// IEEE 802 · ITU-T V.24 · X.21 · RS-232 · Ethernet · DSL · Wi-Fi
Layer 1 — the Physical Layer — is the lowest and most fundamental stratum of the OSI (Open Systems Interconnection) Reference Model, formalized by ISO/IEC 7498-1. It defines everything that must be true about the physical interface between a communicating device and a transmission medium: voltages, frequencies, connector pin assignments, cable types, optical power levels, and timing relationships.
Crucially, the transmission medium itself — copper cable, optical fibre, air (radio) — sits below Layer 1 in the OSI sense. Layer 1 is the interface to that medium, not the medium itself. It is responsible for the raw, unstructured bit stream: converting logical ones and zeros produced by higher layers into physical signals, and recovering bits from incoming signals for delivery upward to Layer 2, the Data Link Layer.
Key Insight: The Physical Layer has no knowledge of frames, packets, or data semantics. It deals exclusively in bits (or, in parallel interfaces, groups of bits sent simultaneously). All error detection, addressing, and framing is the responsibility of higher layers.
The Physical Layer must perform — or define the rules for — the following tasks:
Convert binary data into electrical, optical, or radio signals and transmit them over the medium.
Define how devices are physically connected — bus, ring, star, mesh, or hybrid arrangements.
Provide clocking so sender and receiver sample the medium at the same instants in time.
Specify whether communication is simplex, half-duplex, or full-duplex.
Establish, maintain, and deactivate physical connections between endpoints.
Define connector shapes, pin counts, cable types, and physical dimensions (e.g., RJ-45, SC fibre).
Encode bit patterns into signal waveforms suitable for the medium (NRZ, Manchester, 4B/5B…).
Govern the maximum data rate (bps) achievable, per Shannon's and Nyquist's theorems.
Physical Layer signals take one of two fundamental forms:
Analog signals vary continuously in amplitude, frequency, or phase. Traditional telephony (PSTN), AM/FM radio, and early modems all use analog signalling. Key parameters include:
Digital signals switch between a discrete set of states — most commonly two voltage levels representing logical 0 and 1. Modern LANs, USB, HDMI, and storage interfaces are all digital. Higher-order signalling (PAM-4, QAM) allows more than two states per symbol, increasing the number of bits carried per baud (symbol period).
Two foundational theorems govern what a channel can carry:
| Theorem | Formula | What it says |
|---|---|---|
| Nyquist (noiseless) | C = 2B log₂(M) | Max data rate with bandwidth B and M signal levels |
| Shannon–Hartley (noisy) | C = B log₂(1 + S/N) | Theoretical capacity limit given signal-to-noise ratio S/N |
The Physical Layer must specify the directionality and timing of the link:
One-way only. Data flows from sender to receiver only. Example: broadcast radio, TV antenna signal.
Two-way but not simultaneous. One side transmits while the other listens. Example: walkie-talkies, classic Ethernet (CSMA/CD).
Simultaneous two-way transmission. Example: telephone, modern switched Ethernet, USB 3.x.
One node communicates with multiple endpoints via a shared medium or hub. Example: Wi-Fi access point, DSL DSLAM.
Synchronous: Both sender and receiver share the same clock signal (or recover it from the data stream). Bits are transmitted at a constant rate in a continuous stream. Used in high-speed links: T1/E1, SDH/SONET, Gigabit Ethernet. High throughput; low overhead.
Asynchronous: Each character or byte is framed by start and stop bits, allowing transmission at irregular intervals. The receiver re-synchronizes on each character. Used in RS-232 serial ports, UART-based devices. Simple but adds ~20% overhead per byte.
In serial transmission one bit is sent at a time — the dominant form in modern long-distance and high-speed links (USB, SATA, PCIe, Ethernet). In parallel transmission multiple bits travel simultaneously on multiple wires (e.g., 8 bits across an old IDE/ATA ribbon cable). Parallel suffers from skew at high speeds, which is why modern buses have largely migrated to serial.
Line encoding converts raw bit sequences into waveforms suitable for the medium. A good encoding must: provide self-clocking (receiver can extract timing), minimize DC component (important for AC-coupled links), and detect some errors.
| Scheme | Used In | Key Property |
|---|---|---|
| NRZ-L (Non-Return to Zero Level) | RS-232, basic serial | Simple; long runs cause DC wander, no self-clocking |
| NRZI (NRZ Inverted) | USB, FDDI | Transition on 1; solves long-zero runs only |
| Manchester | 10BASE-T Ethernet, IEEE 802.4 | Mid-bit transition; self-clocking; doubles bandwidth requirement |
| Differential Manchester | Token Ring (802.5) | Transition at start of each bit; direction encodes 0/1 |
| 4B/5B | Fast Ethernet (100BASE-TX), FDDI | Maps 4 data bits → 5 code bits; used with NRZI to prevent long idle runs |
| 8B/10B | Gigabit Ethernet, USB 3.0, SATA | DC balance guaranteed; 20% overhead; rich error detection |
| PAM-4 | 400GbE, 25G/100G interfaces | Four amplitude levels; 2 bits/symbol; doubles spectral efficiency |
Although the medium itself is formally below the Physical Layer in OSI terminology, Layer 1 specifications are tightly coupled to the medium they target. The three broad categories are:
| Medium | Typical Bandwidth | Max Distance | Notes |
|---|---|---|---|
| Twisted Pair (UTP Cat 5e) | 100 MHz | 100 m | 100BASE-TX; common LAN wiring |
| Twisted Pair (UTP Cat 6A) | 500 MHz | 100 m | 10GBASE-T; reduced crosstalk |
| Coaxial Cable (75Ω) | 1 GHz+ | 500 m (10BASE5) | Cable TV, legacy thicknet/thinnet |
| Single-Mode Fibre | 100+ THz | 80–120 km (unamplified) | Long-haul; narrow 9 µm core; laser source |
| Multi-Mode Fibre (OM4) | 4.7 GHz·km | 400 m @ 100G | Short reach datacentre; LED or VCSEL |
| Technology | Frequency Band | Standard | Typical Range |
|---|---|---|---|
| Wi-Fi 6 (802.11ax) | 2.4 GHz / 5 GHz / 6 GHz | IEEE 802.11ax | ~300 m outdoor |
| Bluetooth 5.x | 2.4 GHz ISM | IEEE 802.15.1 | ~400 m (class 1) |
| LTE / 4G | 700 MHz – 2.6 GHz | 3GPP Rel. 8+ | Cells 1–30 km |
| 5G NR (sub-6 GHz) | 600 MHz – 6 GHz | 3GPP Rel. 15+ | Cells 100 m – 10 km |
| 5G mmWave | 24–100 GHz | 3GPP Rel. 15+ | <500 m (line-of-sight) |
| Free-Space Optical (FSO) | ~193 THz (1550 nm) | ITU-T G.654 | 1–4 km |
The Physical Layer defines how nodes are physically wired or connected:
All nodes share one cable segment. Obsolete for modern LANs (legacy 10BASE2/5). Simple but any break severs the network.
