Different types of Ethernet (standard Ethernet, Fast Ethernet, Gigabit Ethernet) have different speed of data transfer (10 Mbit, 100 Mbit, 1 Gbit). Why is that?
Progress - technology advances.
Including obsolete and brand new physical layers, Ethernet ranges from 1 Mbit/s to 400 Gbit/s.
On the physical layer, the speed of electrical signals (voltage change) stays the same.
The wave propagation speed depends on the medium, not the signaling rate. Cat-3 is slightly slower than the Cat-5 and fiber, and coax (from ancient 10BASE5) and Cat-7/8 are the fastest - they're all very close though, 60% to 80% speed of light.
However, the frequencies that pass over the medium differ considerably - from 10 MHz for 10BASEx to ca. 1.6 GHz for 40GBASE-T. Of course, higher frequency means faster voltage change. On fiber, the current generation runs 50 Gbit/s per lane.
The cable (twisted pair) is also the same (I think).
No. Cat-3 only supports 10 Mbit/s, Cat-5e up to 1 Gbit/s, 10 Gbit/s requires Cat-6A, and 25/40 Gbit/s require Cat-8 cable rated at 2 GHz. The cable is only the same (Cat-5e) for 100BASE-TX and 1000BASE-T due to a leap in encoding technology.
So why does the maximum bandwidth differ? If the reason is that in the past, we didn't have so much processing power in network cards, why doesn't the protocol allow for bandwidth negotiation (that would suit both devices that communicate)?
Yes. Faster data rates mean faster processing and more elaborate encoding using more silicon (=transistors). Also, back in the 80s and early 90s, computers lacked the internal bandwidth to make use of faster data rates - they even struggled with 10 Mbit/s for a while. Literally nobody would have had a use for 10+ Gbit/s back then.
Ethernet does allow for speed negotiation on twisted pair - faster (and some intermediate) rates have been added for a while:
- 1 Mbit/s (1987)
- 10 Mbit/s (1990)
- 100 Mbit/s (1995)
- 1000 Mbit/s (1999)
- 2500 Mbit/s (2016)
- 5 Gbit/s (2016)
- 10 Git/s (2006)
- 25 Gbit/s (2016)
- 40 Gbit/s (2016, most likely the final speed for twisted pair)
- fiber currently adds 50, 100, 200, and 400 Gbit/s (2017)