11.6.2.3 Later Versions of Ethernet or CSMA/CD. The later versions of Ethernet
came about when users demanded higher and higher data rates. We started at 10 Mbps,
thence to 100 Mbps, 1000 Mbps, and finally 10,000 Mbps (10 Gbps). As these transmission
rates speeded up, there was less and less time to detect carrier presence or a collision
occurrence. The velocity of propagation is essentially constant for all the transmission
rates using the same transmission medium type. The frame lengths remained the same
(1518 octets), but at 100 Mbps the time to transmit a frame is one-tenth that of 10 Mbps.
It would follow that 1 Gbps would be 100 times shorter than the 10-Mbps case.
One way to improve the situation to permit the faster data rates is to reduce the
maximum length of the medium. This shortens the roundtrip propagation time to detect
carrier/collision. We remember that the maximum length of Ethernet at 10 Mbps was 2500
m with repeaters. This length dropped by an order of 10 to about 200 m for 100 Mbps
using two repeaters instead of five. These maximum lengths can be increased to 412 m
if a fiber-optic medium is employed and to 316 m for 1 Gbps.
At 1000 Mbps the developers turned to frame bursting. In this burst mode the MAC
is allowed to send a short burst of frames equal to approximately 5.4 maximum-length
frames without having to relinquish control of the medium. The transmitting MAC station
fills each interframe interval with extension bits so that other stations on the network will
see that the network is busy and will not attempt transmission until the burst is complete.
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