All nodes connect to a central hub or switch. Dominant topology for modern Ethernet. A failed link isolates only one node.
Each node connects to exactly two others forming a closed loop. Used in Token Ring, SONET/SDH rings, and Metro Ethernet.
Nodes interconnect with multiple paths. Full mesh: every node connects to every other. Common in WANs and core internet routers for redundancy.
Star topology extended into a hierarchy of cascaded switches. Typical enterprise LAN: access → distribution → core layers.
A direct dedicated link between exactly two nodes. Used in WAN leased lines (T1, E1), fibre trunks, serial WAN interfaces.
Numerous standards bodies — ITU-T, IEEE, EIA/TIA, ANSI, and ISO/IEC — define Physical Layer specifications. The most significant are listed below.
| Standard | Body | Application | Key Specs |
|---|---|---|---|
| V.24 / RS-232 | ITU-T / EIA | Analog/DTE-DCE serial; modem interfaces | ±3–15 V logic; DB-9/DB-25 connectors; up to ~20 kbps |
| V.35 | ITU-T | High-speed WAN serial (T1/E1 connections to CSU/DSU) | Combined voltage levels; up to 4 Mbps |
| X.21 | ITU-T | Digital circuit-switched WANs (ISDN BRI D-channel, X.25) | 15-pin connector; balanced signals; synchronous |
| 10BASE-T / 100BASE-TX / 1000BASE-T | IEEE 802.3 | Ethernet over twisted pair | RJ-45; Manchester / MLT-3 / PAM-5 encoding |
| 1000BASE-LX / 10GBASE-SR | IEEE 802.3 | Gigabit/10G Ethernet over fibre | 1310 nm / 850 nm; SFP / SFP+ transceivers |
| 802.11ax (Wi-Fi 6) | IEEE 802.11 | Wireless LAN physical layer | OFDMA; 1024-QAM; MU-MIMO; up to 9.6 Gbps theoretical |
| USB 3.2 Gen 2×2 | USB-IF | Peripheral interconnect | 20 Gbps; 128b/132b encoding; USB-C connector |
| PCIe 5.0 | PCI-SIG | Internal system bus (GPU, NVMe SSDs) | 32 GT/s per lane; 128b/130b encoding; ~128 GB/s ×16 |
| SONET / SDH | ANSI / ITU-T | Synchronous optical carrier (WAN backbone) | OC-1 = 51.84 Mbps base; NRZ; fibre; strict timing hierarchy |
| DSL (ADSL2+ / VDSL2) | ITU-T G.992.5 / G.993.2 | Broadband over telephone copper | DMT modulation; up to 100–200 Mbps (VDSL2) |
Note on Hybrid Standards: V.24 and X.21 — cited in the original OSI documents — technically contain elements of higher layers (e.g., connection establishment procedures that touch Layer 2 semantics). In practice, most real-world protocols are not perfectly layer-clean.
Devices that operate exclusively at Layer 1 deal only with signal regeneration and physical connectivity — they have no awareness of MAC addresses, IP addresses, or data frames:
Receives a signal on one port, amplifies it, and broadcasts it out all other ports. No MAC awareness. Obsolete in modern networks; replaced by switches.
Regenerates a degraded signal to extend cable segment length beyond its attenuation limit. Re-clocks and reshapes the bit stream without examining its content.
Boosts analog signal power but also amplifies any noise present. Unlike digital repeaters, amplifiers cannot remove accumulated noise — used in CATV and PSTN trunk lines.
The electronic circuit inside every NIC, switch port, and router interface that encodes/decodes bits to and from the physical signal. Handles line coding, clock recovery, and equalization.
Converts electrical signals into electromagnetic waves for transmission, and converts incoming waves back to electrical signals. Gain, polarization, and beam pattern are key physical parameters.
Modulates digital data onto an analog carrier signal (e.g., telephone line, cable) and demodulates the reverse. DSL modems use DMT; cable modems use DOCSIS QAM.
Converts between different physical media (e.g., twisted-pair copper to single-mode fibre) at Layer 1. Transparent to all higher-layer protocols.
Passive copper or fibre runs forming the physical plant. Governed by TIA-568 structured cabling standards. Patch panels aggregate horizontal runs for flexible patching in the MDF/IDF.