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MEDIUM ACCESS CONTROL. Wireless channel is a shared medium MAC coordinates transmission between users sharing the spectrum Goals: prevent collisions while maximizing throughput and minimizing delay Why Special MAC needed in Wireless Domain? Why not CSMA/CD?. HIDDEN & EXPOSED TERMINAL.

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  1. MEDIUM ACCESS CONTROL • Wireless channel is a shared medium • MAC coordinates transmission between users sharing the spectrum • Goals: prevent collisions while maximizing throughput and minimizing delay • Why Special MAC needed in Wireless Domain? Why not CSMA/CD? Prof.R.K.NADESH;SITE;VIT




  5. Space Division Multiple Access • Allocating Separate Space • Eg.BS-MS User • SDMA is always used in combinations • Basis of SDMA – Cells,Sectorized Antennas which constitute the infrastructure implementing SDM. Prof.R.K.NADESH;SITE;VIT

  6. Frequency Division Multiple Access • Allocating Frequecies to Transmission Channel • FIXED(Radion Station) /Dynamic (Demand Driven) • FDMA+TDMA • Hopping Pattern • FDM-Simultaneous access to network • Duplex Channel ;MS-BS;BS-MS Prof.R.K.NADESH;SITE;VIT

  7. Contd.. • Two Different Directions;Separated by two different frequencies(FDD) • UPLINK-890.2 & 915 MHZ • DOWNLINK – 935.2 to 960 MHZ • 124 Channels Prof.R.K.NADESH;SITE;VIT

  8. Contd… Prof.R.K.NADESH;SITE;VIT

  9. Time Division Multiple Access • Allocates certain time slot for communication • Same frequency • Synchronization must be done (s-r) • Fixed and Dynamic -(WLC) • Combined FDMA->Greater Flexibility & Transmission Capacity Prof.R.K.NADESH;SITE;VIT

  10. Fixed TDMA • Allocating Time Slots in a fixed pattern (Fixed Bandwidth ;BS-MS) Prof.R.K.NADESH;SITE;VIT

  11. Contd.. • Different slots for U/D Link in same frequency (TDD) • Multiple Access & Duplex Channel(BS-MS) • 1O ms the pattern is repeated. Each slot has the time duration of 417µs • 64bit Duplex is inefficient for bursty data • Too Static and too inflexible for data communication Prof.R.K.NADESH;SITE;VIT

  12. Classical Aloha • Scheme from University of HAWAII • If TDM is applied without controlling access • Each station can access medium at any time(Random Access Scheme-No Control) Prof.R.K.NADESH;SITE;VIT

  13. Slotted Aloha • Refinement by introducing Time Slots • All Senders- Synchronized • Transmission starts when each timeslot begins • Still Access is not coordinated • Suited for light loads(18-36%) Prof.R.K.NADESH;SITE;VIT

  14. Carrier Sense Multiple Access • Sensing the carrier and accessing the medium if it is idle • Decreases the probability of Collision • Hidden Terminals not detected Versions Non-Persistent CSMA Sense carrier & Starts Sending if it is idle otherwise repeat sensing after some time P-Persistent CSMA Sense the medium but only transmit with a probability of p. Prof.R.K.NADESH;SITE;VIT

  15. Demand Assigned Multiple Access(Reservation Aloha) • Reservation period followed by transmission period • Eg.Satellite Systems (Station on earth-Satellite ) • Timeslot alloted successfully, no collision • Satellite collects all requests & sends back reservation list indicating access rights • EXPLICIT RESERVATION Prof.R.K.NADESH;SITE;VIT

  16. PACKET RESERVATION MULTIPLE ACCESS(PRMA)DAMA with implicit reservation • Implicit Reservation • Certain slots form a frame • Frame repeated in time • BS broadcasts status of each slot • All MS will know free & reserved slot Prof.R.K.NADESH;SITE;VIT

  17. RESERVATION TDMAEach station has certain bandwidth and fixed delay Prof.R.K.NADESH;SITE;VIT

  18. Multiple Access with Collision Avoidance • Solves Hidden and Terminal Problem Prof.R.K.NADESH;SITE;VIT

  19. MACA-Examples Prof.R.K.NADESH;SITE;VIT

  20. BASESTATION FOR COORDINATION • POLLING ONE STATION TO BE HEARED BY ALL STATIONS • One Master with several slaves • Round Robin & Randomly Accessed Polling Prof.R.K.NADESH;SITE;VIT

  21. Inhibit Sense Multiple Access • Current state of the medium is signaled via a “busy tone” • the base station signals on the downlink (base station to terminals) if the medium is free or not • terminals must not send if the medium is busy • terminals can access the medium as soon as the busy tone stops • the base station signals collisions and successful transmissionsvia the busy tone and acknowledgements, respectively Prof.R.K.NADESH;SITE;VIT

  22. Code Division Multiple Access-(CDMA) • All terminals send on the same frequency probably at same time and can use the whole bandwidth of the transmission channel • Uses codes to separate users and enable access to shared medium without interference Prof.R.K.NADESH;SITE;VIT

  23. CDMA • Orthogonal Codes 2 vectors are called orthogonal if the inner product is Zero (2,5,0) & (0,0,17) 2*0+5*0+0*17=0 (3,-2,4) &(-2,3,3) -6-6+12=0 Prof.R.K.NADESH;SITE;VIT

  24. BASIC FUNCTION OF CDMA • Two senders A & B want to send data • CDMA assigns 2 unique orthogonal code • Key Ak = 010011 Bk=110101 • Ad=>1; Bd=>0 • Assumptions 0 as -1 & 1 as 1 Prof.R.K.NADESH;SITE;VIT

  25. CDMA-Spreading Signal • Sender Spreading the Signal (Multiplication of data bits) As =Ad*Ak = 1* (-1,1,-1,-1,1,1) =(-1,1,-1,-1,1,1) Bs= Bd*Bk = -1 * (1, 1, -1, 1, -1, 1) = (-1,-1,1,-1,1,-1) Prof.R.K.NADESH;SITE;VIT

  26. CDMA • Both Signals are transmitted@same time,frequency C= AS+BS (-2,0,0,-2,2,0) Receiver wants A Code :C*AK=(-2,0,0,-2,2,0)*(-1,1,-1,-1,1,1) = 2+0+0+2+2+0 =6 >0 ;Receiver detects as binary 1 Prof.R.K.NADESH;SITE;VIT

  27. CDMA • Receiver wants B Code C*Bk=(-2,0,0,-2,2,0)*(1, 1, -1, 1, -1, 1) = -2+0+0-2-2+0 = -6 <0 detects as ZERO Prof.R.K.NADESH;SITE;VIT

  28. CDMA On Signal - I Prof.R.K.NADESH;SITE;VIT

  29. CDMA On Signal - II Prof.R.K.NADESH;SITE;VIT

  30. CDMA On Signal - III Prof.R.K.NADESH;SITE;VIT

  31. CDMA On Signal - IV Prof.R.K.NADESH;SITE;VIT

  32. CDMA On Signal - V Prof.R.K.NADESH;SITE;VIT


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