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ADV. REAL-WORLD NETWORKING
LECTURE 7 * FIBEROPTICS AND SATELLITE* SPRING 2020 * KESDEN
FIBEROPTICS
FIRST MODULATED COMMUNICATION BY LIGHT:PHOTOPHONE © 1880
• Invented by Alexander Graham Bell &
Charles Tainter in 1880
• Voice modulated mirrors to send
modulated light which was received and
sensed using a selenium photoresistor and
subsequently turned into sound.
https://commons.wikimedia.org/w/index.php?curid=9508996https://commons.wikimedia.org/w/index.php?curid=9508945
FIBEROPTICS: MODERN DEVELOPMENT
• 1950s and 1960s: Glass can transmit light and be made pure enough to do it with little loss
• 1970s: Initial experimental deployments of fiberoptics for telephone system
• 1980s: Systems matured into inter-continental use
• 1990s: Improvements in lasers and fiber composition extended distance between runs to
100+km and increase data rates
• Bandwidth doubled every 6 months during the 1990s
• 2000s: Optical amplifiers directly amplify signal without converting to electric and back
• Current work: Extending wavelength
lower index
of refraction
core
cladding
(note: minimum bend radius of a few cm)
RAY PROPAGATION
MODAL DISPERSION
• Refraction isn’t perfect consistent
• If using different modes, e.g. lights, the
groups don’t stay perfectly together.
• If they are too close, they come together.
http://www.thefoa.org/tech/ref/basic/fiber.html
CHROMATIC DISPERSION
• Glass fiber acts as a prism
• The index of refraction is
dependent on wavelength, e.g. color
• Paths take different lengths.
http://www.thefoa.org/tech/ref/basic/fiber.html
TYPES OF FIBER
• Earliest fiber design
• Light bounces around
• Takes longer or shorter paths
• Dispersion is a significant limitation
• Basically just used for short links between
consumer electronics deviceshttp://www.thefoa.org/tech/ref/basic/fiber.html
TYPES OF FIBER
• Index profile is graded in steps
• These provide grades of refraction
• Bend light more straight.
• Minimizes modal dispersion
http://www.thefoa.org/tech/ref/basic/fiber.html
TYPES OF FIBER
• Single “mode”, e.g. single beam of light
• Achieved via tiny core
• Very limited modal dispersion
http://www.thefoa.org/tech/ref/basic/fiber.html
1000
wavelength (nm)
loss(dB/km)
1500
0.0
0.5
1.0
tens of THz
1.3
1.55
LIGHT TRANSMISSION IN SILICA FIBER
ATTENUATION: ABSORPTION AND SCATTERING
http://www.thefoa.org/tech/ref/basic/fiber.html
• Absorption: Light turns into heat, e.g. via
water molecules in glass.
• Scattering: Bounces in ways other than
basic refraction index.
⚫ Send multiple wavelengths through the same fiber.
– Multiplex and demultiplex the optical signal on the
fiber
⚫ Each wavelength represents an optical carrier that can carry a separate signal.
– E.g., 16 colors of 2.4 Gbit/second
⚫ Like radio, but optical and much faster
Optical
Splitter
Frequency
WAVELENGTH DIVISION MULTIPLEXING
CAN WDM BE USED OVER SINGLE MODE FIBER?
• Yes.
• Multiple colors travel along narrow beam
• Single mode isn’t single wavelength
OPTICAL ELECTRICAL INTERFACE
• Needed to convert light into electrons
• Now mostly just needed at end points
• In the past needed for amplification
• Significant source of latency.
• Clock bits on, process, clock bits off
PURELY OPTICAL AMPLIFIERS
• Erbium Doped Fiber Amplifier (EDFA)
• A laser is used to excite erbium ions in the doped material
• The input signal passes though the excited area, causing the excited ions transition back
to a lower energy state
• The energy lost by that transition is in the form of photons which are collected by the
input signal
• These photons have the same phase and direction as the original signal
SURPRISING FACT:
• Copper – 2.3 × 108 m/s
• Fiber – 2.0 × 108 m/s
• The speed of light in copper is 15% faster than through fiber optics
FIBER OPTIC VS COPPER LATENCY
• Fiber optics have dramatically lower loss
• Fiber optics require less frequent amplification
• Fiber optics suffer less delay from being read, processed, and repeated.
QUESTION?
• If fiberoptics can send light more efficiently than copper wire, can they send energy via
light more efficiently than copy wire?
• Why or why not?
