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As part of my course I’ve been reading the paper Ethernet: Distributed Packet Switching for Local Computer Networks. I understand that “classic” Ethernet (over coaxial cable) has a maximum length of 2500m while Ethernet over twisted pair has a maximum length of just 100m.

While Googling for an answer I found a whose accepted answer is: The specification of 328 feet has to do entirely with collision detection in a CSMA/CD (Carrier Sense Multi Access / Collision Detection network. Amie Notes Section A Pdf here. The length is limited by the fact that the shortest possible frame size (64 bytes) can be sent out on the wire and if a collision occurs, the sending node will still be sending that frame when it hears the collision. Turbocash Manual. However, I understand that full duplex, packet switched Ethernet networks because the connection is point and point (i.e. Your computer is connected to an Ethernet switch - there are no other computers physically sharing the same cable with you) and data is sent and received on separate wires. So, I must ask: why is Ethernet over Cat5 limited to 100m?

It can't be because of collision detection, since full duplex Ethernet (which I suspect make up almost 99% of all LANs, unless anyone is still running a bus network from 1995) does not suffer from collisions. If I had to guess I would guess that it is due to attenuation and signal degradation over the copper wire. First, you're correct in saying that it's not linked to CSMA/CD. Second, you referenced a common, but incorrect belief that CSMA/CD was the reason for 10Base-T [half-duplex] 100m limit. This was a reason for - as you called it - classic Ethernet network length of 2500m (with ample margin - minimum frame of 64 bytes at 10Mb/s would 'occupy' around 11000m of cable - or to word it differently - collision would be heard back by sender at about middle of transmission) 1. It is linked to the electrical interface and signal characteristic outlined in the standard. One of the ideas behind twisted pair was to use existing cabling - and 100m was around max length that still satisfied parameters like attenuation, crosstalk, etc.: 14.4.1 Overview The medium for 10BASE-T is twisted-pair wiring.