07 - Wi-Fi - Learner Guide - CompTIA Network+ (N10-009)

1Why wireless behaves differently

This is your copy to keep, and the last session of this course. The gold Going deeper boxes go past what the session covers, for anyone sitting the CompTIA Network+ exam or who wants the fuller picture.

A cable is a private channel. Two devices at either end of it can both send at the same time, and nothing else on the network is affected by what crosses it.

Radio is neither of those things. Every device on a channel shares the same air, so only one of them can transmit at a time, and every one of them has to wait while any other is talking. Devices take turns: a device with something to send listens first, sends if the air is quiet, and backs off and retries if it is not.

Three things follow, and every wireless fault in this session is one of them.

Capacity is shared, not per device. Twenty devices on one access point are taking turns with each other, so what any one of them gets depends on what the other nineteen are doing.
A weak client slows everyone. A device with a poor signal sends at a lower rate, so it holds the air for longer to move the same data. The time it spends is time nobody else can use.
Interference is not a connection problem. Anything else transmitting on the same channel, including a neighbor's network, takes turns with your devices whether or not it is part of your network.

What is worth measuring, then, is not signal strength on its own but airtime, meaning how much of the available time is being used and by what.

2The bands

Wireless equipment transmits in one of three bands, meaning ranges of radio frequency. A single access point normally serves two or three of them at once, and a device picks the one it can hear.

The three bands
Band	Range through a building	Capacity	Congestion
2.4 GHz	Furthest. Passes through walls better than the others	Lowest	Worst. Shared with every neighbor, and with microwaves, cordless phones and other equipment
5 GHz	Shorter. Loses more to walls and floors	High	Much better. Many more channels available
6 GHz	Shortest	Highest	Least. Only recent devices can use it at all

The trade is the same one every time: lower frequency travels further and carries less, higher frequency carries more and travels less far. There is no band that does both, which is why access points serve several at once and why a site with too few of them ends up relying on the worst band.

That last point is worth stating plainly, because it explains a common complaint. A device at the far edge of coverage falls back to 2.4 GHz, because it is the only one still audible. It connects, so the user reports being connected and slow rather than disconnected. The fix is coverage, not settings.

3Channels and overlap

Each band is divided into channels, and two access points on the same channel share the air. Two on different channels do not, which is the entire reason channel planning exists.

On 2.4 GHz the channels overlap each other. The band is numbered 1 to 11 in the United States, but each channel is wide enough to spill into its neighbors, so channel 2 interferes with channels 1 and 3. Only three of them, 1, 6 and 11, do not overlap at all. Any plan that uses other numbers has access points interfering with each other, and the fix is always to move everything back onto 1, 6 and 11 and arrange them so that no two neighbors share one.

On 5 GHz there are many more channels and they do not overlap in the same way, which is most of why the band performs better. Some of those channels are shared with weather and military radar. An access point using one has to listen for radar and move off immediately if it hears any, which is called DFS (dynamic frequency selection). The effect on the job is a wireless network that drops for everyone at once, for no reason visible in the console, in a building near an airport or a weather station.

Going deeper

The usual reflex when wireless is poor is to turn the transmit power up. It is worth understanding why that often makes things worse. Power only affects what the access point is shouting; it does nothing for the client, which is a phone with a small antenna and a battery. Turn it up and devices hear an access point from further away, stay attached to a distant one instead of moving to a near one, and get a connection they can hear and barely answer. It also makes your access points interfere with each other across a larger area. More coverage comes from more access points at lower power, not from fewer at higher power.

4Channel width

Channel width is how much of the band one network occupies, measured in megahertz. A wider channel carries more data and consumes more of the air, so the setting is a trade between the speed one device can reach and how many devices can be served without colliding.

The vendor's stated guidance for its equipment is 20 MHz on 2.4 GHz, to avoid excessive interference; 80 MHz on 5 GHz; and 160 or 320 MHz on 6 GHz.

The reason 2.4 GHz is held at 20 is arithmetic. The band has only three non-overlapping channels at that width. Double the width to 40 and a single network covers what were two of those three, so one access point can leave one usable channel for everything else in the building and everything in the neighboring building. Width bought on 2.4 GHz is taken directly from everyone, including yourself.

The vendor is also explicit about what to do when a widened channel causes trouble: if connections become unreliable or latency rises, revert the change and return to the defaults, which are set to favor reliable connections over peak speed.

