CS 453 Computer Networks Lecture 6 Layer 1 – Physical Layer.

CS 453CS 453Computer NetworksComputer Networks

Lecture 6Lecture 6

Layer 1 – Physical LayerLayer 1 – Physical Layer

Physical LayerPhysical Layer

LasersLasers Directional - point to point linksDirectional - point to point links Line of siteLine of site Equipment relatively cheapEquipment relatively cheap No right of way, no license (not so for No right of way, no license (not so for

microwave)microwave) Very high bandwidthVery high bandwidth Very low error rates (in good conditions)Very low error rates (in good conditions) No EMF interferenceNo EMF interference

Physical Layer - LasersPhysical Layer - Lasers

Unidirectional – each side of link requires Unidirectional – each side of link requires a laser transmitter and receivera laser transmitter and receiverGreat for cross campus, metropolitan Great for cross campus, metropolitan applicationsapplicationsCan be difficult to targetCan be difficult to target Defocusing is sometimes used to make Defocusing is sometimes used to make

targeting easiertargeting easier Targeting gets to be a bigger problem at Targeting gets to be a bigger problem at

greater distancesgreater distances


Wind, building movement, heat convection Wind, building movement, heat convection can disturb the laser beamcan disturb the laser beam

Works better in flat country than hilly Works better in flat country than hilly countrycountry

Can be severely degraded by heavy Can be severely degraded by heavy weather or fogweather or fog

Often used with lower bandwidth radio link Often used with lower bandwidth radio link for backupfor backup


Sample products Sample products Full duplex 100 Mbps – 150 metersFull duplex 100 Mbps – 150 meters Full duplex 100 Mbps – 500 metersFull duplex 100 Mbps – 500 meters Full duplex 100 Mbps – 1000 metersFull duplex 100 Mbps – 1000 meters Full duplex 1 Gbps – 200 metersFull duplex 1 Gbps – 200 meters Full duplex 1 Gbps – 2500 metersFull duplex 1 Gbps – 2500 meters Full duplex 100 Mbps – 2500 metersFull duplex 100 Mbps – 2500 meters

From: http://www.laserbandwidth.com/laser_products.htm


Lasers have very high data ratesLasers have very high data rates

Can propagate signals over long distancesCan propagate signals over long distances

Can create very long data links…Can create very long data links…

If line of site and…If line of site and…

……especially if you can mitigate the especially if you can mitigate the atmospheric effects that can degrade atmospheric effects that can degrade performance.performance.

… … But how far?…But how far?…


The European Space Agency (ESA) established The European Space Agency (ESA) established an experimental laser communications links an experimental laser communications links between …between …

An orbiting satellite and an airborne aircraft An orbiting satellite and an airborne aircraft flying at 20,000 and 33,000 ft.flying at 20,000 and 33,000 ft.

The link range distance was about 25,000 milesThe link range distance was about 25,000 miles From : From :

http://www.spacemart.com/reports/First_satellite-http://www.spacemart.com/reports/First_satellite-airplane_laser_link_made.htmlairplane_laser_link_made.html


MIT and NASA are working on the Mars Laser MIT and NASA are working on the Mars Laser Communication Demonstration ProjectCommunication Demonstration Project……a planned test in 2010a planned test in 2010““the first deep-space laser communication link “the first deep-space laser communication link “Data rates 10 times faster than existing radio Data rates 10 times faster than existing radio based space communications linksbased space communications linksPlan to get data rates of 1 Mbps (maybe as high Plan to get data rates of 1 Mbps (maybe as high as 30 Mbs)as 30 Mbs)Current max is about 128,000 bpsCurrent max is about 128,000 bpsUse multiple earth terminals to “work around” Use multiple earth terminals to “work around” weatherweather

Physical Layer- SatellitesPhysical Layer- Satellites

Communications SatellitesCommunications Satellites

The first communications satellites…The first communications satellites…

Arthur C. Clarke (2001: A Space Odyssey)Arthur C. Clarke (2001: A Space Odyssey) in 1945 in 1945 Designed a system of geosynchronous Designed a system of geosynchronous

satellites for communicationssatellites for communications Satellites used vacuum tube powered relaysSatellites used vacuum tube powered relays Satellites were mannedSatellites were manned

See: http://lakdiva.org/clarke/1945ww/

Physical Layer - SatellitesPhysical Layer - Satellites

……OK, not reallyOK, not really

But it was a proposalBut it was a proposal


First real satellites – First real satellites – experimentalexperimentalEcho 1 (actually Echo 1 (actually Echo 1A)Echo 1A)Metalized weather Metalized weather balloonballoon100 feet in diameter100 feet in diameterLauched in 1960Lauched in 1960NASA bounced NASA bounced microwaves off of itmicrowaves off of it

From: http://en.wikipedia.org/wiki/Echo_satellite


U.S. Navy developed a system that used the U.S. Navy developed a system that used the moon as a reflector for radio communicationsmoon as a reflector for radio communications Used for ship-to-shore communicationsUsed for ship-to-shore communications

Real man-made communications satellites had Real man-made communications satellites had to wait for better technology – less power to wait for better technology – less power consumption, more amplificationconsumption, more amplification

