the superhighway in the sky
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The SuperHighway in the SkyThe SuperHighway in the Sky
ISOM 591
Understanding Modern Telecommunications, Chapter 6
March 13, 2000
SatellitesSatellites
• satellites simply reflect and amplify signals transmitted to them
• the transponder provides this function and the typical unit is equivalent to 36 Mhz of bandwidth
• a transponder receives a signal, amplifies it, and transmits it to Earth
• signal to the satellite is called an uplink, from a satellite is called a downlink
SatellitesSatellites
• satellites simply reflect and amplify signals transmitted to them
• the transponder provides this function and the typical unit is equivalent to 36 Mhz of bandwidth
• a transponder receives a signal, amplifies it, and transmits it to Earth
• signal to the satellite is called an uplink, from a satellite is called a downlink
SatellitesSatellites
• cross-strapping allows a satellite to receive a signal on one frequency and transmit it on another
• the size of the footprint (area covered by the signal) can be controlled
• intersatellite links can allow carriers to transmit signals between satellites
• satellites can be characterized by their orbits
Geosynchronous SatellitesGeosynchronous Satellites
• geosynchronous (or geostationary)– rotating at the same angular speed as earth – a stationary target for signals uplinked by Earth stations
• footprint or coverage is about 1/4 to 1/3 of the earth• a single satellite can be used to satisfy demand from many
nations• a 4 degree spacing required to keep satellites from
interfering with one another, thus the number of satellites is limited
Geosynchronous SatellitesGeosynchronous Satellites
• delay for signals travelling at the speed of light (186,00 miles/sec) to the satellite and back would be 2*22,300/186,000 = 0.24 sec.
• latency or delay a real problem in data transmission– transaction-oriented applications and associated
protocols with a large number of requests and responses result in too many roundtrips
• Useful for broadcasting over large areas
WirelessWireless
• Unguided media for which an antenna radiates electromagnetic energy into the medium (air) and another antenna acts as the receptor
• directional (point to point) and omnidirectional (like radio)
• the dish is the most common type of microwave antenna
• a series of microwave antennas can be used
WirelessWireless
• Major applications include long distance telecommunications (long distance international links), point to point trunks between telephone exchange offices, television distribution, direct broadcast satellite (ex: DirecTV), closed circuit TV, VSATs, and data links between LANs
• in international telecom, used to bypass the local telephone company
Wireless: General ApplicationsWireless: General Applications
• microwave provides high capacity links in many established and emerging networks
• geosynchronous orbiting satellites, low orbiting satellites
• analog cellular phones
• digital cellular phones promise more clarity and more capacity
• wireless PCS (personal communication systems)
Advantages of Microwave Advantages of Microwave TransmissionTransmission
• Common frequencies are in the range of 2 to 40 GHz, thus this has high frequency, high bandwidth, and a high potential data rate
• global access to information for a mobile workforce
• mobility within the workplace and home
• ability to overcome environmental obstacles– cable is often impractical
Limitations of Microwave Limitations of Microwave TransmissionTransmission• Attenuation (repeaters or amplifiers need to
be placed 10 to 100 km apart)
• interference (ex: weather, airplanes)– disruption of radio frequency transmissions
• security is a concern
• transmissions areas overlap
• expensive and limited availability
Mobile Satellite SystemsMobile Satellite Systems
• MSAT 1-2 GHz L-band frequency
• American Mobile Satellite Corp., TeleSat Mobile, Inc. of Canada
• mobile communications providers use MEO (middle earth orbit) at altitudes of about 10,000 km or LEO’s
Low Earth-Orbiting SatellitesLow Earth-Orbiting Satellites
• inexpensive satellites which orbit the earth about 200 to 700 miles above the surface
• signal is stronger than that of the geosynchronous satellites
• coverage can be localized so less spectrum can be consumed
• services include personal communication services, radiodetermination service (location services like GPS), two-way messaging, paging, fax, data
Low Earth Orbiting Satellites Low Earth Orbiting Satellites (LEOs)(LEOs)
• for orbits closer to earth, less power is needed
• these are not geosynchronous and orbital speed relative to earth is increased
• footprints are smaller
• thus, a constellation of satellites is needed to provide services like cellular
LEOS: “Teledesic: Internet in LEOS: “Teledesic: Internet in the Sky”the Sky”
• Teledesic, Motorola, Boeing, McCaw, Gates, Matra Marconi Space (France and UK)– http://www.teledesic.com/– http://www.spacer.com/spacecast/news/
teledesic-97d.html• world’s first network to provide fiber-like access to telecom services
including linking enterprise computing networks, broadband Internet access, videoconferencing and other digital needs
• licensed in March 1997 by FCC and in November, 1997 by the ITU, service to begin in 2003
• other projects under development include Odyssey (TRW), GlobalStar (QualComm)
International SatellitesInternational Satellites
• no single location or route needs to generate a sufficient volume of traffic to justify its investment since such a large area is covered
• Satellites can be used for point-to-multipoint service like video distribution, broadcast and news services
• medium for international telecommunications
International SatellitesInternational Satellites
• until the middle 1980s, only the US, the European Space Agency, and the former Soviet Union had satellite launching technology
• recently, China, Israel, Japan, Norway, Australia, to name a few, have launched satellites
• lifetime expectation is about 15 years• satellites and submarine cables are
complementary media
Satellite ApplicationsSatellite Applications
• Globalstar is a low orbiting earth satellite network
– 48 satellites in orbit covering 70 degrees north and south of the equator
– acts as a communications network to ground stations which connect the user to the land network
– this means you can roam the globe with your cellular phone
• work sites in remote areas can use the Immarsat (International Maritime Satellite Organization) satellite to gain access to the global phone system
Satellite ApplicationsSatellite Applications
• Teledesic– proposed by Microsoft and McCaw– global network of 840 low-earth (435 miles)
satellites using ATM to carry voice, data, video, and other multimedia data
– users can access this network using small hand-held devices
Satellite ApplicationsSatellite Applications
• DirecTV– Hughes and USSB
– the DirecTV satellite circumvents cable and broadcast TV to offer interactive television directly to subscribers
– programming is transmitted as digital video with 16-bit stereo and audio, compatible with NTSB and HDTV
– a back channel operates over a modem to make home shopping and interactive games available to the subscriber
Satellite ApplicationsSatellite Applications
• Primestar, AlphaStar, EchoStar, USSB
• American Sky Broadcasting system owned by US partners, including WorldCom
• health applications such as broadcast of live surgery and diagnostics– to be delivered to the home
• need for line-of-sight transmission and launch facilities and insurance!
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