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Free Space Optical Communication
Seminar report submitted for the award of the Degree of
Bachelor of Technology in
Electronics and Communication Engineeringof Siksha ‘o’ Anusandhan University
by
Sweta Mohanty
Department of Electronics & Communication Engineering
Institute of Technical Education & Research
Siksha ‘o’ Anusandhan University, Bhubaneswar - 751030
September,2013
DECLARATION
I hereby declare that the report entitled “Free Space
Optical Communication” submitted to Siksha ‘o’
Anusandhan University for the award of the degree of
Bachelor of Technology in Electronics and
Communication Engineering is absolutely based on my
own literature review and extensive survey. Wherever
contributions of others are involved, every effort has been
made to indicate this clearly, with due reference to the
literature. I also declare that this report in the present
form has not been submitted for award of any degree or
diploma or any other academic award anywhere else
before.
i
Sweta Mohanty
Reg. No.: 1011016060, Section: E
Department of
Electronics and Communication Engineering
ABSTRACT
Free Space Optics (FSO) or Optical Wireless, refers to the transmission of
modulated visible or infrared (IR) beams through the air to obtain optical
communications. Like fiber, Free Space Optics (FSO) uses lasers to transmit data, but
instead of enclosing the data stream in a glass fiber, it is transmitted through the air. It
is a secure, cost-effective alternative to other wireless connectivity options. This form
of delivering communication has a lot of compelling advantages.
Data rates comparable to fiber transmission can be carried with very low error
rates, while the extremely narrow laser beam widths ensure that it is possible to co-
locate multiple transceivers without risk of mutual interference in a given location.
FSO has roles to play as primary access medium and backup technology. It could
also be the solution for high speed residential access. Though this technology sprang
into being, its applications are wide and many. It indeed is the technology of the
future...
Keywords: Free space Optics(FSO), Infrared, Optical Communication, Data rates, Error
rates.
ii
CONTENTS
DECLARATION...............................................................................................................................i
ABSTRACT.......................................................................................................................................ii
CONTENTS.....................................................................................................................................iii
(Free Space Optical Communication).............................................................................................5
1. Introduction..........................................................................................................................5
2. Free Space Optics.................................................................................................................6
3. Relevance of FSO in Present Day Communication.............................................................7
4. Origin of FSO.......................................................................................................................8
5. The Technology of FSO.......................................................................................................8
6. The Working of FSO system................................................................................................9
7. Block Diagram of FSO.......................................................................................................10
7.1 The Transmitter ...........................................................................................................11
7.2 The Atmospheric Channel...........................................................................................12
7.3 The Receiver...............................................................................................................15
8. Architectures of FSO..........................................................................................................17
8.1 Point to Point Architecture..........................................................................................17
8.2 Mesh Architecture.......................................................................................................17
8.3 Point to Multi-Point Architecture.............................................................................189.
FSO Security......................................................................................................................18
10. Merits of FSO.....................................................................................................................20
11. Applications of FSO...........................................................................................................21
12. Challenges of FSO.............................................................................................................22
12.1 Fog.............................................................................................................................22
12.2 Physical Obstruction.................................................................................................22
iii
12.3 Scintillation...............................................................................................................22
12.4 Solar Interference......................................................................................................23
12.5 Scattering...................................................................................................................23
12.6 Absorption ................................................................................................................24
12.7 Building Sway/Seismic .......................................................................................2413.
FSO! as a Future Technology.............................................................................................25
14. Conclusion..........................................................................................................................26
15. References..........................................................................................................................27
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Free Space Optical Communication
1. INTRODUCTION
Communication, as it has always been relied and simply depended upon
speed. The faster the means! The more popular, the more effective the
communication is! Presently in the twenty-first century wireless networking is
gaining because of speed and ease of deployment and relatively high network
robustness. Modern era of optical communication originated with the invention of
LASER in 1958 and fabrication of low-loss optical fiber in 1970.
When we hear of optical communications we all think of optical fibers, what we
have today is AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS
or in other words WIRE FREE OPTICS. Free space optics or FSO –Although it
only recently and rather suddenly sprang in to public awareness, free space optics
is not a new idea. It has roots that 90 back over 30 years-to the era before fiber
optic cable became the preferred transport medium for high speed communication.
