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Wireless Networks


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Wireless Networks

  1. 1. CS 313 Introduction to Computer Networking & Telecommunication Mobile Telephone System and Wireless LANs Chi-Cheng Lin, Winona State University
  2. 2. Topics <ul><li>Mobile Telephone System </li></ul><ul><li>CDMA </li></ul><ul><li>Wireless LANs </li></ul>
  3. 3. Mobile Telephone System <ul><li>First-Generation Mobile Phones </li></ul><ul><ul><li>Analog Voice </li></ul></ul><ul><li>Second-Generation Mobile Phones </li></ul><ul><ul><li>Digital Voice </li></ul></ul><ul><li>Third-Generation Mobile Phones </li></ul><ul><ul><li>Digital Voice and Data </li></ul></ul>
  4. 4. Advanced Mobile Phone System <ul><li>Area is divided into cells with an antenna control by a cell office in each cell </li></ul><ul><li>Cell offices communicate with MTSO </li></ul><ul><li>Transmission frequencies cannot be the same in adjacent cells </li></ul><ul><li>Cell size is not fixed </li></ul><ul><ul><li>Smaller cells used in higher populated area </li></ul></ul>
  5. 5. WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Cellular System
  6. 6. Advanced Mobile Phone System <ul><li>(a) Frequencies are not reused in adjacent cells. </li></ul><ul><li>(b) To add more users, smaller cells can be used. </li></ul>
  7. 7. Frequency reuse patterns <ul><li>The McGraw-Hill Companies, Inc., 2004 </li></ul>
  8. 8. Cellular Transmission <ul><li>Traditionally analog </li></ul><ul><ul><li>FM used to minimized noise </li></ul></ul><ul><li>Digital transmission </li></ul><ul><ul><li>CDPD (Cellular Digital Packet Data) </li></ul></ul><ul><ul><ul><li>Low-speed digital service over existing cellular network </li></ul></ul></ul><ul><ul><ul><li>Based on OSI Model </li></ul></ul></ul><ul><ul><ul><li>Modem needed </li></ul></ul></ul>
  9. 9. Cellular System <ul><li>Handoff </li></ul><ul><ul><li>When a mobile telephone leaves a cell </li></ul></ul><ul><ul><li>1. Its base station notices the signal fading out </li></ul></ul><ul><ul><li>2. The base station asks all the surrounding base stations how much power they are getting from it </li></ul></ul><ul><ul><li>3. Ownership is transferred to the neighbor base station that receives strongest power </li></ul></ul><ul><ul><li>4. The telephone is informed of its new boss </li></ul></ul><ul><ul><li>5. If a call is in progress, it will be asked to switch to a new channel </li></ul></ul>
  10. 10. Channels <ul><li>832 full-duplex channels </li></ul><ul><ul><li>Each channel consists of 2 simplex channels </li></ul></ul><ul><ul><li>Transmission channels </li></ul></ul><ul><ul><ul><li>(849-824)MHz/30KHz  832 </li></ul></ul></ul><ul><ul><li>Receiving channels </li></ul></ul><ul><ul><ul><li>(894-869)MHz/30KHz  832 </li></ul></ul></ul><ul><li>Typically, actual number of voice channel per cell  45 </li></ul>
  11. 11. Channel Categories <ul><li>The 832 channels are divided into four categories </li></ul><ul><ul><li>Control (base to mobile) to manage the system </li></ul></ul><ul><ul><li>Paging (base to mobile) to alert users to calls for them </li></ul></ul><ul><ul><li>Access (bidirectional) for call setup and channel assignment </li></ul></ul><ul><ul><li>Data (bidirectional) for voice, fax, or data </li></ul></ul>
  12. 12. Second-Generation Mobile Phones <ul><li>D-AMP </li></ul><ul><li>GSM </li></ul><ul><li>CDMA </li></ul>
  13. 13. D-AMPS Digital Advanced Mobile Phone System <ul><li>(a) A D-AMPS channel with three users. </li></ul><ul><li>(b) A D-AMPS channel with six users. </li></ul>
  14. 14. GSM Global System for Mobile Communications <ul><li>GSM uses 124 frequency channels, each of which uses an eight-slot TDM system </li></ul>
  15. 15. GSM <ul><li>A portion of the GSM framing structure. </li></ul>
