system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can Ethernet data speeds be far greater than its bandwidth.

I'm currently doing a report on networking and have become really confused when it comes to the difference between data rate and bandwidth within Ethernet cables. Online it says that digital bandwidth is the amount of electrical pulses in 1 second. If that's the case, shouldn't the maximum data rate be equal to the maximum bandwidth? How can a Cat 6a Ethernet cable have 10 Gigabit speeds at a 500Mhz bandwidth? Maybe I've misunderstood this entirely.
gpt
At it's most simple, each pulse is one bit per wire. But a cat 6 cable has (for Gigabit and 10Gigabit) 4 pairs. Each of those pairs can carry one signal at a time. And since it's not strictly restricted to digital signals (that is, it can be more than a 1 or 0), they can use pulse amplitude modulation (PAM) to make each "signal" represent more than one bit by encoding.

So while the most simple scheme might have +1V equal a binary 1 and 0V equal a binary 0, a simple PAM scheme might allow for more signals to be valid. Let's say a PAM scheme as four distinct levels, and they encode two bits per level. -1V might be 00, -.5V could be 01, +.5V could be 10 and +1V could be 11. Now each symbol represents two bits rather than one. 10GB Ethernet uses a scheme that allows for 16 distinct levels. It also uses other schemes to enable more symbols to get transmitted than one would expect at 500MHz rated cable, it actually sends at a peak of 800Msymbols/sec, and that's per pair.