14-760: adv. real-world networking

14-760:

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.


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


TYPES OF FIBER

• Single “mode”, e.g. single beam of light

• Achieved via tiny core

• Very limited modal dispersion


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


• 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


IRIDIUM: FREQUENCY DIVISION

• Frequency re-use factor of 12

• 20 channels per cluster

• 240 channels / 12 cells/cluster = 20 channels/cluster


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.


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

14-760: adv. real-world networking

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