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Ethernet packet collision over fiber optic?

I heard that lightwaves do not interfere with each other when intersecting. Does this mean there will be no collisions when using Ethernet with fibre? 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.

This was last published in June 2002

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