Wi-Fi: Wireless Fidelity
IEEE 802.11 enabled wireless local networking, untethering devices from ethernet cables.
The 802.11 Protocol Stack
Wi-Fi operates at the physical and data-link layers of the OSI model, with the MAC sublayer handling access control and the PHY sublayer handling radio transmission. The 802.11 standard defines two MAC mechanisms: DCF (Distributed Coordination Function) for contention-based access, and PCF (Point Coordination Function) for centralized polling. Modern networks use EDCA (Enhanced Distributed Channel Access), which extends DCF with four access categories—Voice, Video, Best Effort, and Background—each with distinct contention window sizes and inter-frame spacing to prioritize time-sensitive traffic.
CSMA/CA and MAC Layer Access
Unlike Ethernet's collision detection (CSMA/CD), Wi-Fi cannot reliably detect collisions over the air while transmitting. Instead, it uses CSMA/CA (Collision Avoidance). Before transmitting, a station senses the channel. If idle for a DIFS (Distributed Inter-Frame Space) period, it transmits. If busy, it defers and sets a random backoff timer within the contention window. EDCA refines this by assigning shorter AIFS (Arbitration Inter-Frame Space) values and smaller contention windows to higher-priority access categories. An optional RTS/CTS handshake prevents the hidden node problem by reserving the channel before data transmission.
Physical Layer Evolution
The PHY layer has undergone radical transformation. Early standards used spread-spectrum techniques—FHSS (Frequency-Hopping Spread Spectrum) and DSSS (Direct-Sequence Spread Spectrum)—to hop or spread signals across the 2.4 GHz band. 802.11a and later standards adopted OFDM, which divides a wide channel into many narrow orthogonal subcarriers, each modulated independently. This dramatically improves spectral efficiency and multipath resilience. Modern PHY layers use higher-order modulation (1024-QAM in Wi-Fi 6) and wider channel bandwidths (320 MHz in Wi-Fi 7) to achieve multi-gigabit throughput.
Wi-Fi Generations: 802.11b through 802.11ax
- 802.11b (1999): 11 Mbps, DSSS with CCK coding, 2.4 GHz. Uses Barker code modulation at lower rates (1, 2 Mbps) and CCK at 5.5 and 11 Mbps. Only 3 non-overlapping channels.
- 802.11a (1999): 54 Mbps, OFDM, 5 GHz UNII band. 52 subcarriers with BPSK/QPSK/16-QAM/64-QAM. Shorter range than 2.4 GHz due to higher frequency attenuation and regulatory power limits.
- 802.11g (2003): 54 Mbps, OFDM at 2.4 GHz. Backward-compatible with 802.11b via CCK fallback. Still only 3 non-overlapping channels, limiting dense deployments.
- 802.11n (2009): Up to 600 Mbps with 4 spatial streams (MIMO). Introduced frame aggregation (A-MSDU, A-MPDU), block ACK, and optional 40 MHz channels. Greenfield mode removes legacy overhead.
- 802.11ac (2013): Up to 6.9 Gbps. 80/160 MHz channels, up to 8 spatial streams, 256-QAM modulation. MU-MIMO for simultaneous downlink transmission to multiple clients. Beamforming via compressed beamforming feedback.
- 802.11ax (2019): Up to 9.6 Gbps. OFDMA divides channels into Resource Units for simultaneous multi-user transmission. BSS Coloring reduces co-channel interference by tagging frames with a color code. Target Wake Time (TWT) enables scheduled access for IoT power savings. 1024-QAM modulation.
2.4 GHz Channel Plan (ISM Band)
The 2.4 GHz ISM band spans 2400–2495 MHz. Channel center frequencies follow F(MHz) = 2407 + 5 × N where N is the channel number. Only channels 1, 6, and 11 are truly non-overlapping in the US. The band is heavily congested — Bluetooth, microwaves, cordless phones, and neighboring Wi-Fi networks all compete for spectrum.
