OSI Layer: Physical Layer (L1)
Definition
The Physical Layer (OSI Layer 1) is responsible for the actual transmission of raw bits over a physical medium — the lowest layer, dealing purely with hardware, electrical/optical/radio signals, and physical connections, with no awareness of addressing, framing, or error control (those begin at the Data Link layer).
Core Functions
1. Bit-to-Signal Encoding
Represents binary 0s/1s as physical signals appropriate to the medium:
- Digital-to-digital encoding — e.g., NRZ (Non-Return-to-Zero), Manchester encoding (each bit has a guaranteed transition, aiding clock synchronisation), Differential Manchester.
- Digital-to-analogue modulation — e.g., ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), PSK (Phase Shift Keying) — used when digital data must travel over an analogue channel (e.g., traditional modems).
- Analogue-to-digital conversion — e.g., PCM (Pulse Code Modulation), used to digitise analogue voice for transmission over digital telephone networks.
2. Physical Topology
Defines how devices are physically connected (bus, star, ring, mesh) — see the dedicated Network Topologies page for full classification and trade-offs.
3. Transmission Mode (Direction of Flow)
- Simplex — data flows in one direction only (e.g., a keyboard to a computer, or a broadcast TV signal).
- Half-duplex — data flows both ways, but only one direction at a time (e.g., a walkie-talkie).
- Full-duplex — data flows both ways simultaneously (e.g., a telephone call, modern Ethernet).
4. Multiplexing (sharing one physical medium among multiple signals)
- FDM (Frequency Division Multiplexing) — the available bandwidth is split into non-overlapping frequency bands, each carrying one signal continuously; classic analogue technique (e.g., radio/TV broadcast, older telephone trunking).
- TDM (Time Division Multiplexing) — the full bandwidth is used by each signal, but only during its own dedicated, rotating time slot; the standard digital technique (e.g., T1/E1 telephone trunks).
- Synchronous TDM — fixed time slots assigned regardless of whether a source has data to send (can waste slots).
- Statistical (Asynchronous) TDM — slots are dynamically assigned only to sources that actually have data, improving efficiency.
- WDM (Wavelength Division Multiplexing) — the fibre-optic analogue of FDM: multiple signals are carried simultaneously on the same fibre using different light wavelengths (colours).
- CDM (Code Division Multiplexing) — every signal is transmitted across the full bandwidth simultaneously, but each is distinguished using a unique spreading code (basis of CDMA mobile technology).