Wireless LAN¶

Practical 3.6 Site survey PDF

Take out your phone and open the Wi-Fi list. How many networks do you see? Ten? Twenty? All of them are talking through the same air, in this room, right now. How is that possible? And why is the Wi-Fi sometimes slow with five bars?

Until now every bit you sent in this unit travelled through a cable you could follow with your finger. Now the cable disappears. The good news: everything else you know (frames, MAC addresses, DHCP, the bridge) stays exactly the same. Only the medium changes, and the medium brings its own rules: generations, bands, channels and passwords.

Radio instead of copper: the canteen¶

More people at the table means a longer wait for your turn. That is why five bars can be slow: the bars measure how loud the AP sounds, not how many people are waiting to speak.

The family names: 802.11 and Wi-Fi numbers¶

The standard behind Wi-Fi is IEEE 802.11. The first version, in 1997, reached 2 Mbit/s. Each improvement got letters (b, a, g, n...), and nobody could remember them. So in 2018 the Wi-Fi Alliance, the industry group that certifies devices, introduced simple numbers, starting at 4.

Two honest notes:

• Products arrive before the paper. Wi-Fi 6 devices were certified from 2019 and Wi-Fi 7 devices from January 2024, before their standards were final.
• The last column is a laboratory maximum, with many antennas and nobody else at the table. Your phone gets a fraction of it. And remember the ÷8 from Unit 1: a link of 600 Mbit/s is at most 75 MB/s.

Decode the box¶

Product boxes rarely say "Wi-Fi 5". They say things like this:

Bands: three rooms of the canteen¶

A band is a range of radio frequencies that Wi-Fi is allowed to use.

In the European Union the 6 GHz band (5945–6425 MHz) was opened to Wi-Fi by a Commission decision of June 2021.

Channels: the tables in each room¶

A channel is the slice of a band that one access point uses.

In Europe the 2.4 GHz band has channels 1 to 13, their centres 5 MHz apart: channel 1 = 2412 MHz, channel 6 = 2437 MHz, channel 11 = 2462 MHz. But one Wi-Fi signal is about 20 MHz wide. Picture it as a hill: a signal on channel 1 covers the marks up to channel 3. A signal on channel 3 lands right on top of it.

Find the hills that do not touch: 1, 6 and 11. Those are the only three that do not overlap.

flowchart LR
    C1["<b>Channel 1</b><br>2412 MHz"] ~~~ C6["<b>Channel 6</b><br>2437 MHz"] ~~~ C11["<b>Channel 11</b><br>2462 MHz"]
    C3["<b>Channel 3</b><br>2422 MHz<br>overlaps 1 and 6"] -.->|"noise"| C1
    C3 -.->|"noise"| C6

At 5 GHz the channels are numbered 36, 40, 44, 48... and are already 20 MHz apart, so they do not overlap. A router can join several into a wider channel (40 or 80 MHz): faster, but it occupies more tables.

To turn a frequency into a channel number:

Security: who may sit at the table¶

Radio goes through walls: anyone in the street can listen. So the frames are encrypted, and joining the network needs a secret.

Two flavours:

• Personal (home, small office): one passphrase for everyone, 8 to 63 characters. Long beats complicated.
• Enterprise (school, company): each person logs in with their own user name. eduroam at universities works like this.

Configuring the access point on RouterOS¶

Your group router becomes an AP called TRAMUNTANA-G. Eight APs share one classroom, so each group gets a channel from the plan on the board: groups 1, 4, 7 on channel 1 (2412 MHz) · groups 2, 5, 8 on channel 6 (2437 MHz) · groups 3, 6 on channel 11 (2462 MHz).

RouterOS v7 has two different wireless menus, depending on the driver package. Look at the left menu of WinBox: WiFi (interfaces wifi1, wifi2) or Wireless (interfaces wlan1, wlan2). /system package print confirms it: wifi-qcom or wifi-qcom-ac means the WiFi menu; wireless means the Wireless menu.

