Carrier extension concept
To preserve the 200 m network diameter of 100Base-T using half-duplex links, the 802.3z working group for Gigabit Ethernet implemented a techniques called carrier extension.
Whenever the Gigabit Ethernet adapter transmits a frame less than 512 Bytes the carrier extension mechanism maintains the total frame size at 512 Bytes by adding non-data carrier extension Bytes. If Gigabit Ethernet detects a collision during this period it sends out a jam signal so offending stations back-off and try again.
The 512 Byte limit (instead of 640 Byte) is possible because the number of repeater hops is reduced from two (100Base-T) to one (Gigabit Ethernet) and the built-in engineering safety margin (extra time) is eliminated.
Recognizing the potential impact on sustained throughput, the 802.3z working group also implemented a frame bursting mode in which the transmitting station may sequentially transmit frames without contending for the medium for each frame for a period up to the burstLength.