The document outlines different multiple access mechanisms for data link layers, including random access, controlled access, and channelization. It then describes the sublayers of the data link layer and various random access protocols like ALOHA, slotted ALOHA, CSMA, CSMA/CD, and CSMA/CA. It provides details on how each protocol handles channel access and collisions.

1. Outline
 Multiple access mechanisms
 Random access
 Controlled access
 Channelization
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2. Sublayers of Data Link
Layer
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3. Multiple Access
Mechanisms
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4. Random Access

5. Random Access
 Also called contention-based access
 No station is assigned to control another
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6. ALOHA Network
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7. Frames in Pure ALOHA
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8. ALOHA Protocol
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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}
 :
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10. ALOHA: Vulnerable Time
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11. Slotted ALOHA
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12. Slotted ALOHA: Vulnerable Time
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13. CSMA
 C arrier S ense M ultiple A ccess
 “Sense Before Transmit"
 Reduce the possibility of collision
 But cannot completely eliminate it
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14. Collision in CSMA
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15. CSMA: Vulnerable Time
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16. Persistence Methods
 What a station does when channel is idle or busy
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17. Persistence Methods
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18. CSMA/CD
 C arrier S ense M ultiple A ccess with
C ollision D etection
 Station monitors channel when sending a
frame
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19. Energy Levels
ZERO, NORMAL AND ABNORMAL
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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 A's signal arrives
A B C D
Long enough to
hear colliding signal
from D
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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?
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22. CSMA/CD: Flow Diagram
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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 LAN
IFS – Interframe Space
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24. CSMA/CA: Flow Diagram
contention window
size is 2K-1
After each slot:
- If idle, continue counting
- If busy, stop counting
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