/interface wifi set wifi1 configuration.mode=ap configuration.ssid=TRAMUNTANA-3
/interface wifi set wifi1 configuration.country=Spain
/interface wifi set wifi1 channel.frequency=2462 channel.width=20mhz
/interface wifi set wifi1 security.authentication-types=wpa2-psk,wpa3-psk
/interface wifi set wifi1 security.passphrase="group passphrase"
/interface wifi set wifi1 disabled=no
/interface bridge port add bridge=bridge-lan interface=wifi1
/interface wifi registration-table print

/interface wireless security-profiles add name=tramuntana mode=dynamic-keys
/interface wireless security-profiles set tramuntana authentication-types=wpa2-psk
/interface wireless security-profiles set tramuntana wpa2-pre-shared-key="group passphrase"
/interface wireless set wlan1 mode=ap-bridge band=2ghz-b/g/n
/interface wireless set wlan1 channel-width=20mhz frequency=2462
/interface wireless set wlan1 ssid=TRAMUNTANA-3 security-profile=tramuntana
/interface wireless set wlan1 frequency-mode=regulatory-domain country=spain
/interface wireless enable wlan1
/interface bridge port add bridge=bridge-lan interface=wlan1
/interface wireless registration-table print

Three things that matter:

• Country = Spain is not decoration. It sets the legal channels and transmit power. An AP with the wrong country can transmit where it must not.
• The bridge port is the whole trick. The radio is just one more port of bridge-lan, like ether4. Without it, phones connect and get no address.
• A passphrase from the first minute, at least 12 characters. Never leave an AP open "just to test".

Join TRAMUNTANA-G with your phone. It will say "no internet": true, the router has no way out yet. Stay connected. The phone gets a 10.G.0.x address from the same DHCP server as the wired PCs. Ping it from a wired PC: the reply proves that wired and wireless are one LAN.

Measuring the signal: dBm¶

The registration table lists every connected client and its signal strength in dBm (decibels relative to one milliwatt). The numbers are negative: closer to zero is stronger.

The scale is logarithmic: every −3 dB is half the power, and −10 dB is one tenth. So −70 is not "a bit worse" than −50: it is a hundred times less power. Walk away from the AP and you lose half the power every few steps; a wall costs more than ten metres of air.

Site survey: measure before you choose¶

Your channel came from a table on the board. That was a guess. A professional never guesses: before installing an AP they walk the building and measure.

A site survey answers three questions before you install anything: which networks are already in the air, on which channels, and how strong?

You cannot see radio, so you need an instrument. Ours is the AP itself: it can stop talking for a moment and listen. In WinBox, select the interface and press Scan; in the CLI:

/interface wifi scan wifi1
/interface wireless scan wlan1

(The first for the WiFi menu, the second for the Wireless menu. Stop with Q.) While the radio scans it does not serve its clients, so phones drop for a few seconds. Each line is one AP heard:

The school's network name may appear four times, with four different BSSIDs. Four APs announce the same SSID so that your phone can move between them as you walk: that is roaming. The SSID is the name of the table; the BSSID identifies the box.

From a scan to a decision¶

The Site survey sheet walks you through it: scan for 30 seconds, turn each frequency into a channel, tally one mark per AP, draw a bar chart. Then choose 1, 6 or 11 with these rules:

• Count only the strong ones, above about −75 dBm. Twelve networks at −85 disturb less than two at −40: ten whispers bother you less than one shout.
• Never pick an in-between channel. Channel 3 looks free, but it overlaps 1 and 6.
• Neighbours get different channels. Colour the classroom plan like a map. When two APs must share, make it the two farthest apart.

Example: channel 1 has two networks at −45 dBm; channel 6 one at −50 and six at −88; channel 11 four at −86. The best choice is 11: it is the only one of the three with no strong signal.

The surprise: turn the power down¶

A louder AP is a person shouting in the canteen: their own table hears them, and so do three other tables that now have to wait. Offices use many quiet APs, not one loud one.

/interface wifi set wifi1 channel.frequency=2412 configuration.tx-power=5
/interface wireless set wlan1 frequency=2412 tx-power-mode=all-rates-fixed tx-power=5

5 dBm of transmit power is plenty for one table. Scan again: the neighbours' signal drops by 10 dB or more, and your own phone still connects. Nobody has more signal than before; everybody has better Wi-Fi.

Key takeaways¶

• Wi-Fi is IEEE 802.11: 802.11n = Wi-Fi 4, ac = Wi-Fi 5, ax = Wi-Fi 6/6E, be = Wi-Fi 7. Speeds on the box are laboratory maximums.
• 2.4 GHz reaches farther but is slower and crowded; 5 GHz and 6 GHz are faster with shorter range. At 2.4 GHz only channels 1, 6 and 11 do not overlap.
• Use WPA2/WPA3 with a long passphrase. WEP and WPA are broken; a hidden SSID protects nothing.
• On RouterOS the radio is one more port of the bridge: set mode, SSID, country, channel and security, then add it to bridge-lan.
• Signal is in dBm: closer to zero is stronger, and −3 dB is half the power.
• A site survey lists APs (SSID, BSSID, channel, signal). Choose by counting strong signals, and lower the power so neighbours wait less.

Quick check¶

Next: connect the eight group islands to each other — Routing.