Wi-Fi: Wireless Fidelity

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.

Channel Plans and Spectrum

Wi-Fi operates across three frequency bands, each with distinct channel allocations and regulatory constraints:

• 2.4 GHz (802.11b/g/n): 14 channels, each 20-25 MHz wide. Channels 1, 6, and 11 are the only three non-overlapping channels in the US. Crowded due to Bluetooth, microwaves, and cordless phones. Maximum 40 MHz (HT40) bonding available but rarely practical.
• 5 GHz (802.11a/n/ac/ax): Up to 24 UNII channels (UNII-1: 36-48, UNII-2: 52-64, UNII-2e: 100-144, UNII-3: 149-165). DFS (Dynamic Frequency Selection) required for UNII-2/2e. Wider channels (80, 160 MHz) dramatically increase throughput. Less penetration through walls than 2.4 GHz.
• 6 GHz (Wi-Fi 6E/7): 59 channels (4 x 20 MHz + 12 x 40 MHz + 6 x 80 MHz + 3 x 160 MHz). No legacy devices, no DFS. Clean spectrum but shorter range due to higher frequency. Maximum 160 MHz channels. Only Wi-Fi 6E/7 clients supported.

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 46 Gbps aggregate throughput. 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.