Layer 1 — Physical
Bits on a medium: voltage, light, radio, connectors, and the limits of Shannon.
Level: beginner
What is it?
Layer 1 is the analog (or optical) world: how a bit is a voltage, a light pulse, or a radio symbol. Cables, connectors, repeaters, hubs, NICs’ PHYs. No addresses, no frames — just a stream of symbols and a link that is up or down.
Why does it matter?
If the link light is off, nothing above exists. Bandwidth, latency (propagation), and BER start here.
Mental model
The road. No road, no trucks, no letters.
How it works
- Purpose: transmit raw bits over a medium.
- PDU: bits / symbols. No addressing.
- Media: twisted pair, coax, fiber, radio. Encoding and modulation.
- Devices: hubs, repeaters, transceivers, cables, the PHY side of a NIC.
- Limits: Shannon capacity, attenuation, noise, duplex vs half-duplex.
Real-world usage
- Cabling plants
- Optical networks
- Wi-Fi radios
Common mistakes
- Calling a switch L1
- Confusing bandwidth (Hz or bps) with throughput
Interview questions
Hub vs switch?
Hub: L1 repeater, one collision domain. Switch: L2, per-port collision domain.
Name four latency components.
Propagation, transmission, queueing, processing. L1 is mostly the first two.