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In an Ethernet local area network, which one of the following statements is TRUE?

  1. A station stops to sense the channel once it starts transmitting a frame.
  2. The purpose of the jamming signal is to pad the frames that are smaller than the minimum frame size.
  3. A station continues to transmit the packet even after the collision is detected.
  4. The exponential back off mechanism reduces the probability of collision on retransmissions.
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37 votes

On Ethernet:

  1. This is false because station need not stop to listen to stuff !
     
  2. No, this is not purpose of jamming signal.
     
  3. No, stations sends jamming signal if collision is detected. This is reason why (B) is false.


So, answer is (D).

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A : FALSE, because if it stops sensing channel then how it sense if collision occur.

B: FALSE, because the purpose of jamming signal is to tell all other station that collision has occurred.

C: FALSE, station stops transmit as soon as collision has detected.

D: TRUE, probability of collision on retransmission reduces exponentially.

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16 votes

Option D.

Exponential backoff algorithm is used to reduce the probability of collision on retransmission by selecting waiting time for each station = k*rtt

where k will be 0 to 2n-1.

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13 votes

Ethernet uses CSMA/CD.

In CSMA/CD, when a collision is detected the involved stations must immediately stop transmitting, so as to not waste bandwidth and time. The collided messages are garbled irretrievably.


Any station that wants to transmit a frame, or is currently transmitting a frame must continuously keep sensing the channel — to detect if the channel is idle and to detect collisions respectively.


The stations keep sensing the channel while transmitting. When their "sensed" data does not resemble the data they're sending; they know collision has occured. But sometimes, for example, when a collison of 0 Volts occurs, it is impossible to detect collision. Hence, we use a maximum 48-bit jamming signal which announces the occurrence of collision


Options A, B and C are gone.

 


Any algorithm that's meant for minimising collisions obviously reduces the chances of collisions. :P

So, D is correct.

Well, the binary exponential back-off algorithm makes the colliding stations choose a random number between $0 \ to \ 2^i-1$ where $i$ is the number of collisions. This random number is the number of slots the stations will skip.

Practically, after $i=2$, chances of collision are negligible.

Answer:

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