Wireless LANs

Computer Network & Network Security System — Data Link Layer, NEC licence examination syllabus (Nepal Engineering Council).

Wireless LANs: why Wi-Fi can't just copy Ethernet's rules

A station can't reliably "listen while talking" over radio the way a wire allows — so wireless needs a different strategy entirely.

📶 IEEE 802.11 Architecture

BSSBasic Service Set — one access point plus the devices connected to it (like your home Wi-Fi router and your phone/laptop).

ESSExtended Service Set — multiple BSSs connected together, letting you roam between access points (like walking through a mall with Wi-Fi that never disconnects).

CSMA/CA and the terminal problems

Since a wireless station's own transmission drowns out its ability to hear a collision, Wi-Fi uses CSMA/CA (Collision Avoidance) instead of CD — using techniques like waiting a random backoff period even BEFORE the first transmission attempt, and optional RTS/CTS handshaking.

👻 Hidden and Exposed Terminal Problems

Hidden terminalTwo stations can't hear each other directly, but both can reach the same access point — they might transmit simultaneously without knowing, causing a collision at the AP.

Exposed terminalA station holds back from transmitting because it hears a NEIGHBOUR transmitting, even though its own transmission wouldn't actually have caused a collision at the intended receiver.

💡 Draw a simple diagram with 3 stations in a line (A–B–C) to explain both hidden terminal (A and C can't hear each other, both talk to B) and exposed terminal (B holds back because it hears A, even though B→C wouldn't collide with A).

Syllabus points

IEEE 802.11 architecture (BSS, ESS)
CSMA/CA; hidden & exposed terminal

Create a free account to tick topics off, take notes as you read, watch the video lessons and get a day-by-day study plan built around your exam date.