| Ch | Center (MHz) | Range (MHz) | US | EU | Notes |
|---|---|---|---|---|---|
| 1 | 2412 | 2401–2423 | Yes | Yes | Non-overlapping (20 MHz) |
| 2 | 2417 | 2406–2428 | Yes | Yes | |
| 3 | 2422 | 2411–2433 | Yes | Yes | |
| 4 | 2427 | 2416–2438 | Yes | Yes | |
| 5 | 2432 | 2421–2443 | Yes | Yes | |
| 6 | 2437 | 2426–2448 | Yes | Yes | Non-overlapping (20 MHz) |
| 7 | 2442 | 2431–2453 | Yes | Yes | |
| 8 | 2447 | 2436–2458 | Yes | Yes | |
| 9 | 2452 | 2441–2463 | Yes | Yes | |
| 10 | 2457 | 2446–2468 | Yes | Yes | |
| 11 | 2462 | 2451–2473 | Yes | Yes | Non-overlapping (20 MHz) |
| 12 | 2467 | 2456–2478 | Low-power | Yes | |
| 13 | 2472 | 2461–2483 | Low-power | Yes | |
| 14 | 2484 | 2473–2495 | Forbidden | No | 802.11b only, Japan |
5 GHz UNII Band Channels
The 5 GHz band uses the formula F(MHz) = 5000 + 5 × N for center frequencies. Four UNII sub-bands have different power limits and DFS requirements:
UNII-1 (5150–5250 MHz) — Indoor Only
| Ch | Center (MHz) | 40 MHz | 80 MHz |
|---|---|---|---|
| 36 | 5180 | 38 | 42 |
| 40 | 5200 | — | — |
| 44 | 5220 | 46 | — |
| 48 | 5240 | — | — |
UNII-2 (5250–5350 MHz) — DFS Required
| Ch | Center (MHz) | 40 MHz | 80 MHz |
|---|---|---|---|
| 52 | 5260 | 54 | 58 |
| 56 | 5280 | — | — |
| 60 | 5300 | 62 | — |
| 64 | 5320 | — | — |
UNII-2 Extended (5470–5725 MHz) — DFS Required
| Ch | Center (MHz) | 40 MHz | 80 MHz | 160 MHz |
|---|---|---|---|---|
| 100 | 5500 | 102 | 106 | 114 |
| 104 | 5520 | — | — | — |
| 108 | 5540 | 110 | — | — |
| 112 | 5560 | — | — | — |
| 116 | 5580 | 118 | 122 | — |
| 120 | 5600 | — | — | — |
| 124 | 5620 | 126 | — | — |
| 128 | 5640 | — | — | — |
| 132 | 5660 | 134 | 138 | — |
| 136 | 5680 | — | — | — |
| 140 | 5700 | 142 | — | — |
| 144 | 5720 | — | — | — |
UNII-3 (5725–5850 MHz)
| Ch | Center (MHz) | 40 MHz | 80 MHz |
|---|---|---|---|
| 149 | 5745 | 151 | 155 |
| 153 | 5765 | — | — |
| 157 | 5785 | 159 | — |
| 161 | 5805 | — | — |
| 165 | 5825 | — | — |
DFS (Dynamic Frequency Selection) is required for UNII-2 and UNII-2 Extended bands to avoid interfering with weather radar (NEXRAD) and military radar systems. The equipment must detect radar within 10 seconds and switch to a non-interfered channel, which cannot be reused for 30 minutes.
6 GHz Band (Wi-Fi 6E / Wi-Fi 7)
The 6 GHz band (5925–7125 MHz) provides 59 channels with no DFS requirement and no legacy devices. Channel formula: F(MHz) = 5955 + 5 × (CH − 1). This band supports channel widths up to 320 MHz (Wi-Fi 7). Output power is limited to 24 dBm (LPI mode) or 30 dBm with AFC (Automated Frequency Coordination) for standard-power access points.
U-NII-5 (5925–6425 MHz) — Primary 6 GHz Band
Primary channels: 5, 9, 13, 17, 21, 25, 29, 33, 37, 41, 45, 49, 53, 57, 61, 65, 69, 73, 77, 81, 85, 89, 93
U-NII-6 (6425–6525 MHz) — Standard Power with AFC
Channels 97, 101, 105, 109, 113 — requires AFC coordination for standard-power operation
U-NII-7 (6525–6875 MHz)
Channels 121, 125, 129, 133, 137, 141, 145, 149, 153, 157, 161, 165, 169, 173, 177, 181, 185
U-NII-8 (6875–7125 MHz)
Channels 189, 193, 197, 201, 205, 209, 213, 217, 221, 225, 229, 233
Security Evolution: WEP to WPA3
- WEP (1997): Rivest Cipher 4 (RC4) stream cipher with 64/128-bit keys. Initialization Vector (IV) was only 23 bits, leading to collisions after ~5,000 frames. Vulnerable to statistical attacks—aircrack-ng could recover keys in minutes.
- WPA (2003): Interim fix using TKIP (Temporal Key Integrity Protocol). Dynamic per-frame key rotation via Michael MIC to prevent tampering. Still used RC4 but with longer IVs and per-packet key mixing.
- WPA2 (2004): Mandatory CCMP (Counter Mode with CBC-MAC Protocol) using AES-128-CCM. 4-Way Handshake for key derivation (PSK or 802.1X/EAP). Vulnerable to KRACK attack (2017) which forced reinstallation of session keys.