First active satellite used for data First active satellite used for data communications and telephone was Telstar communications and telephone was Telstar (1960) from: http://en.wikipedia.org/wiki/Telstar(1960) from: http://en.wikipedia.org/wiki/Telstar

Physical Layer – SatellitesPhysical Layer – Satellites

Keplar’s third law Keplar’s third law The orbital period of a satellite is a function of The orbital period of a satellite is a function of

radius of the orbit to 3/2 powerradius of the orbit to 3/2 power The space shuttle is in LEO – Low Earth The space shuttle is in LEO – Low Earth

Orbit…Orbit… … … it has a short orbital period…it has a short orbital period… … … about 90 minutesabout 90 minutes


So, as the radius of the orbit gets larger So, as the radius of the orbit gets larger (i.e. the satellite gets further away)…(i.e. the satellite gets further away)…

The orbital period gets longerThe orbital period gets longer

At an orbit radius of about 22,000 miles or At an orbit radius of about 22,000 miles or 35,800 km the orbital period is 24 hours…35,800 km the orbital period is 24 hours…

So, what happensSo, what happens

And why is this a good thing?And why is this a good thing?


Three classes of satellites based on their Three classes of satellites based on their orbital positionorbital position GEO – Geostationary Earth OrbitGEO – Geostationary Earth Orbit MEO – Medium Earth OrbitMEO – Medium Earth Orbit LEO – Low Earth OrbitLEO – Low Earth Orbit


From: Tanenbaum (2003)


Geostationary Earth Orbit Geostationary Earth Orbit Geosynchronous SatellitesGeosynchronous Satellites Stay in fixed position over the earthStay in fixed position over the earth No need to track themNo need to track them

Satellites can do broad beam transmission Satellites can do broad beam transmission or spot beam transmissionor spot beam transmission

Receive on one frequency – transmit on a Receive on one frequency – transmit on a different frequencydifferent frequency

Physical Layer - SatellitesPhysical Layer - SatellitesFrequency bands assigned to guard against Frequency bands assigned to guard against interference with microwave transmissionsinterference with microwave transmissions

Physical Layer - SatellitePhysical Layer - Satellite

VSAT - Very Small Aperture TerminalsVSAT - Very Small Aperture Terminals Uses very small antennas - one meter or Uses very small antennas - one meter or

smallersmaller Widely used for dataWidely used for data Cheap, easy to install, setupCheap, easy to install, setup Low powerLow power Uses ground stations for relaysUses ground stations for relays

““Star” topologyStar” topology Fairly good data rates (up to 4 Mbps), but Fairly good data rates (up to 4 Mbps), but

poor latencypoor latency

Physical Layer - SatellitesPhysical Layer - SatellitesYou see these everywhereYou see these everywhere

http://www.vsat-systems.com/satellite-internet/how-it-works.html

http://www.es.northropgrumman.com/SATCOM/vsat/vsat.htm

Physical Layer - SatellitePhysical Layer - Satellite

MEO – Medium Earth OrbitMEO – Medium Earth Orbit Short orbital periodShort orbital period Must be trackedMust be tracked GPS satellitesGPS satellites


LEO – Low Earth OrbitLEO – Low Earth Orbit Requires a lot of satellites because the orbital Requires a lot of satellites because the orbital

period is short …period is short … And they zip in and out of viewAnd they zip in and out of view However, because of the LEO they require However, because of the LEO they require

little power and have small communication little power and have small communication latencieslatencies


IridiumIridium Motorola and investors proposed in 1990Motorola and investors proposed in 1990 … … 77 LEO satellites 77 LEO satellites For voice communications in remote placesFor voice communications in remote places Launched satellites (scaled back to 66)Launched satellites (scaled back to 66) Went operational in 1998Went operational in 1998 Went bankrupt in 1999Went bankrupt in 1999


IridiumIridium Bought by new investorsBought by new investors Restarted service in 2001Restarted service in 2001 Worldwide communications servicesWorldwide communications services

Maritime, aviation, jungles, mountains, etc.Maritime, aviation, jungles, mountains, etc.


IridiumIridium Comm traffic is routed satellite to satelliteComm traffic is routed satellite to satellite Orbit a little under 500 milesOrbit a little under 500 miles Polar orbitPolar orbit Each satellite has 48 spot beamsEach satellite has 48 spot beams Combines 1628 beams cover the earth’s Combines 1628 beams cover the earth’s

surfacesurface Each satellite has 3840 channelsEach satellite has 3840 channels 252,440 channels total252,440 channels total


GlobalstarGlobalstar Uses 48 satellitesUses 48 satellites While Iridium routes traffic from satellite to While Iridium routes traffic from satellite to

satellitesatellite Globalstar routes traffic through ground Globalstar routes traffic through ground

stationsstations


TeledesicTeledesic Created by Bill Gates and Craig McCawCreated by Bill Gates and Craig McCaw Goal to provide high speed internet access to Goal to provide high speed internet access to

…… To everyone, regardless of location…To everyone, regardless of location… Using small VSAT type antennaUsing small VSAT type antenna 30 satellites – large footprints30 satellites – large footprints Packet switching in spacePacket switching in space Bandwidth dynamically allocated on requestBandwidth dynamically allocated on request Not “off the ground” Not “off the ground”


What’s all mean?What’s all mean?

CS 453 Computer Networks Lecture 6 Layer 1 – Physical Layer.

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