How Wi-Fi works

Everything from the Introduction still applies. Your laptop on Wi-Fi sends the same packets, with the same MAC and IP addresses, to the same default gateway as a laptop on a cable. The only difference is the bottom layer: instead of electrical pulses on copper, the bits travel as radio waves to an access point, which passes them onto the wired network.

Two names to know:

• SSID (service set identifier): the network name you pick from the list, like HomeNet. The access point broadcasts it in a small beacon about ten times a second, which is how your phone knows the network is there.
• BSSID: the MAC address of one access point's radio. One SSID can have many BSSIDs. A house with a mesh system, or an office with twenty ceiling access points, shows one network name, but every radio has its own BSSID, and your device picks one.

See yours

netsh wlan show interfaces

Mac: hold Option and click the Wi-Fi icon in the menu bar. The grey details under your network show the same things: PHY Mode, BSSID, Channel, Security, RSSI (signal), Noise and Tx Rate.

nmcli dev wifi list

Keep this output open; every step below explains one or two of its lines. Notice the Linux example: HomeNet appears twice, with BSSIDs one digit apart. That's the same router's two radios, one on channel 36 (5 GHz) and one on channel 6 (2.4 GHz), both announcing the same name.

Wi-Fi uses three slices of the radio spectrum, called bands, named after their frequency: 2.4 GHz, 5 GHz and 6 GHz. The physics comes down to one trade-off. Lower frequencies travel further and pass through walls better. Higher frequencies can carry more data but fade faster, especially through walls, floors and people.

Most routers run 2.4 and 5 GHz at the same time, often under one name, and let each device pick (band steering). Rule of thumb: use 5 GHz when you're in the same room or next door, and 2.4 GHz for the far bedroom, the garden, and cheap smart-home gadgets, many of which only speak 2.4. Look at the Band or Channel line from Step 01 to see which one you're on now. Channels 1–14 are 2.4 GHz and 36 and up are 5 GHz. 6 GHz channels restart from 1, so on 6E hardware check the band line.

Each band is divided into channels, like lanes on a motorway. Two networks on the same channel have to take turns, and two networks on overlapping channels garble each other. In the 2.4 GHz band this is a real problem, because the channels were laid out before anyone expected every flat to have a router:

So the 2.4 GHz band really only has three clean lanes: 1, 6 and 11. (Europe and much of the world also allow 12 and 13; the 1/6/11 rule still works everywhere.) If your neighbour is on 6 and you're on 6, you share the lane politely. If you're on 4, you overlap both 1 and 6 and make things worse for everyone, including yourself.

The 5 GHz band has far more room: dozens of non-overlapping 20 MHz channels, from 36 upward. Routers usually bond several together into one wider channel (40, 80 or 160 MHz) to go faster. A wider channel is like a wider lane: more speed, but fewer lanes to share around the neighbourhood. Some 5 GHz channels are shared with weather and aviation radar (DFS channels), and a router must jump off them if it detects radar, which you may notice as a brief dropout.

See your neighbours

netsh wlan show networks mode=bssid

On Linux, the nmcli dev wifi list from Step 01 already has a CHAN column; on a Mac, hold Option and click Wi-Fi, then choose Open Wireless Diagnostics → Window → Scan. Count how many networks sit on each 2.4 GHz channel. In a block of flats it can be twenty or more. If your router is on a busy 2.4 GHz channel, set it to whichever of 1, 6 or 11 is quietest, or leave it on Auto, which on modern routers usually does this for you.

The bars on your phone are a rough summary of a number engineers measure in dBm: decibels relative to one milliwatt. Wi-Fi signals are tiny, far less than a milliwatt, so the numbers are negative, and closer to zero is stronger. −45 is a great signal; −85 is barely there.

Where to find it: on a Mac it's the RSSI line in the Option-click menu. On Linux, iw dev shows your interface name, then iw dev wlan0 link (with your name in place of wlan0) prints signal: -54 dBm. Windows shows a percentage instead; as a rough guide, 100% is about −50 dBm or better and 50% is about −75.

Try it: walk the house

Run the command once next to the router, once in the next room, and once as far away as you can get, and write down the signal each time. On a Mac or Linux, where you get real dBm, you'll typically see each wall cost anywhere from 3 dB (plasterboard) to 15 dB or more (brick, concrete). Also note how the receive rate from Step 01 falls with it. That's the next step.