QUESTION ANSWERED
• Yes, BUT…
• Total end-to-end, electrical-to-electrical efficiency is probably about 20-30%
• Optical-to-Electrical/Electrical-to-Optical is probably 40-50% efficient.
• Not very efficient
• But, good in specialized applications
• Non-electrically conductive
• EM interference
• Higher temperature (Glass vs copper)
• Etc.
https://www.rp-photonics.com/power_over_fiber.html
SATELLITE
WHY DO WE NEED SATELLITES?
Terrestrial Radio Orbital Radio
ORBITS: LOW EARTH ORBIT (LEO)
• 1200 miles or less above surface
• Approximately 90 minutes to orbit Earth
• Less energy to launch
• Less energy to transmit
• Higher resolution scanning of earth
• Higher data rates and lower latency, generally
• Move quickly across earth’s surface, “covering a lot of ground” quickly.
• Applications
• All (human occupied) space stations
• Most observational satellites, e.g. photographing, spying, etc
• Constellation-based communication systems, e.g. Iridium
• Upside: Reach everywhere
• Downside: Many hand-offs
ORBITS: MEDIUM EARTH ORBIT (MEO)
• 1200 to 22,236 miles above surface
• 2 – 8 hours to orbit earth
• Larger coverage areas, lower resolution/throughput, higher latency
• Higher cost
• Applications
• Same basic uses as LEO
• Except – not used for (human occupied) space stations
ORBITS: GEOSTATIONARY ORBIT
• 22,236 miles above Earth’s surface, specifically the equator
• Rotates along with Earth, maintaining constant relative position
• Can’t be seen near poles
• Area of coverage is constant
• Antennas don’t even need to change angle to maintain contact
• Less stable orbit (asymmetry of earth causes longitudinal drift)
• Fuel required to maintain orbit
• Fixed number of satellite slots
• One ring
• Minimum spacing between satellites
• Best application
• Latency tolerant communication that can’t be interrupted
• 1/8 second each way, 1/4 secondround trip – not so good for humans
• Non-interactive data don’t care
ORBITS: MOLNIYA
• Used mainly by Russians
• Communication in Russian and former Soviet areas
• Spying on US/Canada, esp. early warning of missile launches
• Orbit is about 12 hours
• 6 – 9 hours of use per every other revolution
• The other revolution it is around wrong pole
• 3 satellites provide 24 hour coverage
https://en.wikipedia.org/wiki/Molniya_orbit
SATELLITE MODES AND MULTIPLEXING
• “Bent pipe”
• Transmissions come from Earth to satellite and are broadcast back down
• Addressed communication or some form of multiplexing
• Requires a ground station in each satellite coverage area, possibly with ground relaying
• Intersatellite links
• Novel with Iridium
• Satellites relay messages through space to ground station(s)
SATELLITE BASED TELEPHONE: A PERSPECTIVE
• 1856 first trans-Atlantic telegraph line
• 1866 – the first one that worked for more than a month
• 1956 TAT-1, first trans-Atlantic telephone line came into service
• 36 channels a 4khz, later reallocated to 48 channels at 3khz
• 1969 Intelsat, global organization promoting and organizing satellite usage, had enough coverage to
connect the globe
• 1978 TAT-1 retired, but by then there were plenty, plenty more
• Satellite-based long distance overlaps cable-based long distance.
• Greater latency means lower quality
• Reaches places cables didn’t or don’t.
SATELLITE PHONES TODAY
• Used for global coverage
• Remote areas
• Example: Iridium
• Low earth orbit
• Relatively low call quality
• Interruptions due to satellite movement
• But, satellite will come to you – even if restricted view of the sky from terrain, etc
• Most other providers
• Geostationary
• Often higher call quality and/or data rate
• Not truly global, depends on satellites
• Line-of-site can be a problem in some terrain
SATELLITE-BASED BROADCAST
• Very natural for many-to-one communications
• Television, e.g. DirectTV
• Radio, e.g. SiriusXM
SATELLITE BASED RADIO
• SiriusXM
• Geostationary satellites
• 2x XM
• 2x Sirius
• 2 spare
SATELLITE TV
• 1960s - mid-1970s: Experiments, small deployments
• 1970s: Networks used satellites to distribute their content from each other
• 1976: Taylor Howard became world’s first telephone pirate
• Built a receiving dish to get HBO in his back yard.
• Tried to pay HBO, but they didn’t want to deal with individuals
• He wrote a book describing how, and stealing satellite TV became “a thing” until providers scrambled it.
