Cisco 100-105 Exam

Which two statements describe the operation of the CSMA/CD access method? (Choose two.)

  • A. After a collision, all stations run a random backoff algorithm. When the backoff delay period has expired, all stations have equal priority to transmit data.
  • B. In a CSMA/CD collision domain, multiple stations can successfully transmit data simultaneously.
  • C. After a collision, the station that detected the collision has first priority to resend the lost data.
  • D. The use of hubs to enlarge the size of collision domain is one way to improve the operation of the CSMA/CD access method.
  • E. After a collision, all stations involved run an identical backoff algorithm and then synchronize with each other prior to transmitting data.
  • F. In a CSMA/CD collision domain, stations must wait until media is not in use before transmitting.
Answer: Option A. F.
Explanation: 

Ethernet networking uses Carrier Sense Multiple Access with Collision Detect (CSMA/CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes. And trust me, good collision management is crucial, because when a node transmits in a CSMA/CD network, all the other nodes on the network receive and examine that transmission. Only bridges and routers can effectively prevent a transmission from propagating throughout the entire network! So, how does the CSMA/CD protocol work? Like this: when a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear (no other host is transmitting), the host will then proceed with its transmission. But it doesn’t stop there.
The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data (think, busy signal). The nodes respond to that jam signal by waiting a while before attempting to transmit again. Backoff algorithms determine when the colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit will then time out.

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