core.net

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

• Transmitting and receiving data frames
• Decoding data frames and checking them for valid addresses before passing them to the upper layers of the OSI model
• Detecting errors within data frames 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 nodes detect the collision. When all nodes have detected the collision, the backoff algorithm is invoked and transmission stops. The nodes stop transmitting for a random period of time, determined by the backoff algorithm. When the delay periods expire, each node can attempt to access the networking media. The devices that were involved in the collision do not have transmission priority.

References: Cisco Networking Academy Program-CCNA 1 Networking Basics, 6.2.2 MAC rules and collision detection/backoff