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CSMA vs CSMA/CD

Many LAN protocols are based on either CSMA (Carrier Sense Multiple Access) or CSMA/CD (CSMA with collision detection) for media access. This application shows the difference between the two algorithms in a space-time-diagram. The horizontal axis shows 4 nodes (A, B, C, D) attached to a linear broadcast bus, the vertical axis represents time. At time t0, node A senses the channel is idle and thus begins transmitting. The packet propagates in both directions along the broadcast medium. Although A is transmitting at time t1, the packet has not reached node D and thus D senses an idle channel and begins transmitting as well. A short time later both signals interfere and cause a collision. CSMA does not perform collision detection and thus both nodes continue to transmit their entire packet. On the other hand, with CSMA/CD the nodes abort transmission a short time after detecting the collision.

The application allows you to detect the differences between the CSMA and CSMA/CD algorithms while transmitting small, medium or large packets. Just press the Start button!

(Example from J. F. Kurose, K. W. Ross: Computer Networking - Ch.: link layer and local area networks)