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Local Area High Speed Networks by Dr. Sidnie Feit

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100Mbps Collision Domain Diameter

The smallest Ethernet frame is 64 bytes (512 bits) in length. To make sure that every station in a collision domain becomes aware of a collision, the round-trip time between the two stations that are farthest apart must be less than 512 bit times.

At 100Mbps, a round trip of 512 bit times translates to .00000512 seconds. This is not much time!

The speed of light (c) is 299,792,458 meters per second. The speed of electrons through a cable is between .6c and .9c for most cable types. In addition to time spent traversing cables, some time is spent processing bits as they enter and leave each DTE and intermediate repeater. As a result, you can't go very far in 512 bit times. The result is that a 100BASE-T collision ...

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