higher transmission rates (bandwidth) compared to radio or microwave. · Generally no license required to operate. · Highest level of transmission security compared to microwave or spread spectrum.
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One solution being tried is to combine FSO with
microwave radio backup. When the lasers are hampered/blocked due to fog or any
other reason, the microwave would step in and restore the connection like done
in the self-healing fiber-loops. The microwave does not work in heavy rain, but
free space optics would. Together, the two technologies could work together
like a tag wrestling team, guaranteeing that connections stay up for 99.999
percent of the time—the so-called "five nines" reliability target for
telecom networks.
And without affecting the eyes of the humans or
the flocks of birds crossing its path!
Niraj K Gupta, from my cell, April 2001.
Graphics: Courtesy LightPointe.com