Round Trip Delay: 51,2 µs

The collision detection function depends on a maximum round trip delay of 51,2 µs. This means: A transceiver has to transmit as long as any collision signal may need to arrive at the sending transceiver (collision detection is done by comparing voltage levels / see above).

Therefore, Ethernet segments are restricted to certain lengths equal to a given maximum round trip delay of 51,2 µs.

1. Slot Time (25,6 µs)

After 25,6 µs the first bit of a frame has to arrive at the logical segment's end (including all repeaters / including all segments connected by repeaters). As long as the frame's header didn't arrive the segment's end, any other station between frame header and the segment's end may begin to transmit a frame (= multiple access). Collisions may only occur within the first 25,6 µs of a current transmission (slot time; collision window); this is equal to 32 byte times / 256 bit times.

If a collision happens only a few microseconds (1 bit = 0,1 µs) before end of slot time (few microsends less than 25,6 µs), the collision (caused by station B) will take another 25,6 µs until it arrives at the transceiver that transmits the first frame (station A). Station A has still to send bits (of the same frame) at this time in order to ensure that its collision detection function will detect the collision by comparing input/output voltage levels. This means that any transmission has to last for at least 2 * 2,56 µs = 51,2 µs (RTD time) / 2 * 32 byte times = 64 byte times.

3. Mindest-Paket-Länge (64 bytes)

This results in a minimum frame length of 64 bytes. This minimum frame length is related to a RTD of 51,2 µs.

4. Maximale Kabel-Längen

Because Ethernet's RTD of 51,2 µs must not be exceeded, cable lengths are related to Ethernet's slot time of 25,6 µs. Because physical transmission speed depends on the type of cable, the use of different cables results in different length restrictions.

Anyway, signal delay caused by repeaters must be taken into account as well.

5. Collision Domain / Time Domain

All cable segments within the 51,2 µs time domain build a collision domain: Collisions may only occur within these cable segments; and: any collision within this time domain can be detected by any transceiver.

6. Late Kollisions

In case of illegal cable length / exceeded slot time collisions cannot not be detected in time (= within 51,2 µs = within the first 64 bytes) by station A (see above) if station B resides at an illegal (additional) cable segment not covered by the slot time (not part of the collision domain).

These collisions are called late collisions because from station A's point of view they arrive too late (after the 64th byte has been sent) respectively they occured after more than 25,6 µs related to start of transmission by station A.

Protocol analyzers can detect late collisions while generating test frames longer than 64 bytes. Late collisions indicate illegal cable length.