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12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
12 multiple access
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12 multiple access

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Transcript

  • 1. Outline Multiple access mechanisms Random access Controlled access Channelization 1
  • 2. Sublayers of Data LinkLayer 2
  • 3. Multiple AccessMechanisms 3
  • 4. Random Access
  • 5. Random Access Also called contention-based access No station is assigned to control another 5
  • 6. ALOHA Network 6
  • 7. Frames in Pure ALOHA 7
  • 8. ALOHA Protocol 8
  • 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. ALOHA: Vulnerable Time 10
  • 11. Slotted ALOHA 11
  • 12. Slotted ALOHA: Vulnerable Time 12
  • 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. Collision in CSMA 14
  • 15. CSMA: Vulnerable Time 15
  • 16. Persistence Methods What a station does when channel is idle or busy 16
  • 17. Persistence Methods 17
  • 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. Energy Levels ZERO, NORMAL AND ABNORMAL 19
  • 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. 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. CSMA/CD: Flow Diagram 22
  • 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. 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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