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Module 2: Data Link Layer & MAC Sublayer

20202m

In pure ALOHA, the vulnerable time is ______ the frame transmission time.

(i) the same as
(ii) two times
(iii) three times
(iv) None of the above

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(ii) two times

In pure ALOHA, a station transmits a frame whenever it has data, with no coordination with other stations. A collision occurs if any other station starts transmitting within one frame-time before OR after this station's transmission begins.

         <-- Tfr -->|<-- Tfr -->|<-- Tfr -->
   ...---[frame X starts]------[frame X ends]---...
         ^                                  ^
   vulnerable window                 vulnerable window
   (1 frame-time before)             (1 frame-time after)

So the total vulnerable period = 1 frame-time (before) + 1 frame-time (after) = 2 x frame transmission time. This large vulnerable window is why pure ALOHA has a low maximum throughput (~18.4%), compared to slotted ALOHA's vulnerable time of just 1 frame-time (throughput ~36.8%).

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