FSO technology has been revived to offer high band width last mile connectivity
for today’s converged network requirements.
(Free Space Optical Communication)
2. FSO! FREE SPACE OPTICS
Free space optics or FSO, free space photonics or optical wireless, refers to
the transmission of modulated visible or infrared beams through the atmosphere to
obtain optical communication. FSO systems can function over distances of several
kilometers.
FSO is a line-of-sight technology, which enables optical transmission up to
2.5 Gbps of data, voice and video communications, allowing optical connectivity
without deploying fiber optic cable or securing spectrum licenses. Free space optics
require light, which can be focused by using either light emitting diodes (LED) 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 being the medium.
As long as there is a clear line of sight between the source and the destination
and enough transmitter power, communication is possible virtually at the speed of
light. Because light travels through air faster than it does through glass, so it is fair to
classify FSO as optical communications at the speed of light. FSO works on the
same basic principle as infrared television remote controls, wireless keyboards or
wireless palm devices.
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(Free Space Optical Communication)
3. RELEVANCE OF FSO IN PRESENT DAY
COMMUNICATION
Presently we are facing with a burgeoning demand for high bandwidth and
differentiated data services. Network traffic doubles every 9-12 months forcing the
bandwidth or data storing capacity to grow and keep pace with this increase. The
right solution for the pressing demand is the untapped bandwidth potential of optical
communications.
Optical communications are in the process of evolving Giga bits/sec to
terabits/sec and eventually to pentabits/sec. The explosion of internet and internet
based applications has fuelled the bandwidth requirements. Business applications
have grown out of the physical boundaries of the enterprise and gone wide area
linking remote vendors, suppliers, and customers in a new web of business
applications. Hence companies are looking for high bandwidth last mile options. The
high initial cost and vast time required for installation in case of OFC speaks for a
wireless technology for high bandwidth last mile connectivity there FSO finds its
place.
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4. ORIGIN OF FSO
It is said that this mode of communication was first used in the 8 th century by
the Greeks. They used fire as the light source, the atmosphere as the transmission
medium and human eye as receiver.
FSO or optical wireless communication by Alexander Graham Bell in the late
19th century even before his telephone! Bells FSO experiment converted voice
sounds to telephone signals and transmitted them between receivers through free air
space along a beam of light for a distance of some 600 feet, this was later called
PHOTOPHONE. Although Bells photo phone never became a commercial reality, it
demonstrated the basic principle of optical communications.
Essentially all of the engineering of today’s FSO or free space optical
communication systems was done over the past 40 years or so mostly for defense
applications.
5. THE TECHNOLOGY OF FSO
The concept behind FSO is simple. FSO uses a directed beam of light
radiation between two end points to transfer information (data, voice or even video).
This is similar to OFC (optical fiber cable) networks, except that light pulses are sent
through free air instead of OFC cores.
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An FSO unit consists 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 (laser) plus a lens that transmits light through the atmosphere
to another lens receiving information. The receiving lens connects to a high
sensitivity receiver via optical fiber. Two FSO units can take the optical connectivity
to a maximum of 4kms.
6. WORKING OF FSO SYSTEM
Optical systems work in the infrared or near infrared region of light and the
easiest way to visualize how the work is imagine, two points interconnected with
fiber optic cable and then remove the cable. The infrared carrier used for
transmitting the signal is generated either by a high power LED or a laser diode.
Two parallel beams are used, one for transmission and one for reception, taking a
standard data, voice or video signal, converting it to a digital format and transmitting
it through free space.
Today’s modern laser system provide network connectivity at speed of 622
Mega bits/sec and beyond with total reliability. The beams are kept very narrow to
ensure that it does not interfere with other FSO beams. The receive detectors are
either PIN diodes or avalanche photodiodes.
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The FSO transmits invisible eye safe light beams from transmitter to the
receiver using low power infrared lasers in the tera hertz spectrum. FSO can function
over kilometers.
WAVELENGTH:
Currently available FSO hardware is of two types based on the operating
wavelength – 800 nm and 1550 nm. 1550 FSO systems are selected because of
more eye safety, reduced solar background radiation and compatibility with
existing technology infrastructure.
