| Like Fast Ethernet, Gigabit Ethernet
supports both full- and half-duplex modes with flow control. In
half-duplex mode, the system operates using CSMA/CD and must
consider the reduced slotTime even more than Fast Ethernet. The
slotTimes for 10BASE-X and 100BASE-X networks are 51.2 and 5.12
microseconds, respectively. These are derived from the smallest
frame size of 64 octets. In the 100BASE-X network, the slotTime
translates into a network diameter of about 200 meters. If the same
frame size is used in Gigabit Ethernet, the slotTime reduces to
0.512 microseconds and about 20 meters in diameter. This is not
feasible. Therefore, 802.3z developed a carrier extension that
enables the network distance to extend further in half-duplex mode
and still support the smallest 802.3 packets.
The carrier-extension process
increases the slotTime value to 4096 bits or 4.096 microseconds. The
transmitting station expands the size of the transmitted frame to
ensure that it meets the minimal slotTime requirements by adding
nondata symbols after the frame check sequence (FCS) field of the
frame. Not all frame sizes require carrier extension. The 8B/10B encoding scheme
used in Gigabit Ethernet defines various combinations of bits called
symbols. Some symbols signal real data, whereas the rest indicate
nondata. The station appends these nondata symbols to the frame. The
receiving station identifies the nondata symbols, strips off the
carrier extension bytes, and recovers the original message. The
figure shows the anatomy of an extended frame.
The addition of the carrier extension
bits does not change the actual Gigabit Ethernet frame size. The
receiving station still expects to see no fewer than 64 octets and
no more than 1518 octets.
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