CS 453 Computer Networks Lecture 6 Layer 1 – Physical Layer

Physical Layer Lasers Directional - point to point links Directional - point to point links Line of site Line of site Equipment relatively cheap Equipment relatively cheap No right of way, no license (not so for microwave) No right of way, no license (not so for microwave) Very high bandwidth Very high bandwidth Very low error rates (in good conditions) Very low error rates (in good conditions) No EMF interference No EMF interference

Physical Layer - Lasers Unidirectional – each side of link requires a laser transmitter and receiver Great for cross campus, metropolitan applications Can be difficult to target Defocusing is sometimes used to make targeting easier Defocusing is sometimes used to make targeting easier Targeting gets to be a bigger problem at greater distances Targeting gets to be a bigger problem at greater distances

Physical Layer - Lasers Wind, building movement, heat convection can disturb the laser beam Works better in flat country than hilly country Can be severely degraded by heavy weather or fog Often used with lower bandwidth radio link for backup

Physical Layer - Lasers Sample products Full duplex 100 Mbps – 150 meters Full duplex 100 Mbps – 150 meters Full duplex 100 Mbps – 500 meters Full duplex 100 Mbps – 500 meters Full duplex 100 Mbps – 1000 meters Full duplex 100 Mbps – 1000 meters Full duplex 1 Gbps – 200 meters Full duplex 1 Gbps – 200 meters Full duplex 1 Gbps – 2500 meters Full duplex 1 Gbps – 2500 meters Full duplex 100 Mbps – 2500 meters Full duplex 100 Mbps – 2500 meters From:

Physical Layer - Lasers Lasers have very high data rates Can propagate signals over long distances Can create very long data links… If line of site and… …especially if you can mitigate the atmospheric effects that can degrade performance. … But how far?…

Physical Layer - Lasers The European Space Agency (ESA) established an experimental laser communications links between … An orbiting satellite and an airborne aircraft flying at 20,000 and 33,000 ft. The link range distance was about 25,000 miles From : airplane_laser_link_made.html From : airplane_laser_link_made.html

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

Physical Layer- Satellites Communications Satellites The first communications satellites… Arthur C. Clarke (2001: A Space Odyssey) in 1945 in 1945 Designed a system of geosynchronous satellites for communications Designed a system of geosynchronous satellites for communications Satellites used vacuum tube powered relays Satellites used vacuum tube powered relays Satellites were manned Satellites were manned See:

Physical Layer - Satellites …OK, not really But it was a proposal

Physical Layer - Satellites First real satellites – experimental Echo 1 (actually Echo 1A) Metalized weather balloon 100 feet in diameter Lauched in 1960 NASA bounced microwaves off of it From:

Physical Layer - Satellites U.S. Navy developed a system that used the moon as a reflector for radio communications Used for ship-to-shore communications Used for ship-to-shore communications Real man-made communications satellites had to wait for better technology – less power consumption, more amplification First active satellite used for data communications and telephone was Telstar (1960) from:

Physical Layer – Satellites Keplar’s third law The orbital period of a satellite is a function of radius of the orbit to 3/2 power The orbital period of a satellite is a function of radius of the orbit to 3/2 power The space shuttle is in LEO – Low Earth Orbit… The space shuttle is in LEO – Low Earth Orbit… … it has a short orbital period… … it has a short orbital period… … about 90 minutes … about 90 minutes

Physical Layer - Satellites So, as the radius of the orbit gets larger (i.e. the satellite gets further away)… The orbital period gets longer At an orbit radius of about 22,000 miles or 35,800 km the orbital period is 24 hours… So, what happens And why is this a good thing?

Physical Layer - Satellites Three classes of satellites based on their orbital position GEO – Geostationary Earth Orbit GEO – Geostationary Earth Orbit MEO – Medium Earth Orbit MEO – Medium Earth Orbit LEO – Low Earth Orbit LEO – Low Earth Orbit

Physical Layer - Satellites From: Tanenbaum (2003)

Physical Layer - Satellites Geostationary Earth Orbit Geosynchronous Satellites Geosynchronous Satellites Stay in fixed position over the earth Stay in fixed position over the earth No need to track them No need to track them Satellites can do broad beam transmission or spot beam transmission Receive on one frequency – transmit on a different frequency

Physical Layer - Satellites Frequency bands assigned to guard against interference with microwave transmissions

Physical Layer - Satellite VSAT - Very Small Aperture Terminals Uses very small antennas - one meter or smaller Uses very small antennas - one meter or smaller Widely used for data Widely used for data Cheap, easy to install, setup Cheap, easy to install, setup Low power Low power Uses ground stations for relays Uses ground stations for relays “Star” topology Fairly good data rates (up to 4 Mbps), but poor latency Fairly good data rates (up to 4 Mbps), but poor latency

Physical Layer - Satellites You see these everywhere internet/how-it-works.html m/SATCOM/vsat/vsat.htm

Physical Layer - Satellites

Physical Layer - Satellite MEO – Medium Earth Orbit Short orbital period Short orbital period Must be tracked Must be tracked GPS satellites GPS satellites

Physical Layer - Satellites LEO – Low Earth Orbit Requires a lot of satellites because the orbital period is short … Requires a lot of satellites because the orbital period is short … And they zip in and out of view And they zip in and out of view However, because of the LEO they require little power and have small communication latencies However, because of the LEO they require little power and have small communication latencies

Physical Layer - Satellites Iridium Motorola and investors proposed in 1990 Motorola and investors proposed in 1990 … … 77 LEO satellites 77 LEO satellites For voice communications in remote places For voice communications in remote places Launched satellites (scaled back to 66) Launched satellites (scaled back to 66) Went operational in 1998 Went operational in 1998 Went bankrupt in 1999 Went bankrupt in 1999

Physical Layer - Satellites Iridium Bought by new investors Bought by new investors Restarted service in 2001 Restarted service in 2001 Worldwide communications services Worldwide communications services Maritime, aviation, jungles, mountains, etc.

Physical Layer - Satellites Iridium Comm traffic is routed satellite to satellite Comm traffic is routed satellite to satellite Orbit a little under 500 miles Orbit a little under 500 miles Polar orbit Polar orbit Each satellite has 48 spot beams Each satellite has 48 spot beams Combines 1628 beams cover the earth’s surface Combines 1628 beams cover the earth’s surface Each satellite has 3840 channels Each satellite has 3840 channels 252,440 channels total 252,440 channels total

Physical Layer - Satellites Globalstar Uses 48 satellites Uses 48 satellites While Iridium routes traffic from satellite to satellite While Iridium routes traffic from satellite to satellite Globalstar routes traffic through ground stations Globalstar routes traffic through ground stations

Physical Layer - Satellites Teledesic Created by Bill Gates and Craig McCaw Created 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 antenna Using small VSAT type antenna 30 satellites – large footprints 30 satellites – large footprints Packet switching in space Packet switching in space Bandwidth dynamically allocated on request Bandwidth dynamically allocated on request Not “off the ground” Not “off the ground”

Physical Layer - Satellites What’s all mean?