GATE CS 2016 Set 2 Q.34 (CS Q.24): Medium Access Control (ALOHA, CSMA/CD) and Ethernet/Switching

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

A station stops to sense the channel once it starts transmitting a frame.

The purpose of the jamming signal is to pad the frames that are smaller than the minimum frame size.

A station continues to transmit the packet even after the collision is detected.

The exponential backoff mechanism reduces the probability of collision on retransmissions.

Official answer and solution

Solution

Evaluate each statement regarding Ethernet CSMA/CD operation and backoff mechanisms.

Analyze each statement about Ethernet (IEEE 802.3 CSMA/CD):

Option (A): False. A transmitting station continues to sense the channel during transmission to detect collisions (Carrier Sense Multiple Access with Collision Detection).

Option (B): False. The jamming signal is transmitted immediately after a collision is detected to ensure that all other transmitting stations detect the collision. Padding is done within the frame header/payload, not via jamming.

Option (C): False. Once a collision is detected, transmission of the frame is aborted immediately to avoid wasting channel bandwidth.

Option (D): True. The binary exponential backoff algorithm dynamically expands the range of retransmission slot times $[0, 2^k - 1]$ after the $k$ -th collision (for $k \le 10$ ). By spreading out the random retransmission times of colliding stations over a larger window, it reduces the probability of subsequent collisions.

Therefore, statement (D) is TRUE.

Final answer

Common trap

Thinking a station sends the entire packet regardless of collision.

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