7. FSO BLOCK DIAGRAM
The block diagram of a typical terrestrial FSO link is shown below. Like any
other communication technology, an FSO system essentially comprises the
following three parts: transmitter, channel and receiver. Further discussion of each
of these blocks is given below.
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7.1 The transmitter
This functional block has the primary duty of modulating the source data onto
the optical carrier, which is then propagated through the atmosphere to the receiver.
The most widely used modulation type is the intensity modulation (IM), in which the
source data is modulated onto the irradiance of the optical radiation. This is achieved
by varying the driving current of the optical source directly in sympathy with the
data to be transmitted or via an external modulator, such as the symmetric Mach-
Zehnder interferometer. The use of an external modulator guarantees a higher data
rate than direct modulation, but an external modulator has a nonlinear response.
Other properties of the radiated optical field such as its phase, frequency and state of
polarisation can also be modulated with data/information through the use of an
external modulator. The transmitter telescope collects, collimates and directs the
optical radiation towards the receiver telescope at the other end of the channel.
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7.2 The atmospheric channel
An optical communications channel differs from the conventional Gaussian-
noise channel in that the signal x(t) represents power rather than amplitude. This
leads to two constraints on the transmitted signal: i) x(t) must be non-negative; and
ii) the average value of x(t) must not exceed a specified maximum power Pmax, that
is . In contrast to the conventional channels, where the signal-to-noise ratio (SNR) is
proportional to the power, in optical systems the received electrical power and the
variance of the shot noise are proportional to Ad2 and Ad, respectively, where Ad is
the receiver detector area. Thus, for a shot noise limited optical system, the SNR is
proportional to Ad. This implies that for a given transmitted power, a higher SNR
can be attained by using a large area detector. However, as Ad increases so does its
capacitance, which has a limiting effect on the receiver bandwidth.
The atmospheric channel consists of gases, and aerosols – tiny particles
suspended in the atmosphere. Also present in the atmosphere are rain, haze, fog and
other forms of precipitation. The amount of precipitation present in the atmosphere
depends on the location (longitude and latitude) and the season. The highest
concentration of particles is obviously near the Earth surface within the troposphere
and this decreases with increasing altitude up through to the ionosphere. With the
size distribution of the atmospheric constituents ranging from sub-micrometres to
centimetres, an optical field that traverses the atmosphere is scattered and/or
absorbed resulting in power loss.
Another feature of interest is the atmospheric turbulence. When radiation
strikes the Earth from the Sun, some of the radiation is absorbed by the Earth’s
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(Free Space Optical Communication)
surface thereby heating up its (Earth’s) surface air mass. The resulting mass of warm
and lighter air then rises up to mix turbulently with the surrounding cooler air mass.
This culminates in small (in the range of 0.01 to 0.1 degrees) but spatially and
temporally fluctuating atmospheric temperature. The temperature inhomogeneity of
the atmosphere causes corresponding changes in the index of refraction of the
atmosphere, resulting in eddies, cells or air packets having varying sizes from ~0.1
cm to ~10 m. These air packets act like refractive prisms of varying indices of
refraction. The propagating optical radiation is therefore fully or partially deviated
depending on the relative size of the beam and the degree of temperature
inhomogeneity along its path. Consequently the optical radiation traversing the
turbulent atmosphere experiences random variation/fading in its irradiance
(scintillation) and phase. Familiar effects of turbulence are the twinkling of stars
caused by random fluctuations of stars. irradiance, and the shimmer of the horizon
on a hot day caused by random changes in the optical phase of the light beam
resulting in reduced image resolution .Atmospheric turbulence depends on i)
atmospheric pressure/altitude, ii) wind speed, and iii) variation of index of refraction
due to temperature inhomogeneity.
Known effects of atmospheric turbulence include:-
a) Beam steering – Angular deviation of the beam from its original LOS causing
the beam to miss the receiver.
b) Image dancing – The received beam focus moves in the image plane due to
variations in the beam.s angle of arrival.
