Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
31
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.
Fig. 3-4 Gigabit Ethernet Frame
64 bytes
