Cisco CCNA mock exam questions sample test – Question 9

Which Ethernet LAN contention or access method listens for a signal on the channel before transmitting data, and stops transmitting if a collision is detected?


Correct Answer: B

The Carrier Sense Multiple Access – Collision Detection (CSMA/CD) contention method verifies that a channel is clear before transmitting, and stops transmitting data when it detects a collision on the channel in use.

Carrier Sense Multiple Access (CSMA) is the channel access mechanism used by Ethernet LANs. CSMA defines when and how to access the channel to transmit data. There are two variants of CSMA: CSMA with Collision Avoidance (CSMA/CA) and CSMA/CD.

With CSMA/CD, the transmitting station waits to detect channel traffic before sending the first packet over the channel. If the channel happens to be idle, the station transmits its packets. Despite the process of checking the channel before transmitting, it is still possible for two stations to transmit at once, resulting in collisions. If a collision occurs, the transmitting stations perform a retransmission. This retransmission uses a back-off algorithm by which a station waits for a random amount of time before retransmitting. As soon there is a collision on the network, the transmitting station stops transmitting and waits for a random interval of time before attempting the transmission again.

You should not select CSMA/CA. With Carrier Sense Multiple Access – Collision Avoidance (CSMA/CA), the transmitting station listens for a signal on the channel, then only transmits when the channel is idle. If the channel is busy, it waits a random amount of time before re-attempting transmission. CSMA/CA protocol is used in 802.11-based wireless LANs, while CSMA/CD is used in Ethernet LANs. Collisions are more often avoided with CSMA/CA than with CSMA/CD because sending stations signal non-sending stations to “wait” a specific amount of time and then check for clearance again before sending. The cost of these mechanisms is reduced throughput.

CSMA/CB and CSMA/CS are invalid Ethernet contention methods, and are therefore incorrect options.

LAN Switching Fundamentals

Sub-Objective: Describe and verify switching concepts

Cisco Documentation > Internetwork Troubleshooting Handbook > Troubleshooting Ethernet
Cisco > Tech Notes > Troubleshooting Ethernet Collisions > Document ID: 12768
Cisco > Technology Support > Ethernet > Carrier Sense Multi-Access/Collision Detection (CSMA/CD)