Wireless Fundamentals

On switched Ethernet each device gets its own cable and can send and receive at once. Over the air, every station within range hears every transmission on the channel, and a radio cannot cleanly transmit and receive on the same channel at the same time. So Wi-Fi is half-duplex and contention-based: only one station should be talking on a channel at a time.

When two stations cannot hear each other but can both hear the AP — the hidden-node problem — an optional RTS/CTS exchange reserves the medium before the data frame is sent, so the station that could not hear the other still knows to wait.

Radio waves also weaken and bend on the way. Absorption by walls, reflection off metal, refraction through glass or water, diffraction around obstacles, and scattering off rough surfaces all degrade the signal and the usable data rate. The practical takeaway is that coverage is never a clean circle: materials shrink and distort each cell, which is why site design matters more in wireless than anywhere else in the course.

The 2.4 GHz band spans only about 2.400 to 2.4835 GHz, and each channel is wide enough that adjacent channel numbers overlap. Fitting nonoverlapping cells into that space means using 1, 6, and 11 and nothing between them. The 5 GHz band is much wider with many clean channels, so the higher frequency buys throughput and planning headroom at the cost of range and wall penetration. (6 GHz is Wi-Fi 6E — awareness only at CCNA.)

The SSID is the human-readable network name that every AP in the network advertises. The BSSID is the MAC address of one AP radio, identifying a single cell. One SSID can be served by dozens of BSSIDs — and when it is, you are looking at an ESS, which is what makes roaming possible.

A honeycomb channel plan tiles cells so no two touching cells share a channel, drawing only from 1, 6, and 11. Neighbouring cells deliberately overlap by roughly 10 to 15 percent, so a moving client always has signal from the next AP before it loses the current one — that overlap is what makes roaming smooth rather than a drop and reconnect.

Association is a sequence, not a single event. A client scans — hearing beacons or sending probes — then moves through three states: not authenticated and not associated, then authenticated but not associated, then authenticated and associated. Only the third state can pass user traffic. Behind the APs, the distribution system is the wired network, and each SSID typically maps to a VLAN on that wired side.

• Shared air means half-duplex and CSMA/CA: listen, wait, send, then wait for the ACK. A missing ACK signals loss; RTS/CTS handles hidden nodes.
• In 2.4 GHz only 1, 6, and 11 are nonoverlapping. Anything between them overlaps two channels at once.
• 5 GHz has many nonoverlapping channels and higher throughput, but shorter range and weaker wall penetration.
• SSID is the network name; BSSID is one AP radio's MAC. One SSID with many BSSIDs is an ESS, which is what supports roaming.
• BSS is one AP, ESS is many APs on one SSID, IBSS is ad-hoc with no AP.
• A client must reach authenticated *and* associated before any user traffic passes.
• Cells overlap about 10 to 15 percent on purpose, so roaming clients never lose coverage.