Faster Physical layer implementations of Ethernet introduce complexities to the management of collisions.
As discussed, each device that wants to transmit must first "listen" to the media to check for traffic. If no traffic exists, the station will begin to transmit immediately. The electrical signal that is transmitted takes a certain amount of time (latency) to propagate (travel) down the cable. Each hub or repeater in the signal's path adds latency as it forwards the bits from one port to the next.
This accumulated delay increases the likelihood that collisions will occur because a listening node may transition into transmitting while the hub or repeater is processing the message. Because the signal had not reached this node while it was listening, it thought that the media was available. This condition often results in collisions.
Timing and Synchronization
In half-duplex mode, if a collision has not occurred, the sending device will transmit 64 bits of timing synchronization information, which is known as the Preamble.
The sending device will then transmit the complete frame.
Ethernet with throughput speeds of 10 Mbps and slower are asynchronous. An asynchronous communication in this context means that each receiving device will use the 8 bytes of timing information to synchronize the receive circuit to the incoming data and then discard the 8 bytes.
Ethernet implementations with throughput of 100 Mbps and higher are synchronous. Synchronous communication in this context means that the timing information is not required. However, for compatibility reasons, the Preamble and Start Frame Delimiter (SFD) fields are still present.
For each different media speed, a period of time is required for a bit to be placed and sensed on the media. This period of time is referred to as the bit time. On 10-Mbps Ethernet, one bit at the MAC layer requires 100 nanoseconds (nS) to transmit. At 100 Mbps, that same bit requires 10 nS to transmit. And at 1000 Mbps, it only takes 1 nS to transmit a bit. As a rough estimate, 20.3 centimeters (8 inches) per nanosecond is often used for calculating the propagation delay on a UTP cable. The result is that for 100 meters of UTP cable, it takes just under 5 bit times for a 10BASE-T signal to travel the length the cable.
For CSMA/CD Ethernet to operate, the sending device must become aware of a collision before it has completed transmission of a minimum-sized frame. At 100 Mbps, the device timing is barely able to accommodate 100 meter cables. At 1000 Mbps, special adjustments are required because nearly an entire minimum-sized frame would be transmitted before the first bit reached the end of the first 100 meters of UTP cable. For this reason, half-duplex mode is not permitted in 10-Gigabit Ethernet.