Ethernet is a shared-media broadcast technology. The access method CSMA/CD used in Ethernet performs three functions:

• Transmitting and receiving data packets
• Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model
• Detecting errors within data packets or on the network

In the CSMA/CD access method, networking devices with data to transmit work in a listen-before-transmit mode. This means when a node wants to send data, it must first check to see whether the networking media is busy. If the node determines the network is busy, the node will wait a random amount of time before retrying. If the node determines the networking media is not busy, the node will begin transmitting and listening. The node listens to ensure no other stations are transmitting at the same time. After completing data transmission the device will return to listening mode.

Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all devices see the collision. Once all the devices have detected the collision a backoff algorithm is invoked and transmission is stopped. The nodes stop transmitting for a random period of time, which is different for each device. When the delay period expires, all devices on the network can attempt to gain access to the networking media. When data transmission resumes on the network, the devices that were involved in the collision do not have priority to transmit data.

Interactive Media Activity Drag and Drop: Collision Detection Flowchart After completing this activity, the student will be able to identify the procedure of collision detection in an Ethernet network.

   

Web Links Carrier Sense Multiple Access (CSMA) http://www.cisco.com/en/US/tech/tk389/ tk214/tk125/ tech_protocol_ home.html