WiFi (IEEE 802.11)

Overview

WiFi is a family of wireless network protocols based on the IEEE 802.11 standard. It’s the most common wireless LAN technology, used in homes, offices, and public hotspots.

WiFi Standards

WiFi Architecture

graph TD
    subgraph "Basic Service Set (BSS)"
        AP[Access Point]
        C1[Client 1]
        C2[Client 2]
        C3[Client 3]
        C1 --> AP
        C2 --> AP
        C3 --> AP
    end
    subgraph "Extended Service Set (ESS)"
        AP1[AP 1] --> DS[Distribution System]
        AP2[AP 2] --> DS
        AP3[AP 3] --> DS
    end
    DS --> IN[Internet]

CSMA/CA (Collision Avoidance)

WiFi uses CSMA/CA instead of CSMA/CD (Ethernet) because wireless stations can’t detect collisions:

sequenceDiagram
    participant A as Station A
    participant M as Medium
    participant B as Station B
    A->>M: Sense medium
    Note over M: Medium idle
    A->>M: Wait DIFS (Distributed IFS)
    A->>A: Random backoff
    A->>M: RTS (Request to Send)
    M->>B: RTS
    B->>M: CTS (Clear to Send)
    M->>A: CTS
    A->>M: Data
    M->>B: Data
    B->>M: ACK

Why Collision Avoidance?

• Hidden terminal problem: A and C can’t hear each other but both reach B
• Exposed terminal problem: B can hear A but C’s transmission to D wouldn’t interfere
• RTS/CTS solves hidden terminal by reserving the medium

WiFi Security

WPA2 4-Way Handshake

sequenceDiagram
    participant C as Client
    participant AP as Access Point
    AP->>C: ANonce (AP nonce)
    C->>C: Derive PTK from PMK + ANonce + SNonce
    C->>AP: SNonce + MIC
    AP->>C: GTK + MIC (encrypted)
    C->>AP: ACK
    Note over C,AP: Encrypted communication begins

WiFi Channel Allocation

2.4 GHz (3 non-overlapping channels)

Channel: 1  2  3  4  5  6  7  8  9  10 11
         [====1====]
                   [====6====]
                              [====11====]

Best practice: Use channels 1, 6, or 11 only.

5 GHz (24+ non-overlapping channels)

Much more spectrum available, reducing interference significantly.

Interview Questions

1. Q: What’s the difference between WiFi 5 and WiFi 6? A: WiFi 6 adds: OFDMA (multiple users per channel), BSS coloring (reduces interference), TWT (Target Wake Time for IoT battery life), 1024-QAM (higher data rates), and better performance in dense environments.

2. Q: What is CSMA/CA and why not CSMA/CD? A: CSMA/CA (Collision Avoidance) is used because wireless stations can’t detect collisions (can’t send and listen simultaneously on the same frequency). CSMA/CD (Collision Detection) works on wired Ethernet where collisions are detectable.

3. Q: What is the hidden terminal problem? A: Two stations can’t hear each other but both can reach a third station. They may transmit simultaneously, causing a collision at the third station. Solved by RTS/CTS: the access point reserves the medium.

4. Q: Why is WEP insecure? A: WEP uses RC4 with a 24-bit IV (too small, causing reuse). The key can be cracked in minutes with tools like Aircrack-ng. WPA2 (AES-CCMP) is the minimum secure standard.

5. Q: What is MIMO? A: Multiple-Input Multiple-Output uses multiple antennas at both transmitter and receiver to increase throughput and reliability. MU-MIMO (WiFi 5+) allows serving multiple clients simultaneously.

6. Q: What is OFDMA in WiFi 6? A: Orthogonal Frequency Division Multiple Access divides a channel into smaller sub-channels (Resource Units). Multiple users can transmit simultaneously on different sub-channels, improving efficiency in dense environments.

Common Mistakes

• Using 2.4 GHz channel other than 1, 6, or 11 (causes interference)
• Using WEP or WPA (insecure)
• Not knowing that WiFi speed degrades with distance and obstacles
• Confusing bandwidth (Hz) with data rate (Mbps)
• Forgetting that WiFi is half-duplex (one transmission at a time per channel)

Summary

WiFi has evolved from 11 Mbps (802.11b) to 46 Gbps (WiFi 7). Key concepts: CSMA/CA, MIMO, OFDMA, channel planning, and WPA3 security. WiFi 6/6E brings OFDMA and 6 GHz spectrum for better performance in dense environments.

Cross-References

• Wireless Overview
• 5G — Cellular alternative
• SDN — Software-defined networking
• TLS — Application-layer security

Cross References

• 5G
• Physical Layer
• Data Link Layer