Wireless Links & 802.11 WiFi
A wired link is a controlled, private medium: the signal stays on the cable and arrives more or less as it left. A wireless link is the opposite - the signal radiates through open air that everyone shares, and the physics work against you at every step. Understanding WiFi starts with understanding why the wireless channel is so much harder than a wire.
What makes a wireless link hard
- Signal attenuation - radio power falls off sharply with distance (roughly with the square of it, and worse indoors). A signal that is strong at the access point may be barely readable across the building, so the usable range is small and the data rate drops as you move away.
- Interference - the unlicensed bands WiFi uses are crowded. Other WiFi networks, Bluetooth, cordless phones, and microwave ovens all add noise on the same frequencies, and that noise competes with your signal.
- Multipath propagation - the signal bounces off walls, floors, and furniture, so copies of it arrive at the receiver at slightly different times. These delayed echoes overlap and distort each other, blurring the received waveform.
- Hidden terminal problem - two stations can both be in range of the access point yet out of range of each other, often because a wall or distance sits between them. Neither can hear the other transmit, so each thinks the channel is idle and they collide at the AP without ever knowing why.
The practical consequence: wireless bit-error rates are far higher and far more variable than on a wire, and a station cannot assume that "I hear nothing" means "the channel is truly free."
802.11 architecture
WiFi is standardized as the IEEE 802.11 family. Its building blocks:
- Access point (AP) - the base station. It bridges wireless stations to the wired network and coordinates the local wireless cell.
- BSS (Basic Service Set) - the cell itself: one AP plus the stations associated with it. In infrastructure mode all traffic flows through the AP, even between two stations in the same BSS.
- SSID - the human-readable network name (for example
CampusWiFi) that an AP advertises. Multiple APs can share one SSID to present a single logical network across a large area.
Every AP transmits on a specific channel within a band (2.4 GHz or 5 GHz). Neighboring APs are configured on non-overlapping channels to limit interference.
The 2.4 GHz band illustrates why this is harder than "11 channels means 11 APs." Each channel is about 22 MHz wide, but channels are spaced only 5 MHz apart - channel 1 is centered at 2412 MHz, channel 2 at 2417 MHz, and so on. A 22-MHz-wide signal on channel 1 spills well past channel 2, 3, 4, and even 5. Only channels spaced at least 22 MHz apart avoid overlapping at all, which in an 11-channel band leaves exactly three non-overlapping channels: 1, 6, and 11. That is why virtually every 2.4 GHz deployment guide says "use 1, 6, or 11" - it is not a preference, it is the entire set of channels that do not interfere with each other.
Many 5 GHz channels sit inside spectrum shared with weather and airport radar and are governed by Dynamic Frequency Selection (DFS). An AP using a DFS channel must continuously listen for radar pulses, and if it detects one it is legally required to vacate that channel immediately - it stops serving clients for up to a minute while it silently switches to a new channel and clients scramble to reassociate. Network engineers who pick a DFS channel purely because it looked clean during a site survey get blindsided weeks later by radar events near an airport or coastline, which read to end users as random, unexplained WiFi dropouts with no obvious cause in casual troubleshooting. Production deployments near airports either avoid DFS channels entirely or plan for the brief but real disruption when a radar hit forces a channel change.
Scanning and association
Before a station can send data it must associate with an AP. There are two ways to find one:
- Passive scanning - the station listens for beacon frames, which each AP broadcasts periodically to announce its SSID and capabilities.
- Active scanning - the station broadcasts a probe request, and nearby APs reply with a probe response.
The station picks an AP (usually the strongest signal), then exchanges association request / response frames, and typically authenticates. Once associated it has joined that BSS and can send and receive through the AP. Moving to a different AP means associating again.
CSMA/CA: avoiding collisions instead of detecting them
Wired Ethernet uses CSMA/CD - it detects a collision by listening while it transmits. WiFi cannot do that reliably, so 802.11 uses CSMA/CA - Carrier Sense Multiple Access with Collision Avoidance. A station:
- Senses the channel. If it is idle for a required interval, the station may transmit; if busy, it waits.
- Backs off randomly even after the channel goes idle - it picks a random count and only transmits when that countdown reaches zero, which spreads out stations that were all waiting for the same busy channel to clear.
- Waits for an ACK. Because it cannot detect collisions itself, the receiver sends an explicit acknowledgement for every frame received correctly. No ACK means the frame was lost, so the sender backs off further and retransmits.
Collision detection requires a station to transmit and listen at the same time and compare what it hears to what it sent. On a wireless radio the station's own outgoing signal is enormously stronger than any incoming one - it drowns out everything else, so the station simply cannot hear a competing transmission while it is sending. Detection is impossible, so WiFi does the next best thing: it tries to avoid collisions up front (random backoff) and confirms success after the fact (ACKs), retransmitting when an ACK does not arrive.
RTS/CTS: taming the hidden terminal
To handle hidden terminals, 802.11 offers an optional handshake. A station that wants to send first transmits a short RTS (Request To Send) frame to the AP. The AP replies with a CTS (Clear To Send) frame. Because every station in range of the AP hears the CTS - including the hidden terminal that could not hear the original sender - they all learn the channel is reserved and stay quiet for the announced duration.
The RTS/CTS exchange costs airtime, so it is normally used only for larger frames where a collision would waste far more than the handshake does.
WiFi generations
The 802.11 standard has evolved through several generations, each raising speed and efficiency. The WiFi Alliance now uses simpler "WiFi N" names.
| Standard | Marketing name | Band(s) | Max rate (approx) | Notes |
|---|---|---|---|---|
| 802.11a | - | 5 GHz | 54 Mbps | Early 5 GHz standard |
| 802.11b | - | 2.4 GHz | 11 Mbps | Long range, low speed |
| 802.11g | - | 2.4 GHz | 54 Mbps | 2.4 GHz at 802.11a speeds |
| 802.11n | WiFi 4 | 2.4 / 5 GHz | 600 Mbps | Introduced MIMO (multiple antennas) |
| 802.11ac | WiFi 5 | 5 GHz | ~3.5 Gbps | Wider channels, more MIMO streams |
| 802.11ax | WiFi 6 | 2.4 / 5 GHz | ~9.6 Gbps | Efficiency in dense environments (OFDMA) |
Higher generations do not just add raw speed - the biggest gains from WiFi 5 and 6 come from serving many stations more efficiently, which matters far more in a crowded office or stadium than the headline peak rate a single device could ever reach.