What is the physical layer?

Ultimately, the internet is physical, material. So we have all these computers lying around. They're all over the world, and we want them all to talk to each other.

Before we can do anything, we have to make sure they can send physical signals to each other. How can we do this?

These days, the internet probably comes to you through the air. Probably from the WiFi router closest to you. Maybe from the cell tower closest to your phone. If you're very rich, or live in a very remote place, perhaps the internet comes to you by satalite. But these are last-mile solutions.

If you were using the internet in the 90s, you'll remember not being able to get online when someone was talking on the phone, and vice versa. That's because, in those days, the internet traveled through the same physical media as the phone: copper wire. A modem (modulator/demodulator) turned data packets into sounds. You could quite literally hear it "dialing up" (opens new window) when it connected to the broader internet.

Undersea cables carry between 95 and 99% of all internet traffic (Starosielski, 2015; Sherman, 2021). About ten trillion US dollars depend on them (Vatanparast, 2020; Starosielski, 2015). They're hugely impactful to infrastructure providers and end-users alike (Liu et al, 2020).

References

Sherman, J. (2021). Cyber-defense across the ocean floor: the geopolitics of submarine cable security.

Starosielski, N. (2015). The undersea network. : Duke University Press.

Vatanparast, R. (2020). The infrastructures of the global data economy: undersea cables and international law. Harvard International Law Journal Frontiers, 61(2020), 1–9.

Liu, S., Bischof, Z. S., Madan, I., Chan, P. K., & Bustamante, Fabi'an E. (2020). Out of sight, not out of mind: a user-view on the criticality of the submarine cable network. In , Proceedings of the ACM Internet Measurement Conference (pp. 194–200). New York, NY, USA: Association for Computing Machinery.