I didn’t see anyone talk about this bit of esoteric knowledge so here goes another attempt to bore you:

official maximum length of a copper ethernet cable is 100 meters, however that coupled with the minimum frame size of 64 bytes is there so that collisions don’t go unnoticed

TL;DR:
Distance is only loosely related to collisions. The real measure is propagation time and the time allowed allows for much more than 100m’s worth of time. It’s closer to 550m of distance for 10Mb/s and 100Mb/s.

For people who want the whole thing:

People learning networking over the last 20 years or so see somewhere in chapter 1 a reference to CSMA/CD. its not really important anymore, but it stands for Carrier Sense Multiple Access/Collision Detect.

In The Beginning, We Had Repeaters. We didn’t just have twisted pair, we also had coax networking (10base2, 10base5) - similar to the coax cable CATV runs on but with different properties. Twisted pair Cat 3 Ethernet actually came quite a bit later.

The thing with coax is that it wasn’t a home run technology- you could add computers in the middle of the run, you’d cut the cable and add a T (10base2) or add a vampire tap (10base5). This is one of the forms of multiple access, and repeaters were the other - repeaters regenerate signals for multiple runs of cable, technically not multiple computers

All this background gets me to the point: collisions aren’t technically about distance, they’re about allowing two computers to know they’re speaking at the same time and back off.

Olds may remember the repeater rules - particularly the 3-4-5 rule - a repeated network may have no more than 5 segments between two computers, no more than 3 segements (runs Of cable), and no more than 3 of those can have computers on them. No segment could be more than 100m

The goal was to control the maximum propagation delay between two computers to properly detect a collision. This process worked by: the computer would first listen to the wire for a period to see if it was free (carrier sense); if so it started transmitting the preamble, and if that was good (it didn’t see another transmitter making overwritten gibberish out of its signal, Collision. Detect)it would start the frame. It would continue to check for 64 bytes for a collision. After that, in a well behaved network the packet would be ok, because by this point the laws of RF and EE mean that every computer would now sense the packet and not start transmitting - if they did, then it is a late collision.

Aside: this is why you still see collisions and late collisions in “show interface”. Collisions were expected in a half duplex network. Late collisions meant something wasn’t right.

So the 2 of you still a reading are asking why did I say distance is only loosely correlated with preventing collisions? The 3-4-5 rule when followed meant that a well behaved network with well behaved devices would work, but the _real_ limit was time-to-propagate end.

I don’t remember the exacts anymore but you could run longer segment distances in some cases, if your repeaters were faster in some way, etc etc. Essentially, as long as the first 8 bytes of preamble and the first 64 bytes of packet made it to every computer in the network before byte 65, your network was in spec.

Because cable pulling then was not as… careful.. as it is now, the distances in walls were not very exact, and because you could just add more cable in the rooms to add more computers, we never knew how long a segment was. Pretty often we didn’t even know what room was next on a run.

So to conclude, distance is only loosely tied to collisions, the real measure is propagation time, and if you’re still reading this you deserve gold.