Collisions in an Ethernet network have nothing to do with the physical medium. Ethernet by design is a CSMA/CD technology. CSMA/CD means Carrier Sense, Multiple Access / Collision Detection. What this means is that each device connected to an Ethernet network listens for a carrier and sends data once that carrier or data signal is detected. Since it's multiple access, there is no restriction to when a device sends data. Therefore, two devices can transmit at the same time, causing a collision. After a collision occurs, both devices stop transmitting, wait a specified amount of time (called jabber control) and then start the process all over again. Even though in today's technology an Ethernet network is physically a star or spoke topology, it is still logically a bus topology so collisions can occur. This applies regardless of whether the medium is copper to fiber.
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