The Wireless Difference

In net-models you met the Data Link and Physical layers. Wired links (Ethernet) assume a shared, bounded medium where every node can hear every other node. Wireless replaces that cable with radio, which changes three fundamental assumptions:

1. Half-duplex & shared air — all stations on a channel share one radio band; only one can transmit at a time.
2. No collision detection — a radio cannot reliably listen while transmitting, so “listen before talking” is the rule (Collision Avoidance, not Detection).
3. Reach is asymmetric — a station may be in range of the Access Point but not of another station (the hidden-node problem).

Everything about Wi-Fi is a workaround for these three constraints.

The 802.11 Standard Family

802.11 is the IEEE standard family that defines Wi-Fi. Each generation adds data rate, capacity, or spectrum:

The “Wi-Fi N” branding came later from the Wi-Fi Alliance; the underlying standards are the 802.11 amendments.

Bands, Channels, and Width

• Wi-Fi runs in ISM bands: 2.4 GHz, 5 GHz, and the newer 6 GHz (Wi-Fi 6E/7).
• Each band is carved into channels (e.g. 2.4 GHz has channels 1–14, 20 MHz wide). Adjacent channels overlap, which is why you pick channels 1, 6, or 11 (the non-overlapping set).
• Channel bonding combines adjacent channels (20→40→80→160 MHz) for more throughput, but burns more of the scarce spectrum.
• 5/6 GHz bands have more room, so denser channel plans and fewer interference collisions are possible than in crowded 2.4 GHz.

CSMA/CA: Collision Avoidance

Ethernet uses CSMA/CD — it detects a collision and back off. Wireless cannot do this (a radio can’t transmit and listen at the same time), so 802.11 uses CSMA/CA — it tries to avoid collisions:

1. Carrier Sense — a station listens and waits until the channel is idle.
2. DIFS — after idle for the Distributed Inter-Frame Space, the station may contend.
3. Random Backoff — the station picks a random slot count in a contention window (CW) and decrements it each idle slot. Because each station picks a different random number, transmissions stagger and collisions are rare.
4. Transmit & ACK — the receiver verifies the frame’s FCS and replies with an ACK. A missing ACK means someone collided.

Try the CSMA/CA · Collision Avoidance scenario in the interactive studio below — notice how A’s random backoff staggers it ahead of B so the two frames don’t collide.

If two nodes DO collide, each doubles its contention window (exponential backoff) and draws a new, longer backoff — exactly the recovery you saw for TCP timeouts, but at the link layer.

The Hidden-Node Problem

Two stations (A and C) may both be in range of the Access Point but out of range of each other. When A transmits, C never hears the carrier and thinks the channel is free — so C transmits too, and the two frames collide at the AP even though neither sender “saw” the other.

Wi-Fi solves this with RTS/CTS (Request-To-Send / Clear-To-Send): A sends a short RTS; the AP broadcasts a CTS that every station (including hidden C) hears; C defers while A transmits. The studio’s Hidden-Node · RTS/CTS scenario animates exactly this.

MAC Frames and Addressing

The 802.11 MAC frame is similar to Ethernet’s but carries three MAC addresses (not two), because frames are relayed through the AP:

• Address 1 — Receiver (the AP or the destination station)
• Address 2 — Transmitter
• Address 3 — final source/destination (used to know which end actually sends/receives)

The frame includes a Frame Control field (type/subtype — data, ACK, RTS, CTS, beacon), sequence control for reassembly, the payload, and a 4-byte FCS used for error detection (there is no error-correction retry at this layer beyond the ACK mechanism).

Access Points, Associations & Roaming

• SSID — the human-readable network name you connect to.
• BSSID — the MAC address of a specific AP (one SSID may have many APs).
• Association — the handshake by which a station joins a BSS and negotiates rates/capabilities.
• Roaming — as you move, a station reassociates to a stronger AP; 802.11r (Fast BSS Transition) speeds this up so VoIP/video calls survive the hop.

OFDM, MIMO & Spatial Streams

To fit more bits into the air, modern Wi-Fi uses:

• OFDM / OFDMA — divides a channel into many narrow orthogonal subcarriers; OFDMA lets multiple users share them simultaneously (a hallmark of Wi-Fi 6).
• MIMO — multiple antennas transmit multiple spatial streams at once (like several parallel roads), multiplying throughput.
• Beamforming — the AP steers its signal toward a specific client instead of broadcasting in all directions.

Verify It Yourself

1. On your phone, look at the Wi-Fi settings: note the SSID, the band (2.4/5/6 GHz), and (in the router) the channel and channel width.
2. Run the three studio scenarios. Count how many ACKs and collisions each produces — the basic scenario should finish with zero collisions.
3. Pause the Hidden-Node · RTS/CTS scenario at the CTS step: confirm node C defers even though A and C cannot hear each other.