Encyclopedia of Computer Science and Technology

416 satellite Internet serviceFurther ReadingMcDonald, Kevin, et al. Mastering the SAP Business InformationWarehouse: Leveraging the Business Intelligence Capabilities ofSAP NetWeaver. 2nd ed. Indianapolis: Wiley, 2006.Ricadela, Aaron. “SAP’s Down-Market Gamble.” Business Week,September 19, 2007. Available online. URL: http://www.businessweek.com/technology/content/sep2007/tc20070919_181869.htm. Accessed October 5, 2007.SAP.com. Available online. URL: http://www.sap.com/usa/index.epx. Accessed October 8, 2007.“SAP History: From Start-Up Software Vendor to Global MarketLeader.” Available online. URL: http://www.sap.com/company/history.epx. Accessed October 8, 2007.Vogel, Andreas, and Jan Kimbell. mySAP for Dummies. Hoboken,N.J.: Wiley, 2004.Woods, Dan, and Jeffrey Word. SAP NetWeaver for Dummies. Hoboken,N.J.: Wiley, 2004.satellite Internet serviceAs with television, satellite Internet service can provideaccess to areas (such as remote locations, ships, or landvehicles) where wired service is not available (see broadband).Besides the satellite, the system includes a terrestrialfacility that has two connections: routers and proxy serversthat manage the flow of traffic to and from the Internet, andan “uplink” transmitter that communicates with the satellite.In addition, there may be a connection to the publictelephone network.Each user has a satellite dish and associated equipmentsimilar to those used for receiving satellite TV, though thedish is larger and existing TV dishes cannot be used. In theNorthern Hemisphere, the user must have an unobstructedview of the southern sky (most satellites orbit over theequator). The equipment is also adapted for use on shipsand recreational vehicles.The user also has a modem (either external or on a cardin the PC) to convert the satellite signals to data, and softwaresupplied by the satellite service.There are two types of systems for sending data from theuser back to the Internet. In a dial-return system, the userhas a conventional telephone dial-up modem that connectsby phone to a hub at the terrestrial facility. Downloadingis at broadband speeds (comparable to low-end DSL orcable), but uploading is at dial-up speeds. (This is not usuallya problem unless the user is uploading large files.) In atwo-way system, the user has a transmitter that sends datadirectly back to the satellite. This is usually several timesfaster than dial-up, but is more expensive.Because of the time it takes signals to travel between asatellite and the ground, all satellite Internet systems have abuilt-in delay, or latency. Satellites in geosynchronous orbit(about 22,000 miles [35,405.6 km] high) have wider coveragebut higher latency, while using lower orbits reduceslatency but requires more satellites to provide continuouscoverage. Latency can be problematic for applications suchas Internet telephony (see VOIP).Although small compared with that for cable or DSL,the satellite user base is growing, particularly in areas andcountries that lack wired infrastructure. Users who havecable or DSL available in their neighborhood would havelittle reason to obtain satellite Internet, since the initial andmonthly costs are considerably higher and download speedsare somewhat slower. Also, the need to use encrypted virtualprivate networks (VPN) to secure business data canlower effective speeds substantially. Finally, though the satellitesthemselves are very reliable, satellite service is subjectto interruption during heavy rain or snow.Further ReadingBrodkin, Jon. “Satellite Services and Telecommuting Not Alwaysa Pretty Mix.” NetworkWorld, July 24, 2007. Availableonline. URL: http://www.networkworld.com/news/2007/072407-satellites-for-telecommuting.html. Accessed November14, 2007.“How Does Satellite Internet Operate?” HowStuffWorks. Availableonline. URL: http://computer.howstuffworks.com/question606.htm. Accessed November 14, 2007.Kota, Sastri L., Kaveh Pahlavan, and Pentti Leppanen. BroadbandSatellite Communication for Internet Access. Norwell, Mass.:Kluwer Academic Publishers, 2004.Nutter, Ron. “Getting More Performance from a Satellite InternetSystem.” Network World, April 16, 2007. Available online. URL:http://www.networkworld.com/columnists/2007/041607nutter.html. Accessed November 14, 2007.scannerIn order for a computer to work with information, theinformation must be digitized—converted to data thatapplication programs can recognize and manipulate (seecharacters and strings). Computer users have thusbeen confronted with the task of converting millions ofpages of printed words or graphics into machine-readableform. Since it is expensive to re-key text (and impractical toredraw images), some way is needed to automatically convertthe varying shades or colors of the text or images into adigitized graphics image that can be stored in a file.This is what a scanner does. The scanner head containsa charge-coupled device (CCD) like that used in digital cameras(see photography, digital). The CCD contains thousandsor millions of tiny regions that can convert incominglight into a voltage level. Each of these voltage levels, whenamplified, will correspond to one pixel of the scanned image.(A color scanner uses three different diodes for each pixel,each receiving light through a red, green, or blue filter.)The operation of the head depends on the type of scanner.In the most common type, the flatbed scanner, a motormoves the head back and forth across the paper, which liesfacedown on a glass window. In a sheet-fed scanner, thehead remains stationery and the paper is fed past it by a setof rollers. Finally, there are handheld scanners, where thejob of moving the scanner head is performed by the usermoving the scanner back and forth over the page.The resolution of a scanner depends on the number ofpixels into which it can break the image. The color depthdepends on how many bits of information that it can storeper pixel (more information means more gradations of coloror gray). Resolutions of 2,400 dots per inch (dpi) or moreare now common, with up to 36 bit color depth, allowingfor about 68.7 billion colors or gradations (see color incomputing).