Where raw bits travel as copper voltages, light pulses, or radio signals.
PDU: Bit
Focus: Electrical/Optical Signals
Hardware: Hubs, Repeaters, Cables, Modems
Core Purpose & Function
Layer 1 has no understanding of packet headers, addresses, or applications. It is strictly concerned with sending a series of electrical or optical pulses representing "1" or "0" down a wire, and ensuring that a receiver interprets those physical pulses back into matching bits.
Key Responsibilities
Bit Representation: Defining how 1s and 0s are coded (e.g. changes in voltage levels, light brightness, or radio frequency phases).
Bandwidth & Data Rates: Governing the maximum bit rate (transmission speed in Mbps/Gbps) supported by a cable type.
Physical Media Specifications: Standardizing wire thicknesses, fiber core widths, connector shapes (like RJ45 plugs), and shielding specifications.
Line Configuration & Duplexing: Defining whether transmission is point-to-point, simplex (one-way), half-duplex, or full-duplex.
Common Physical Media
| Medium Type | Description | Typical Speed | Pros / Cons |
|---|---|---|---|
| Unshielded Twisted Pair (UTP) | Copper wires twisted to reduce interference | 100 Mbps – 10 Gbps | Cheap, flexible, but short range (~100m) |
| Fiber Optic Cable | Glass strands transmitting light pulses | 1 Gbps – 100+ Gbps | Immune to electromagnetic interference, long range |
| Coaxial Cable | Shielded copper conductor | 10 – 100 Mbps | Resistant to noise, used for cable TV/broadband |
| Radio Frequencies | Electromagnetic waves traveling through air | Up to 10 Gbps (Wi-Fi 7) | No physical wires needed, but prone to walls/noise |