system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do two devices on the internet agree on a transmission rate?

I’m teaching a high school intro to computer science course and this question came up, but I can’t find an answer at the very low level that we’re talking about.

We know that the most basic level of the internet is a wire with an electrical current. You can set or read the wire voltage as high or low, and interpret that as 0 or 1. The only way to consistently send/receive messages is to have an agreed timing protocol of when to read and when to set the wire.

But in the real internet, devices all communicate at different connection speeds...so there are clearly multiple timings. How is that decided? Ideally at the physical/binary layer.

The best I can imagine is sort of like radio systems, where there’s a default channel (timing in this case), and then on that channel you both agree to change to a different frequency. That would also explain why transfer rates fluctuate during the same connection or download, since a computer could be switching timings several thousand times a second if it was needed for the best speed. But can anyone verify that this is an accurate analogy?
gpt
Across the Internet they don't sync up transmission rates. The Internet is packet switched, there is no direct connection. 

One device sends data in packets, which is received by a router, and then retransmitted to another router, and so on until it reaches the intended destination.

Generally speaking Internet protocols have to be tolerant of packets not arriving with consistent timings. Or even packets not arriving or being corrupted along the way.

It's only necessary to sync up with with the device that's the next step along the way. So your PC has to sync up timings with your router, and your router has to sync up with your ISP.