5Reading signal and noise

Wireless measurements are in dBm (decibel-milliwatts), a scale that is always negative for received signal and gets more negative as the signal gets weaker. A reading of -65 dBm is a stronger signal than -80 dBm, because -65 is the larger number.

The scale is logarithmic rather than linear, which is why small-looking differences matter: each step of 3 dBm is roughly a halving or doubling of power, so -68 dBm is about half the power of -65 dBm, and -80 dBm is a small fraction of it.

Two numbers are read together.

Signal

How strongly the access point hears the client, or the client hears the access point. Closer to zero is stronger.

This is the number every console shows and the one people quote. On its own it does not settle anything.

Noise

Everything else on that channel: other networks, other equipment, interference. Here the sign works the other way round, because more negative means less of it. A noise figure of -90 dBm is better than -80 dBm.

What matters is the gap between them, called the signal-to-noise ratio. A client at -65 dBm in a quiet room at -95 dBm of noise has a 30 dB gap and works well. The same client at -65 dBm in a room where the noise is -70 dBm has a 5 dB gap and barely works, with an identical signal reading. Quoting a signal figure without the noise figure explains nothing, which is why two sites with the same signal strength can behave completely differently.

6Roaming

Walking from one end of a building to the other means moving from one access point to another. The decision to move is made by the client device, not by the network. The access points cannot push a device off, and the device stays where it is until its own logic decides to look elsewhere.

Client logic is generally reluctant. A device will hold a weakening connection for a long time rather than switch. This produces a common complaint: a laptop that works fine at the desk is unusable for the first few minutes after someone carries it to the meeting room at the far end of the floor, because it is still clinging to the access point by the desk instead of switching to the one nearby. It works normally again only once it finally gives up and reconnects.

Two settings in the console influence it, and neither commands it. A minimum signal level lets an access point refuse a client below a threshold, so the device is pushed to look for a better one. Settings that advertise neighboring access points to the client help it choose quickly rather than scanning from scratch. Both nudge the client's own decision, and a device that has decided to stay will stay.

The underlying cause of most roaming complaints is coverage rather than settings. A device holds on too long because the alternative is not obviously better, and the fix is another access point rather than another setting.

7Networks, VLANs and security

Each wireless network has a name, called the SSID (service set identifier). In the console, every SSID is mapped to a VLAN, a division of one physical switch into separate networks by configuration. A guest SSID mapped to the guest VLAN puts wireless guests on the same separated network as anything wired into a guest port.

This is where a wireless problem is really a wired one. The switch port feeding an access point has to carry every VLAN that access point broadcasts. If the guest VLAN is not allowed across that port, the guest network appears in the air, clients connect to it, and none of them gets an address, while the staff network on the same access point works perfectly. Clients connecting but getting no address, on one network only, is a port setting rather than a wireless fault.

Security

The encryption on a wireless network is set on the network itself, not on the access point.

WPA2 Personal is the shared passphrase in use nearly everywhere. Anyone with the passphrase can join, and anyone who has ever had it can still join until it is changed.
WPA3 is the current generation, with stronger protection for the passphrase exchange. The vendor's default combines it with WPA2 so that older devices can still connect.
PMF (protected management frames) protects the messages that set up and tear down a connection, which are unencrypted in older equipment. It is what stops a device being knocked off a network by a forged disconnect message.

The decision that actually matters at a client site is not which of these to use, since the defaults are sound, but who holds the passphrase. A staff wireless passphrase that has been given to every visitor for three years is a guest network with a misleading name.

8Diagnosing "the Wi-Fi is slow"

This is the most common wireless report and the least specific. Four questions in this order settle it, and each rules out what is below.

Is it wireless at all? Put the same machine on a cable and repeat the task. If it is still slow, nothing in this session is the cause and the fault is somewhere above the air. This step takes one minute and saves the rest of the hour.
What is the client's signal, and what is the noise? Read both from the console, for that client, on that access point. A weak signal means coverage. A weak signal with high noise means interference as well.
How much of the air is in use? Read airtime or channel utilization on the access point the client is on. A channel that is heavily used is slow for everyone on it regardless of any individual signal being fine, and the causes are other networks on the same channel, a channel width taking more than its share, or too many devices on one access point.
Is it one client or all of them? One client with a good signal on a quiet channel is a problem with that device, not with the network. All clients on one access point point at that access point or its channel. All clients on all access points point at something the whole site shares, which is the wired network or the internet connection, and the air is not involved.

