GATE CSE 2016 Set 2 | Question: 24

Question

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

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

Answer 1

On Ethernet:

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

So, answer is (D).

Comments

@Pranav

The purpose of jamming signal is to indicate the station that collision has occurred. So, even though collision has occurred, until the jamming signal reaches the station which is transmitting, the collision is not detected by it, so the station continues transmitting.

Only after receiving the jamming signal, the station gets to know (detects) that a collision has occurred and thus it stops transmitting the packet.

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@aaradhana98

got it! thanks :)

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Answer 2

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.

Comments

nice explanation sir

Answer 3

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.

Answer 4

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.

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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.

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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

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Options A, B and C are gone.

 

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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.