Introduction to information technology


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Introduction to information technology

  1. 1. IT-101 Section 001 Introduction to Information Technology Lecture #11
  2. 2. Overview  Finish chapter 13   Generations of cellular systems Satellite telephones Chapter 16  Radio-Frequency and Satellite Systems -Satellite systems -GPS
  3. 3. Generations of Cellular Systems  Generations of cellular systems include:      AMPS GSM W-CDMA 1st generation 2nd generation 3rd generation Cellular systems operate based on various protocols, and use RF (radio frequency) waves that propagate through the air for transmission of information. These systems typically use the 800-900 MHz or 1800-1900 MHz frequency band of the radio spectrum. But what is the radio spectrum?
  4. 4. The Radio Spectrum Source:   The Radio spectrum is composed of many frequency bands Communication systems have the liberty to transmit signals at various frequencies, and the FCC (Federal Communications Commission) regulates which frequencies to use
  5. 5. Radio Spectrum Bands Low Frequency=large period High Frequency=small period ~400-2400 MHz frequency range MF •AM Radio •Maritime HF •Maritime •Amateur Radio VHF •Television •FM radio •Aviation UHF/SHF •Satellite •Television •Cell Phones •Microwave FM radio stations between: 88-108 MHz EHF •Astronomy
  6. 6. 1st generation  AMPS: Advanced Mobile Phone System       The only system available in the United States until about 1997 The first system used for cellular telephony- is analog Uses the 800 MHz frequency band of the spectrum Is still being used widely - The number of subscribers began to decrease in 1999 due to migration to digital Utilizes FDMA (Frequency division multiple access) to separate users In FDMA, users are separated in frequency. i.e. mobile phones communicate at different frequencies than the others within each cell. The radio spectrum is shared among users
  7. 7. 2nd generation  GSM: Originally: “Group Special Mobile”, now: “Global System for Mobile Communications”        Is the most popular system worldwide Originally developed in Europe Was later introduced into the United States Is a digital system for both voice and data transmission Uses the 900 and 1800 MHz frequency bands Utilizes TDMA (Time division multiple access) In TDMA, users are separated in time. i.e. mobile phones communicate in a different time slot than the others within each cell. Time is shared among the users
  8. 8. 3rd generation  W-CDMA Wideband code division multiple access:       Provides much higher data rates Supports a larger number of users Enables features such as video, internet access, web browsing and complete worldwide operability Widespread introduction expected to begin by 2005 Utilizes CDMA (Code division multiple access) In CDMA, users are separated by a unique code assigned to them. i.e. each mobile phone can utilize the entire chunk of spectrum, and the unique code of each user is used to encode and decode the information during transmission
  9. 9. Satellite Telephones     Satellite telephones are utilized in circumstances where there is no access to a telephone or cellular network Examples include ships at sea Previously, these remote users would communicate via High frequency (HF) systems. A licensed radio operator would need to be present on board to operate these systems, and would need to know morse code Using radio waves in the HF range of the spectrum, communication could be realized between any two points on the earth
  10. 10.   This was possible due to the unique property of HF waves to travel all the way around the world Under appropriate conditions, HF waves can travel around the world by reflection between the earths surface and the ionosphere (the ionospheric reflection is analogous to the internal reflection that occurs in optical fibers-we will learn about fibers in chapter 15 )
  11. 11.       This method of communication however is unreliable, and currently satellites are utilized for communication in remote places Using satellites, a ship at sea is easily reachable as any point on land INMARSAT is one of the main providers of satellite radio communications for remote users A Satellite based system called the Iridium system, conceived by Motorola corp. was proposed as an alternative to the conventional cellular systems In the Iridium system, 66 low earth orbiting satellites are used for relaying information, analogous to a switching center in conventional cellular and telephone systems The call does not pass through land lines, increasing the efficiency over other cellular systems
  12. 12.   The cell sites in the Iridium system are overhead and are in constant motion Let us see how iridium works:
  13. 13.      The Iridium system began operations in 1999, and presented the first generally available global system However, the relatively high cost of service (around $3 per minute) and the relatively large size of the handsets drove the system to bankruptcy Iridium failed to attract a sufficient amount of customers in competition with the rapidly spreading cellular systems across the globe Shortly after this failure, a group of investors bought the system for $25 million, which had actually cost Motorola more than $ 5 billion to build The system is expected to be profitable within a couple of years
  14. 14. Satellite Systems    Communication satellites provided the first long-distance, widebandwidth communication service Testar 1 was the first commercial communications satellite to be launched into orbit Satellites can be distinguished into 3 primary categories, depending on the type of orbit they reside on:    LEO MEO GEO
  15. 15.   LEO: Low Earth Orbit satellites have a small area of coverage. They are positioned in an orbit approximately 3000km from the surface of the earth  They complete one orbit every 90 minutes  The large majority of satellites are in low earth orbit  The Iridium system utilizes LEO satellites  The satellite in LEO orbit is visible to a point on the earth for a very short time MEO: Medium Earth Orbit satellites have orbital altitudes between 3,000 and 30,000 km.  They are commonly used used in navigation systems such as GPS
  16. 16. GEO: Geosynchronous (Geostationary) Earth Orbit satellites are positioned over the equator. The orbital altitude is around 30,000-40,000 km  They complete one orbit every 24 hours. This causes the satellite to appear stationary with respect to a point on the earth, allowing one satellite to provide continual coverage to a given area on the earth's surface  One GEO satellite can cover approximately 1/3 of the world’s surface  They are commonly used in communication systems  Let’s look at a GEO animation: 
  17. 17. The Global Positioning System      GPS is funded and controlled by the Department of Defense (DOD). Although there are many thousands of civil users of GPS worldwide, the system was originally designed for the U. S. military GPS provides special satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position and velocity Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver For around $100, you can get a hand held gadget that will tell you exactly what point you are on the earth
  18. 18. Source:
  19. 19.      The orbital period of the GPS satellites is around 12 hours The satellites move with respect to the receivers on the earth, but at a slower speed compared to LEO’s Typical civilian accuracy is around 100 ft, with occasional errors of up to 300 ft For applications requiring higher accuracy such as aircraft landing etc., enhancements in the systems are made that can reduce the error to around 1 cm Some applications of GPS include:  Aircraft navigation  Marine navigation  Driving  Surveying  Farming