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(Free Space Optical Communication)
c) Beam spreading – Increased beam divergence due to scattering. This leads to a
reduction in received power density.
d) Beam scintillation – Variations in the spatial power density at the receiver plane
caused by small scale destructive interference within the optical beam.
e) Spatial coherence degradation – Turbulence also induces losses in phase
coherence across the beam phase fronts. This is particularly deleterious for
photomixing (e.g. in coherent receiver) .
f) Polarisation fluctuation – This results from changes in the state of polarisation
of the received optical field after passing through a turbulent medium. However
for a horizontally travelling optical radiation, the amount of polarisation
fluctuation is negligible.
The modelling of the fluctuation of an optical radiation traversing a turbulent
atmosphere will be examined in Chapter Four, with the view to understanding the
statistical behaviour of the signal received at the receiver plane.
14
(Free Space Optical Communication)
7.3 The receiver
This essentially helps recover the transmitted data from the incident optical
field. The receiver is composed of the following:-
a) Receiver telescope – collects and focuses the incoming optical radiation onto the
photodetector. It should be noted that a large receiver telescope aperture is desirable
as it collects multiple uncorrelated radiations and focuses their average on the
photodetector. This is referred to as aperture averaging but a wide aperture also
means more background radiation/noise.
b) Optical bandpass filter – to reduce the amount of background radiation.
c) Photodetector – p-i-n diode (PIN) or avalanche photodiode (APD) that converts
the incident optical field into an electrical signal. Germanium only detectors are
generally not used in FSO because of their high dark current.
d) Post-detection processor/decision circuit – this is where the required
amplification, filtering and signal processing necessary to guarantee a high fidelity
data recovery are carried out.
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(Free Space Optical Communication)
FSO TRANSMITTER
FSO RECEIVER
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8. FSO ARCHITECTURES
8.1 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..
8.2 MESH ARCHITECTURE
Mesh architectures may offer redundancy and higher reliability with easy
node addition but restrict distances more than the other options.
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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.
8.3 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.
9. FREE SPACE OPTICS (FSO) SECURITY
Security is an important element of data transmission, irrespective of the
network topology. It is especially important for military and corporate applications.
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Building a network on the SONA beam platform is one of the best ways to ensure
that data transmission between any two points is completely secure. Its focused
transmission beam foils jammers and eavesdroppers and enhances security.
Moreover, SONAR systems can use any signal-scrambling technology that optical
fiber can use.
The common perception of wireless is that it offers less security than wire
line 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.
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10. MERITS OF FSO
1. Free space optics offers a flexible networking solution that delivers on the
promise of broadband.
2. Straight forward deployment-as it requires no licenses.
3. Rapid time of deployment.
4. Low initial investment.
5. Ease of installation even indoors in less than 30 minutes.
6. Security and freedom from irksome regulations like roof top rights and
spectral licenses.
7. Re-deploy ability.
Unlike radio and microwave systems FSO is an optical technology and
no spectrum licensing or frequency co-ordination with other users is required.
Interference from or to other system or equipment is not a concern and the point to
point laser signal is extremely difficult to intercept and therefore secure. Data rate
comparable to OFC can be obtained with very low error rate and the extremely
narrow laser beam which enables unlimited number of separate FSO links to be
installed in a given location.
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11. APPLICATIONS OF FSO
Optical communication systems are becoming more and more popular as
the interest and requirement in high capacity and long distance space
communications grow. FSO overcomes the last mile access bottleneck by sending
high bit rate signals through the air using laser transmission.
Applications of FSO system are many and varied but a few can be listed.
1. Military and government: Secure and undetectable FSO systems can
connect large areas safely with minimal planning and deployment time
2. Wireless service provider: unlike microwaves or fiber ,FSO does not require
spectrum licensing, physical disruption to a location or government zoning
approval. Carriers are free to grow their business
3. Enterprise connectivity: As FSO links can be installed with ease, they
provide a natural method of interconnecting LAN segments that are housed in
buildings separated by public streets or other right-of-way property.
4. Fiber backup: FSO can 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: Instant services to the customers before fiber being
laid.
21
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12. FSO CHALLENGES
The advantages of free space optics come without some cost. As the medium
is air and the light pass through it, some environmental challenges are inevitable.
