What does OSI Layer 1 (Physical) do?
OSI Layer 1 (Physical) transmits raw bits over the media — defining cables, connectors, signals, and bit encoding/timing. Hubs, repeaters, and cables live here.
Layer 1 = Physical Layer
This is where the frame from Layer 2 finally becomes an actual signal on a medium — raw 1s and 0s pushed onto a wire, fibre, or the air. Everything above it is logical; Layer 1 is where bits become physics. To do that it defines:
- Cables and connectors — the mechanical specs (what plug fits what socket).
- Signals — how a "1" versus a "0" is represented electrically/optically.
- Bit timing and encoding — how fast bits are clocked and how they're encoded so the receiver can recover them.
- Physical topology — how devices are physically wired together.
The same bits look different depending on the medium:
| Media type | Signal carrying the bits |
|---|---|
| Copper | Electrical impulses |
| Fiber | Light pulses |
| Wireless | Radio waves |
Key point: there is no addressing at Layer 1 — it neither knows nor cares who the data is for; it just transmits and receives signals. That's why Layer 1 devices (hubs, repeaters, the cables themselves) can't make forwarding decisions and simply repeat everything.
Memory tip: Layer 1 is "Physical stuff" - wires, signals, and hardware!
Go deeper:
Physical layer — cables, connectors, signaling, bit encoding/timing, and why there's no addressing here.