system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does the Ethernet cable from the router to the modem need many conductors when the coaxial cable from the modem to the "Internet" only need 1(plus sheath)?
gpt
Coaxial cable carries the same radio-frequency signals that otherwise could go through the air. Channeling the signal through the cable is just a more efficient means of transmission. The cable company's transmitter is similar equipment to a TV or radio station, just with less amplification. For uploading, the modem on your end does the same.

Radio frequency means that the transmitter mixes the signal with a carrier wave, and the receiver extracts the signal using the same carrier wave. Many signals can be carried on different carriers on the same cable, but they need expensive equipment to combine all their customers' signals, and you need a somewhat expensive modem to extract your particular signals from the mix. You wouldn't want to need a cable modem in every internet-connected device.

Ethernet is designed to run for shorter distances, and to connect to simpler, cheaper circuits. Only one data stream can be carried at any moment, and the router divides time on the cable between your different devices. The usual transmission protocol is called 1000BASE-T:

* **1000** is the total capacity in megabits per second (Mbps).
* **BASE** stands for baseband, as opposed to radio-frequency. The signals are carried as digital pulses of varying voltage, not mixed with a carrier wave. So no modem is needed.
* **T** stands for *twisted pair*, as opposed to coaxial. Coaxial geometry is higher quality, so it could be used just as well, but when wiring an office building one wants to avoid the expense.

OK, so both systems send signals over pairs of conductors. Why does Ethernet use 8 conductors instead of 2? Really, just because of tradition. In the old days, you could put four phone lines on four twisted pairs, and the wires and connectors for this became commonplace.

Today, four pairs carry four times the data as one pair, which is good because 1000 Mbps is a lot of information. The preceding standard, 100BASE-T, only relied on two pairs, potentially leaving half the cable unused. Essentially, 8 conductors instead of 2 is just the difference between having 1000BASE-T and having "250BASE-T" (not a real thing).

---

TL;DR: Coaxial cable allows sending signals to different customers simultaneously. It can do this because it is higher-quality and more expensive. Common "ethernet" cables have the same number of conductors as the pre-ethernet cables they evolved from. But the extra conductors do help to improve their capacity.