12.1 FOG
Fog substantially attenuates visible radiation, and it has a similar affect
on the near-infrared wavelengths that are employed in FSO systems. Rain and snow
have little effect on FSO. Fog being microns in diameter, it hinder the passage of
light by absorption, scattering and reflection. Dealing with fog – which is known as
Mie scattering, is largely a matter of boosting the transmitted power. Fog can be
countered by a network design with short FSO link distances. FSO installation in
foggy cities like San Francisco has successfully achieved carrier-class reliability.
12.2 PHYSICAL OBSTRUCTIONS
Flying birds can temporarily block a single beam, but this tends to cause
only short interruptions and transmissions are easily and automatically re-assumed.
Multi-beam systems are used for better performance.
12.3 SCINTILLATION
Scintillation refers the variations in light intensity caused by atmospheric
turbulence. Such turbulence may be caused by wind and temperature gradients
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which results in air pockets of varying diversity act as prisms or lenses with time
varying properties. This scintillation affects on FSO can be tackled by multi beam
approach exploiting multiple regions of space- this approach is called spatial
diversity.
12.4 SOLAR INTERFERENCE
This can be combated in two ways:
The first is a long pass optical filter window used to block all wavelengths
below 850nm from entering the system.
The second is an optical narrow band filter proceeding the receive detector
used to filter all but the wavelength actually used for intersystem
communications.
12.5 SCATTERING
Scattering is caused when the wavelength collides with the scatterer.
The physical size of the scatterer determines the type of scattering.
When the scatterer is smaller than the wavelength-Rayleigh scattering.
When the scatterer is of comparable size to the wavelength -Mie scattering.
When the scatterer is much larger than the wavelength-Non-selective
scattering
In scattering there is no loss of energy, only a directional re-distribution of
energy which may cause reduction in beam intensity for longer distance.
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12.6 ABSORPTION
Absorption occurs when suspended water molecules in the terrestrial
atmosphere extinguish photons. This causes a decrease in the power density 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 helps to maintain the required level of network availability.
12.7 BUILDING SWAY / SEISMIC ACTIVITY
One of the most common difficulties that arises when deploying FSO
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 spread, forming optical cones
which can take many perturbations.
Active tracking is based on movable mirrors that control the direction in which
beams are launched.
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13. FSO! AS A FUTURE TECHNOLOGY
Infrared technology is as secure or cable applications and can be more
reliable than wired technology as it obviates wear and tear on the connector
hardware. In the future it is forecast that this technology will be implemented in
copiers, fax machines, overhead projectors, bank ATMs, credit cards, game consoles
and head sets. All these have local applications and it is really here where this
technology is best suited, owing to the inherent difficulties in its technological
process for interconnecting over distances.
Outdoors two its use is bound to grow as communications companies,
broadcasters and end users discovers how crowded the radio spectrum has become.
Once infrared’s image issue has been overcome and its profile raised, the medium
will truly have a bright, if invisible, future!
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14. CONCLUSION
FSO enables optical transmission of voice video and data through air at very
high rates. It has key roles to play as primary access medium and backup
technology. Driven by the need for high speed local loop connectivity and the cost
and the difficulties of deploying fiber, the interest in FSO has certainly picked up
dramatically among service providers world wide. Instead of fiber coaxial systems,
fiber laser systems may turn out to be the best way to deliver high data rates to your
home. FSO continues to accelerate the vision of all optical networks cost effectively,
reliably and quickly with freedom and flexibility of deployment.
15.REFERENCES
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[1] Wasiu Oyewole Papoola, “Subcarrier intensity modulated free-space optical communication systems”, School of Computing, Engineering and Information Sciences, University of Northumbria at Newcastle for the degree of Doctor of Philosophy,September 2009
[2] Manzur, T. “Free Space Optical Communications (FSO)”Avionics, Fiber-Optics
and Photonics Technology Conference, 2007 IEEE
[3] Juarez, J.C. ; Dwivedi, A. ; Hammons, A.R. ; Jones,S.D. ;Weerackody, V. ; Nichols,R.A. “FreeSpace Optical Communications for Nextgeneration Military Networks”Communications Magazine, IEEE Volume: 44 , Issue: 11
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