# How can 100Base-T achieve 100 meter distance at 125Mhz clock?

I admit I have some large holes in my understanding of this topic, but it seems like you run into physical limitations of the speed of light.

If the speed of electricity through copper is about 280,000,000 meters per second, and the clock on the ethernet card is running at 125Mhz (found this online, not sure if this is the right speed), the signal should only be able to travel about `280,000,000 * (1/125,000,000) = ~2.24 meters` before the next clock cycle starts.

Assuming only 1 bit can be on a wire at a time (possibly a bad assumption), how is 100 meters possible?

• Bad assumption ;-p – Ron Trunk Jan 7 '17 at 17:49
• Hah, figures. Could you elaborate? I'm guessing some kind of multiplexing is involved? – ConditionRacer Jan 7 '17 at 17:50
• This question is probably better asked on Electrical Engineering. The IEEE has the complete description of this in the 802.3u standard. By the way, ethernet will fill a link with bits traveling on the link. The original minimum ethernet frame size was so that the entire link would be full in order to detect collisions. – Ron Maupin Jan 7 '17 at 17:50

## 1 Answer

Your assumption that only one bit can be on the wire at a time is completely incorrect. Your calculations can compute how far apart the signals on the wire are, but all the bits of the entire frame can be on the wire at the same time.

You also need to understand that one data bit does not equal one bit on the wire. You will need to understand the encoding used to figure that out.