Wireless links and Wi-Fi
Wireless networks free devices from cables but must cope with fading signals, interference, and hidden terminals. Wi-Fi is the most common wireless LAN technology.
Wireless link characteristics
Radio signals weaken with distance (path loss), are absorbed or reflected by walls and objects (multipath propagation, causing fading), and face interference from other devices in the same frequency band. The signal-to-noise ratio (SNR) determines how fast data can be sent reliably: higher SNR allows denser modulation and higher bit rates, so wireless devices adapt their rate to conditions.
Hidden and exposed terminals
In wireless networks, not every node can hear every other. If A and C are both in range of B but not of each other, they cannot sense each other's transmissions and may collide at B: the hidden terminal problem. Because a transmitter cannot easily listen while sending, wireless links also cannot detect collisions the way wired Ethernet could.
802.11 basics
Wi-Fi is the IEEE 802.11 family. In infrastructure mode, devices associate with an access point (AP) that connects to the wired network; a basic service set is an AP with its clients, identified by a network name (SSID). Devices find APs by listening for beacon frames or sending probe requests, then authenticate and associate. Ad hoc mode lets devices communicate directly.
Wi-Fi generations
Wi-Fi standards have advanced steadily: 802.11b (1999), 802.11g, 802.11n (Wi-Fi 4, 2009), 802.11ac (Wi-Fi 5), 802.11ax (Wi-Fi 6 and 6E, with 6E adding the 6 GHz band), and 802.11be (Wi-Fi 7, finalised in 2024). They operate in the 2.4 GHz, 5 GHz, and 6 GHz bands, using wider channels, more antennas (MIMO), and denser modulation to reach multi-gigabit rates.
CSMA/CA
Wi-Fi uses carrier sense multiple access with collision avoidance (CSMA/CA). A station senses the channel; if idle for a short period, it transmits, and if busy, it waits a random backoff that counts down only while the channel is idle. The receiver acknowledges each correctly received frame, and missing ACKs trigger retransmission with a larger backoff range.
RTS and CTS
To reduce hidden-terminal collisions on long frames, a station can first send a short request-to-send (RTS); the receiver replies with clear-to-send (CTS), which every station in range of the receiver hears, so they stay quiet for the announced duration. This reservation costs extra overhead, so it is usually used only for large frames.
Frames, addresses, and security
802.11 frames carry up to four MAC addresses, because frames pass between a wireless station, an AP, and the wired network. Wi-Fi security evolved from the broken WEP to WPA2 (2004), using AES encryption, and WPA3 (2018), which adds protection against offline password-guessing attacks with the SAE handshake and individualised encryption on open networks.
Worked example
In an office, laptops A and C sit at opposite ends, both within range of the AP but not of each other. Each senses an idle channel and starts a large transmission to the AP, and the frames collide at the AP; neither laptop knows until no ACK arrives. With RTS/CTS enabled, A's RTS prompts a CTS from the AP, which C hears, so C defers for A's announced transmission time and the collision is avoided.
Key takeaways
Wireless links suffer path loss, multipath fading, and interference; SNR determines achievable rates, which devices adapt. Hidden terminals and the difficulty of listening while transmitting rule out collision detection. Wi-Fi (IEEE 802.11) connects stations to access points identified by SSIDs, via beacons, probes, and association. Generations from 802.11b (1999) to Wi-Fi 7 (2024) use 2.4, 5, and 6 GHz bands, wider channels, and MIMO. CSMA/CA uses sensing, random backoff, and ACKs; RTS/CTS reserves the channel. Frames carry up to four addresses. WPA2 and WPA3 secure Wi-Fi; WEP is broken.
Flashcard
Signal weakening with distance
Check your understanding
Why does Wi-Fi use collision avoidance and acknowledgements instead of Ethernet-style collision detection?