Network, Computer and Programming Resources

Network - Collision Detection and Avoidance

In an Ethernet network, a collision is the simultaneous presence of signals from two nodes on the network. A collision can occur when two nodes each think the network is idle and both start transmitting at the same time. Both packets involved in a collision are broken into fragments and must be retransmitted. To detect a collision, nodes check the DC voltage level on the line. A voltage level two or more times as high as the expected level indicates a collision, since this means there are multiple signals traveling along the wires at the same time. Collision detection in broadband networks involves a separate bandwidth for collision detection and is somewhat more complex, since there may not be any DC voltage to test. In the CSMA/CD (carrier sense multiple access/collision detection) media-access method, for example, collision detection involves monitoring the transmission line for Collision Detection special signals that indicate that two packets were sent onto the network at the same time and have collided. When this happens, special actions are taken (as described in the CSMA/CD article). To avoid collisions, nodes can send special signals that indicate a line is being used for a transmission. For example, the CSMA/CD media-access method uses RTS (Ready To Send) and CTS (Clear To Send) signals before sending a frame onto the network. A node transmits only after the node has requested access to the line and been granted access. Other nodes will be aware of the RTS/CTS transmission and will not try to transmit at the same time. BROADER CATEGORIES CSMA/CD (Carrier Sense Multiple Access/Collision Detect); Ethernet