Bits on the wire

You click a link. 200 milliseconds later a page appears in Tokyo. That gap is not servers being slow — it is physics. Every byte your browser sent started as a physical perturbation: voltage on copper, a laser pulse through glass, or a radio wave through air.

What the physical layer actually does

The physical layer has one job: turn bits into a signal, push that signal through a medium, and decode it back to bits on the other side. Every higher layer — IP, TCP, TLS, HTTP — waits for this to happen.

Three media carry virtually all the world’s bits:

• Copper wire (Ethernet, DSL, USB) — voltage changes on metal conductors.
• Optical fibre (LAN, submarine cables) — laser pulses through a glass core.
• Radio (Wi-Fi, 4G/5G, satellite) — modulated electromagnetic waves through air.

Think of any link as a hose. It has two properties: thickness (bandwidth — how many bits per second fit) and length (propagation delay — how long it takes a bit to travel from one end to the other). Every network decision either deals with the hose being too thin, or with the hose being too long.

The latency floor you cannot escape

Light travels at roughly 300,000 km/s in vacuum and about 200,000 km/s in glass fibre (the glass slows it by about 33%). That finite speed creates an absolute floor:

Real cables add routing overhead, so the actual RTT NYC → London is 70–90 ms and NYC → Sydney is 200–220 ms. No software, no hardware, no protocol makes a photon travel faster. You can only design around this floor.

End-to-end journey of a bit

When you click a link, here is what happens at the physical layer:

1. Your OS hands packet bytes to the network driver.
2. Your NIC encodes bytes as voltage / light / radio signal.
3. The signal travels through local cable or Wi-Fi to your router.
4. Your router decodes back to bytes, processes the IP packet, re-encodes for the WAN uplink — usually fibre going to your ISP.
5. The ISP’s backbone fibre carries the signal across submarine cables to the destination region.
6. Destination NIC decodes signal back to bytes; OS delivers to application.

Each hop involves decode → process → re-encode. Long-distance hops use submarine fibre with optical amplifiers (EDFAs) every ~80 km to keep the signal strong without converting to electrical.