Your Receive rate of 1,201 Mbps is the link rate: how fast the bits are sent while your device is actually talking. It is not how fast your downloads go. Three things eat into it:

• Only one device talks at a time. A cable has its own wires for each direction; radio doesn't. On each channel, every device (yours, your family's, and your neighbours' on the same channel) listens first and only transmits when the air is quiet. This is called CSMA/CA, “listen before you talk”. Everyone gets a turn, so everyone shares the speed.
• Every packet is acknowledged. Radio loses packets far more often than cable, so each one is confirmed and resent if lost. That checking costs time.
• Slow devices use up airtime. An old phone at the edge of the house might be talking at 20 Mbps. Sending it one packet takes 60 times longer than sending the same packet to your laptop at 1,201, and nobody else can use the channel while that happens.

In practice, real throughput is often around half the link rate on a good day, and less when the channel is busy. Newer standards (Wi-Fi 6 and later) soften this by letting the access point talk to several devices in one turn, but the basic rule stands: the air is shared.

Try it: link rate compared with real speed

Run the site's Boz Bandwidth test next to the router, then again from the far room, and compare each result with the receive rate netsh or your Mac showed at that spot. The gap between the two is the cost of sharing the air. If your Internet plan is slower than your Wi-Fi, the test measures the plan instead, which is useful to know too.

When you pick a network and type the password, four things happen in about a second. The clever part is step 3: your device proves it knows the password without ever sending it.

1. Find. Your device hears beacons and may ask “is HomeNet here?” with a probe. The beacon says which security the network uses.
2. Join. The device asks to join (authentication and association) and the access point accepts it. With WPA3, this is also where the password check begins.
3. The 4-way handshake. Both sides mix the password with fresh random numbers to create encryption keys that are new for this session. Each proves it got the same answer. Anyone listening sees random numbers, never the password.
4. Get an address. Only now does the device send its DHCP request, and the router hands out an IP address, mask, gateway and DNS server, exactly as in the Introduction's Step 03.

A cable only reaches the people in the building. Radio reaches the car park. Wi-Fi security exists so that people nearby can't join your network or read what you send. Here are the options your router will offer, from worst to best:

Three more settings worth a minute:

• Password: long beats clever. Four random words is far stronger than P@ssw0rd1, and easier to type on a TV.
• WPS (the push-button or 8-digit PIN pairing): turn it off. The PIN version in particular can be guessed quickly.
• Guest network: turn it on for visitors and smart-home gadgets. It's a separate subnet that can reach the Internet but not your laptops. That's the subnetting guide's first reason, security, built into your router.

Wi-Fi standards are named by the IEEE as 802.11 plus letters, which nobody could remember, so the Wi-Fi Alliance added plain numbers in 2018. Your Radio type line from Step 01 is the letters version:

Every generation connects to every older one at the older one's speed. The headline maximums on the box (“9.6 Gbps!”) add up every antenna on every band under perfect conditions; a single laptop never sees them. The real-world gains come from the extra room and the better handling of crowds, which is why a Wi-Fi 6 router still helps an older laptop in a busy house.

Nearly every Wi-Fi complaint comes down to distance, walls, or a crowded channel. In order of how much they help:

1. Move the router. Central, high, and in the open: not in a cupboard, behind the TV, on the floor, or next to the fish tank. Water, metal, mirrors and brick absorb signal; a microwave in use floods 2.4 GHz.
2. Use a cable where you can. A desktop, games console or TV that never moves should be wired. It gets a faster, steadier connection and frees airtime for everything that has to be wireless.
3. Fix the channel. 2.4 GHz on 1, 6 or 11 (whichever is quietest), and let 5 GHz pick automatically.
4. Add access points the right way. A mesh system, or extra access points connected by cable, covers a big house properly. A cheap plug-in extender is the last resort; see the prompt below.

Check yourself

• You found your SSID, BSSID, band, channel and signal on your own device
• You can say why 2.4 GHz reaches further but 5 GHz is faster
• You know why 1, 6 and 11 are the 2.4 GHz channels to use
• You measured signal in at least three places and saw it fall through walls
• You compared your link rate with a real speed test and can explain the gap
• Your own router is on WPA3 or WPA2/WPA3, with WPS off

Wi-Fi is the bottom layer: radio in place of copper. Everything above it is what the first two guides covered, and what the router guides let you build by hand.