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CSMA/CD: Transmission Process

On a half-duplex Ethernet network, multiple nodes share a single transmission medium. A MAC protocol is used to allow the nodes to gain access to the shared transmission medium and to ensure error-free data transfer.

CSMA/CS is the MAC protocol used on half-duplex Ethernet networks. It manages the access to the shared transmission medium and prevents multiple nodes from transmitting frames simultaneously. If two or more nodes simultaneously transmit frames, CSMA/CD ensures that the frames are detected and retransmitted.

To ensure reliable frame transfer, CSMA/CD follows a sequence of steps. The first step for transmitting a frame by using CSMA/CD is carrier sensing or listen before talking. A node that has to transmit a frame monitors the transmission medium to detect whether the medium is free. The node does this by sensing a carrier on the medium. A carrier is a specific level of voltage used to transmit a frame. The presence of a carrier on the transmission medium indicates that the medium is being used. If a carrier is detected, the transmission is deferred until the carrier ceases and the transmission medium is free. The node continues to monitor the transmission medium until the carrier ceases.

However, if the node does not detect a carrier, it indicates that the transmission medium is free. The node can then transmit the frame. The node begins the transmission after waiting for a minimum period of time known as an interframe gap (IFG). At any specific point in time, multiple nodes may sense a carrier on the transmission medium. At times, more than one node may simultaneously determine that the transmission medium is free and begin to transmit frames.

In a situation where two or more nodes simultaneously transmit frames a collision occurs. A collision corrupts the frames that are transmitted together. To detect a collision, the node that has initiated a transmission needs to monitor the transmission medium for a possible collision. This process is known as listen while talking. During the process of listen while talking, the transmission is complete if a collision is not detected. However, if a collision is detected, the node stops the transmission of the frame. This process is known as collision detection.

After stopping the transmission of the frame, the node sends a collision enforcement jam signal to the other nodes on the network. When the other nodes receive the jam signal, they discard the frames that are corrupted by the collision. After sending the jam signal, the nodes involved in the collision wait for an indefinite period of time before retransmitting the frames. This process in which the nodes wait before transmitting frames is known as backoff.



The backoff period is determined by a random number generated by a collision counter located on each node. After a backoff, the nodes involved in the collision try to transmit data again. This may cause repeated collisions. The possibility of repeated collisions is reduced if each node backs off for a different period of time. The value of the backoff period is cumulative until a node retransmits a frame without a collision.

Finally, the last step in the process of transmitting a frame by using CSMA/CD is retransmitting the frame that is corrupted because of a collision. This step is performed after a collision is detected, and the node backs off for a period of time. If the retransmission is successful, the node clears its collision counter.


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Content copyright © 2013 by Cathy Spearmon. All rights reserved.
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