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Carrier Sense and Collision Detection

Ethernet uses the CSMA/CD access method. CSMA/CD is best understood by breaking it down into its component parts:

  • Carrier Sense

  • Multiple Access

  • Collision Detection

Carrier Sense means that the Ethernet stations monitor the Ethernet segment and don't transmit until they sense that no one else is transmitting. This is also called "listen before talk." This prevents one station from transmitting while another is already transmitting.

Multiple Access means that two or more stations share the same Ethernet segment. This is also called shared media. Shared media implies that on a 10Mb Ethernet segment with ten stations, all ten stations share the same 10Mb of bandwidth. This is also known as half-duplex operation. This is in contrast to full-duplex operation where two stations are connected point-to-point and are capable of using the entire bandwidth of the segment. Full-duplex operation also allows stations to transmit and receive at the same time. Chapter 6 discusses full-duplex operation in more detail.

In a half-duplex shared-media environment such as Ethernet, you have multiple stations competing for the same bandwidth. Even with Carrier Sense, two stations can transmit simultaneously. This will cause a collision on the Ethernet media, and both frames will be corrupted. The method used to determine that a collision has taken place is called collision detection. This means that the transmitting stations listen to the Ethernet segment for the frame that it just transmitted. If the frame was corrupted, the transmitting station will know this because the Frame Check Sequence (FCS) will be invalid. The station will continue to transmit for a specified period so that all the other stations on the segment detect that the frame is corrupted. This corruption of the frames is called a collision.

After a collision has occurred, the transmitting station will wait a random amount of time before retransmitting. This is called random backoff. If two stations are transmitting at the same time, detect a collision, and wait the same fixed amount of time before retransmitting, they will collide the next time they both try to transmit.

Key Concept

Ethernet is a CSMA/CD protocol supporting half- and full-duplex operation.

Fast Ethernet interfaces on the Cisco router support both half- and full-duplex operation. They also support auto-sensing of speed and duplex. Following is a sample router configuration of three Fast Ethernet interfaces—the first one is configured for 10Mb half-duplex operation, the second is configured for 100Mb full-duplex operation, and the third is configured for auto-sense of both speed and duplex:

interface FastEthernet0 
  speed 10 
  duplex half 
 
interface FastEthernet1 
  speed 100 
  duplex full 
 
interface FastEthernet2 
  speed auto 
  duplex auto
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