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.
GATE 2016 · Computer Networks · Ethernet · medium
Answer: Statement D is TRUE. The exponential backoff mechanism reduces the probability of collision on retransmissions. Answer: D.
- Evaluate Statement A: A says a station stops sensing once it starts transmitting. This is FALSE. In CSMA/CD, a station monitors the channel throughout the entire transmission to detect collisions.
- Evaluate Statement B: B says the jamming signal pads small frames. This is FALSE. The jam signal (typically 32 bits) is broadcast after a collision is detected to ensure all stations learn of the collision. Padding short frames to the minimum Ethernet frame size (64 bytes) is a separate requirement unrelated to jamming.
- Evaluate Statement C: C says a station continues transmitting after detecting a collision. This is FALSE. The 'CD' in CSMA/CD means the station aborts transmission immediately upon collision detection.
- Evaluate Statement D: D says exponential backoff reduces probability of collision on retransmissions. This is TRUE. After each successive collision, the window size doubles. With a larger window, two stations are less likely to choose the same slot time, reducing collision probability. For example, after 3 collisions, each station picks from 8 slots; the probability both pick the same slot is 1/8, much less than 1/2 after the first collision.