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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