Which statement is true about the CSMA/CD access method that is used in Ethernet?

  • When a device hears a carrier signal and transmits, a collision cannot occur.
  • A jamming signal causes only devices that caused the collision to execute a backoff algorithm.
  • All network devices must listen before transmitting.
  • Devices involved in a collision get priority to transmit after the backoff period.
Answers Explanation & Hints:

Legacy bus-topology Ethernet LAN uses CSMA/CD as network media access control protocol. It works by detecting a collision in the medium and backing off (after transmitting a jam signal) as necessary. When one host wants to transmit a frame, it listens on the medium to check if the medium is busy. After it senses that no one else is transmitting, the host starts transmitting the frame, it also monitors the current level to detect a collision. If it detects a collision, it transmits a special jam signal so that all other hosts can know there was a collision. The other host will receive this jam signal and stop transmitting. After this, both hosts enter an exponential backoff phase and retry transmission.

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