- WPA3 (2018): SAE (Simultaneous Authentication of Equals) replaces PSK with Dragonfly handshake—resistant to offline dictionary attacks. OWE (Opportunistic Wireless Encryption) provides unauthenticated encryption on open networks. 192-bit cryptographic suite for enterprise (CNSA).
MIMO and Beamforming
MIMO (Multiple Input Multiple Output) uses multiple antennas at both transmitter and receiver to create independent spatial streams. Each stream carries separate data, multiplying throughput linearly with the number of antennas (up to the rank of the channel matrix). 802.11n introduced MIMO with up to 4 spatial streams; 802.11ac supports up to 8.
Beamforming focuses RF energy toward specific clients rather than radiating omnidirectionally. Implicit beamforming estimates the channel from received signal strength (no feedback needed, less accurate). Explicit beamforming requires the receiver to measure the channel and feed back a beamforming matrix, enabling precise steering. 802.11ac standardized compressed beamforming (V-matrices). MU-MIMO combines beamforming with spatial multiplexing—transmitting independent data to multiple clients simultaneously using separate beam-steered streams. Downlink MU-MIMO arrived in 802.11ac Wave 2; uplink MU-MIMO came with 802.11ax.
OFDMA: Wi-Fi 6's Breakthrough
OFDMA (Orthogonal Frequency Division Multiple Access) is the defining feature of 802.11ax. It divides a 20 MHz channel into 9 Resource Units (RUs) of varying sizes (26, 52, 106, or 242 subcarriers). Each RU can be assigned to a different user, enabling simultaneous uplink and downlink transmission to multiple stations. This mirrors LTE/5G scheduling and dramatically improves efficiency in dense environments. A typical Wi-Fi 6 AP can serve dozens of IoT devices with small RUs while reserving larger RUs for high-throughput clients—all within the same OFDM symbol.
Wi-Fi 7 and the MLO Future
802.11be (Wi-Fi 7) pushes wireless to 23 Gbps single-band (up to 46 Gbps aggregate with MLO). Key innovations include 320 MHz channels in the 6 GHz band, 4096-QAM modulation, and Multi-Link Operation (MLO)—simultaneously transmitting across multiple bands (2.4 + 5 + 6 GHz) for higher throughput and lower latency. MLO enables real-time load balancing and frequency diversity, making Wi-Fi viable for XR (Extended Reality) and industrial automation. Preamble puncturing allows using fragmented portions of wider channels, avoiding interference without falling back to narrower channels.
The 2.4 GHz Congestion Problem
The 2.4 GHz ISM band is shared by Wi-Fi, Bluetooth Classic, Zigbee, Z-Wave, cordless phones, baby monitors, and microwave ovens. With only 3 non-overlapping 20 MHz channels, dense apartment buildings can have 10+ competing networks on the same channel. Hidden nodes (where two stations can reach the AP but not each other) cause hidden collisions. RTS/CTS mitigates this at the cost of overhead. Many enterprise deployments minimize 2.4 GHz usage, reserving it for legacy IoT devices while directing high-performance clients to 5 GHz and 6 GHz.
CSIRO and the Australian WiFi Patent
The CSIRO (Commonwealth Scientific and Industrial Research Organisation) WiFi patent is one of the most significant Australian contributions to wireless technology. Filed on 27 November 1992 by a team led by Dr. John O'Sullivanat CSIRO's Telecommunications and Industrial Physics division in Sydney, the patent (Australian patent AU 51806/93A; US patent 5,487,069) describes a method for reducing multipath interference in wireless LANs.
The core innovation was the use of fast Fourier transform (FFT)techniques to resolve multipath reflections — the problem of radio signals bouncing off walls, floors, and ceilings and arriving at the receiver at different times, causing inter-symbol interference. CSIRO's method used a bank of orthogonal tones (subcarriers), each modulated with a portion of the data. By applying an FFT at the receiver, the system could decompose the received signal into its frequency components and correct for multipath distortion.
This technique became the foundation of the IEEE 802.11a/g/n/ac standards, which use OFDM (Orthogonal Frequency Division Multiplexing) — a closely related approach using multiple subcarriers to combat multipath. Between 2003 and 2016, CSIRO earned approximatelyA$450 millionin licensing revenue from WiFi patent royalties. The patent was licensed to virtually all major technology companies including Apple, Intel, Dell, Microsoft, HP, Toshiba, and Asus. CSIRO's work also spawned Radiata Networks, a spin-off acquired by Cisco in 2000 for A$567 million.