• 1980s: Distributors began scrambling content and eventually, under regulatory pressure, agreed
to sell descramblers and subscriptions – but they sure weren’t cheap
• By 1986: Distributors scrambling signals was putting satellite folks out of work
• John MacDougall lost his business installing and servicing satellites and was working as a part-time
operator at a satellite company
• He pointed their satellite at the one carrying HBO’s content, over-rode their signal, and delivered a
message for 90 seconds
• By the 1990s, most satellite TV, e.g. Dish, was digital, not analog
IRIDIUM EXAMPLE
• Constellation of satellites in low earth orbit
• 77 were originally planned, Iridium element has an atomic number of 77
• Only 66 proved necessary in practice. Current constellation is 66 + spares
• 100 minute orbit
• Low orbit means low latency and low power, esp. for handsets.
http://www.icao.int/anb/panels/acp/wg/m/iridium_swg/ird-08/ird-swg08-ip05 - ams(r)s
manual part ii v4.0.pdf (Figure 2-2, Page 5)
IRIDIUM EXAMPLE
• 3 phase beam antennas produce 48 spot beams on earth
• 48-beam configuration provides a 4,700 km radius
• When satellites “cramp” due to orbit, outer rings are
deactivated.
• 4 crosslink antennas to route traffic through
constellation
• This is magic. Only one grounds station needed
• Others for backup, US military, etc.
• Each satellites supports up to 1100 phones calls at
2400bps.
http://www.icao.int/anb/panels/acp/wg/m/iridium_swg/ird-08/ird-swg08-ip05 - ams(r)s manual part ii v4.0.pdf (Figure 2-3, Page 6)
IRIDIUM: INTERSATELLITE LINKS
• Relay messages along
“intersatellite links (ISL)” from
one satellite to another
• Routing among satellites is a
complex, dynamic shortest-
path like problem
• To my knowledge, algorithm
has never been published
• Very much a special sauce,
and likely a secret one.
http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf
IRIDIUM: COMPLETING CALLS
• Handset turns on and transmits “Ready to receive”
• Routing across inter-satellite links as needed and down to subscriber’s home Earth gateway
• Now home gateway knows about handset’s location, etc.
• All call setup goes to/from a user, including satellite to satellite go through home gateway
• On hook, Off hook, ringing, forwarding, etc
• Satellite-to-satellite call data need not pass through Earth
IRIDIUM: MOTION
• Satellites move quickly
• About 16655 Mph
• Users are relatively stationary – even if moving in an airplane
• Commercial plane is about 3% of satellite speed
• Orbit of each satellite is about 100 minutes
• Constellation is the same over Earth about every 1,440 minutes
• Satellites are in approximately the same position and direction over sky every 24 hrs
• Worse case for 1 satellite failure is 37 minute outage/24 hrs
IRIDIUM: FREQUENCY DIVISION
• 1616 – 1625.5Mhz Frequencies
• 10.5Mhz bandwidth
• 240 channels @ 41.67Khz each
• 500 Khz of guard bandwidth, ~ 2k between each pair of channels
http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf
IRIDIUM: FREQUENCY DIVISION
• Frequency re-use factor of 12
• 20 channels per cluster
• 240 channels / 12 cells/cluster = 20 channels/cluster
http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf
IRIDIUM: TIME DIVISION
• Time Division Multiple Access (TDMA)
• 90ms frame
• 4 full-duplex channels
• 50kbs
• Voice is 4800bps
• 2400bps each way
• 4800 b/s * 90ms = 432 bits/user/time slot
• 432 bits/sec / 50 kb/s = 8.64 ms
• 8 simultaneous users
• 8 users * 8.64 ms = 69.12 ms for user data frames
• 20.88ms for framing, etc.
http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf
IRIDIUM: CAPACITY
• 3,168 users per satellite
• 48 cells/satellite * 66 satellites
• Reduced to 2,150 due to overlapping satellites
• 2,150 users/satellite
• 80 users/cell
• 20 frequencies / cell
• 8 users/ frequency
• 172,000 simultaneous users
• Maximum theoretical
IRIDIUM NEXT
• Launched over last 2 years
• SpaceX did the launches
• 12 satellites, 12 months (Wow!)
• Satellite construction < 5 weeks/satellite (Wow!)
• 66 satellites + 6 orbit spares + 9 ground spares
• Same basic architecture
• Backward compatible
• Additional frequency space
• Speeds up to 1.4Mbps
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