Ethernet MAC transmits frames in half-duplex and full-duplex ways. In half- duplex operation mode, the MAC can either transmit or receive frame at a moment, but cannot do both jobs at the same time.
As the transmission of a MAC frame with the half-duplex operation exists only in the same collision domain, the carrier signal needs to spend time to travel to reach the targeted device. For two most-distant devices in the same collision domain, when one sends the frame first, and the second sends the frame, in worst- case, just before the frame from the first device arrives. The collision happens and will be detected by the second device immediately. Because of the medium delay, this corrupted signal needs to spend some time to propagate back to the first device. The maximum time to detect a collision is approximately twice the signal propagation time between the two most-distant devices. This maximum time is traded-off by the collision recovery time and the diameter of the LAN.
In the original 802.3 specification, Ethernet operates in half duplex only. Under this condition, when in 10Mbps LAN, it’s 2500 meters, in 100Mbps LAN, it’s approximately 200 meters and in 1000Mbps, 200 meters. According to the theory, it should be 20 meters. But it’s not practical, so the LAN diameter is kept by using to increase the minimum frame size with a variable-length non-data extension bit field which is removed at the receiving MAC. The following tables are the frame format suitable for 10M, 100M and 1000M Ethernet, and some parameter values that shall be applied to all of these three types of Ethernet.
Actually, the practice Gigabit Ethernet chips do not feature this so far. They all have their chips supported full-duplex mode only, as well as all network vendors’ devices. So this criterion should not exist at the present time and in the future. The switch’s Gigabit module supports only full-duplex mode.
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Fig. 3-4 Gigabit Ethernet Frame
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