2.3 Gigabit Ethernet
2.3.3 Full-duplex and half-duplex support
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.