  16. 16. Third-Generation Mobile Phones: Digital Voice and Data <ul><li>Basic services an IMT-2000 network should provide </li></ul><ul><ul><li>High-quality voice transmission </li></ul></ul><ul><ul><li>Messaging </li></ul></ul><ul><ul><ul><li>Replace e-mail, fax, SMS, chat, etc. </li></ul></ul></ul><ul><ul><li>Multimedia </li></ul></ul><ul><ul><ul><li>Music, videos, films, TV, etc. </li></ul></ul></ul><ul><ul><li>Internet access </li></ul></ul><ul><ul><ul><li>Web surfing, w/multimedia </li></ul></ul></ul><ul><li>2.5G, 4G, … </li></ul>
  17. 17. Code Division Multiple Access (CDMA) <ul><li>In FDMA, the bandwidth is divided into channels. </li></ul><ul><li>In TDMA, the bandwidth is just one channel that is timeshared. </li></ul><ul><li>In CDMA, one channel carries all transmissions simultaneously. </li></ul>
  18. 18. CDMA <ul><li>Every station has a chip sequence </li></ul><ul><li>Bipolar encoding </li></ul><ul><ul><li>1 is represented by +1 </li></ul></ul><ul><ul><li>0 is represented by -1 </li></ul></ul><ul><ul><li>Silence is represented by 0 </li></ul></ul><ul><li>The set of chip sequences are orthogonal </li></ul><ul><ul><li>A chip sequence is represented by a vector of +1’s and -1’s (for example, 4 sequences A, B, C, & D) </li></ul></ul><ul><ul><ul><li>A ● B = A ● C = A ● D = B ● C = B ● D = C ● D = 0 </li></ul></ul></ul><ul><ul><ul><li>A ● A = B ● B = C ● C = D ● D = length of chip sequence </li></ul></ul></ul><ul><li>Transmission: s = sum of (bit  chip_sequence) </li></ul><ul><li>Receiving: bit = (s ● chip_sequence) / chip_length </li></ul>● : inner product of two vectors
  19. 19. CDMA <ul><li>(a) Binary chip sequences for four stations </li></ul><ul><li>(b) Bipolar chip sequences </li></ul><ul><li>(c) Six examples of transmissions </li></ul><ul><li>(d) Recovery of station C’s signal </li></ul>Q: Why does it work? A: _ _ _ _
  20. 20. Wireless LANs <ul><li>(a) Wireless networking with a base station. </li></ul><ul><li>(b) Ad hoc networking. </li></ul>
  21. 21. <ul><li>The range of a single radio may not cover the entire system </li></ul><ul><li>Multipath fading </li></ul><ul><li>Noisy </li></ul>Wireless LANs Problems
  22. 22. <ul><li>A multicell 802.11 network </li></ul><ul><li>ISM band </li></ul>Wireless LANs 802.11 Broadband Cellular
  23. 23. The 802.11 Protocol Stack (part of)  54Mbps 5GHz ISM band  11Mbps 2.4GHz ISM band  54Mbps 2.4GHz ISM band
  24. 24. The 802.11 MAC Sublayer Protocol <ul><li>The hidden station problem. </li></ul><ul><li>CSMA/CD does not work (why?) </li></ul><ul><li>Hidden station and exposed station problems </li></ul>The exposed station problem.
  25. 25. The 802.11 MAC Sublayer Protocol <ul><li>Two modes of operations </li></ul><ul><ul><li>Contention mode </li></ul></ul><ul><ul><ul><li>DCF: distributed coordination function </li></ul></ul></ul><ul><ul><ul><li>CSMA/CA </li></ul></ul></ul><ul><ul><li>Collision-free mode </li></ul></ul><ul><ul><ul><li>PCF: Point coordination function </li></ul></ul></ul><ul><ul><ul><li>Polling w/ beacon frame </li></ul></ul></ul>NAV: Network Allocation Vector
  26. 26. The 802.11 MAC Sublayer Protocol <ul><li>Noisy, unreliable </li></ul><ul><ul><li> Smaller frames reduce probability of frame damages </li></ul></ul><ul><li>Fragmentation </li></ul><ul><ul><li>Each fragment, with its own checksum and sequence number, is acknowledged individually </li></ul></ul><ul><ul><li>Stop-and-wait </li></ul></ul><ul><ul><li>Fragment burst: sequence of fragments </li></ul></ul>
  27. 27. The 802.11 MAC Sublayer Protocol <ul><li>Problems </li></ul><ul><ul><li>NAV terminates at end of the first fragment </li></ul></ul><ul><ul><li>How do DCF and PCF coexist </li></ul></ul><ul><li>Solution: interframe spacing </li></ul>
  28. 28. The 802.11 Frame Structure <ul><li>Three classes </li></ul><ul><ul><li>Data, control, management </li></ul></ul><ul><li>4 addresses in the frame </li></ul>