12 multiple access

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12 multiple access

  1. 1. Outline Multiple access mechanisms Random access Controlled access Channelization 1
  2. 2. Sublayers of Data LinkLayer 2
  3. 3. Multiple AccessMechanisms 3
  4. 4. Random Access
  5. 5. Random Access Also called contention-based access No station is assigned to control another 5
  6. 6. ALOHA Network 6
  7. 7. Frames in Pure ALOHA 7
  8. 8. ALOHA Protocol 8
  9. 9. Example Calculate possible values of TB, when stations on an ALOHA network are a maximum of 600 km apart Tp = (600 × 103) / (3 × 108) = 2 ms When K=1, TB ∈ {0ms,2ms} When K=2, TB ∈ {0ms,2ms,4ms,6ms} : 9
  10. 10. ALOHA: Vulnerable Time 10
  11. 11. Slotted ALOHA 11
  12. 12. Slotted ALOHA: Vulnerable Time 12
  13. 13. CSMA C arrier S ense M ultiple A ccess  “Sense Before Transmit" Reduce the possibility of collision  But cannot completely eliminate it 13
  14. 14. Collision in CSMA 14
  15. 15. CSMA: Vulnerable Time 15
  16. 16. Persistence Methods What a station does when channel is idle or busy 16
  17. 17. Persistence Methods 17
  18. 18. CSMA/CD C arrier S ense M ultiple A ccess with C ollision D etection Station monitors channel when sending a frame 18
  19. 19. Energy Levels ZERO, NORMAL AND ABNORMAL 19
  20. 20. CSMA/CD: Minimum Frame Size Each frame must be large enough for a sender to detect a collision Worst case scenario:  "A" is transmitting  "D" starts transmitting just before As signal arrives A B C DLong enough tohear colliding signalfrom D 20
  21. 21. Example A CSMA/CD network has a bandwidth of 10 Mbps. If the maximum propagation time is 25.6 μs, what is the minimum size of the frame? 21
  22. 22. CSMA/CD: Flow Diagram 22
  23. 23. CSMA/CA C arrier S ense M ultiple A ccess with C ollision A voidance Used in a network where collision cannot be detected  E.g., wireless LANIFS – Interframe Space 23
  24. 24. CSMA/CA: Flow Diagram contention window size is 2K-1 After each slot: - If idle, continue counting - If busy, stop counting 24

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