The pattern is the same one that runs through this whole course: test in an order where each step rules something out, and stop at the first step that fails. Applied to wireless, the first step is the cable, because it separates a wireless problem from a problem that merely arrived over wireless.

9Practice

In the UniFi console, open a site and list every wireless network, and for each one the network and VLAN it is attached to.
Open an access point and note which bands it is serving and which channel and channel width it is using on each. Compare those widths to the vendor guidance in section 4.
Find your own device in the client list. Note its signal, the band it is on, and which access point it is attached to. Walk to the other end of the building, wait, and look again to see whether it moved.
Find the airtime or channel utilization figure for one access point, and say what it tells you about how much room is left on that channel.
Trace one wireless network from the access point back to the switch port feeding it, and confirm that port allows the VLAN that network needs.
Find the 2.4 GHz channels in use across the site and confirm they are all 1, 6 or 11, and that no two access points within range of each other share one.

10Check for understanding

Why does one device with a weak signal slow down other devices that have a good signal on the same access point?
Why are only 1, 6 and 11 used on 2.4 GHz, and what happens to a site whose access points are on 3, 7 and 9?
A client shows a signal of -65 dBm at one site and -65 dBm at another, and the first works well while the second is unusable. Give the most likely explanation and the number you would ask for.
A user's laptop is fine at their desk and unusable for the first few minutes in a meeting room at the other end of the floor. What is happening, and why is turning the transmit power up the wrong response?
An access point broadcasts a staff network and a guest network. Guests connect to the right name, get prompted correctly, and then receive no address at all. Staff are fine on the same access point. Where is the fault?
The whole wireless network at a site near an airport drops for everyone at once, occasionally, with nothing in the console to explain it and no pattern to the timing. What would you suspect?
A user reports the Wi-Fi is slow. Give the first thing you would do, and say what its outcome rules out either way.

11Where this course ends

Do the practice steps in section 9 on a real site.
Read the Network+ companion, chapter 3, wireless standards and site surveys, and chapter 11, the wireless troubleshooting section.

This is the last session of this course. Across these seven sessions, the same habit has been applied at every layer: test in a fixed order, stop at the first thing that fails, and let what failed choose the next tool. The layers gave the order, addressing decided who is a neighbor, the switch and the VLAN decided who hears whom, the router and its table decided the path, the two services decided whether a device could participate at all, the firewall decided what was permitted, and the air decided how much of it was available. A fault lives at one of those, and naming which one is most of the work.

12Glossary

Band: A range of radio frequency. Wireless equipment uses 2.4 GHz, 5 GHz and 6 GHz.
Channel: A division of a band. Two access points on the same channel share the air; on different channels they do not.
Channel width: How much of the band one network occupies, in megahertz. Wider carries more data and consumes more of the air.
Airtime: How much of the available transmission time is in use on a channel. The measure that actually predicts wireless performance.
Channel utilization: The same idea as reported in the console: the proportion of the channel already in use.
DFS: Dynamic frequency selection. The requirement on some 5 GHz channels to listen for radar and move off immediately if any is heard.
dBm: Decibel-milliwatts. The scale for signal and noise, always negative, where each 3 dBm is roughly a halving or doubling of power.
Signal: How strongly one end hears the other. Closer to zero is stronger.
Noise: Everything else on the channel. More negative is better, because it means less of it.
Signal-to-noise ratio: The gap between signal and noise. The number that decides whether a connection works, rather than signal alone.
Roaming: A client moving from one access point to another. The client decides, not the network.
SSID: Service set identifier. The name of a wireless network.
VLAN: Virtual local area network. A division of one physical switch into separate networks by configuration. Each wireless network is attached to one.
WPA2 Personal: Wireless encryption using a shared passphrase. Anyone with the passphrase can join.
WPA3: The current generation of wireless encryption, with stronger protection for the passphrase exchange.
PMF: Protected management frames. Protection for the messages that set up and tear down a connection, which are otherwise unencrypted.
Transmit power: How strongly an access point broadcasts. It does not change what the client can send back.

13Sources

IEEE, 802.11, Wireless LAN Medium Access Control and Physical Layer Specifications, for the shared medium, taking turns, and the bands and channels.
Ubiquiti, Maximizing Wireless Speeds, for the stated channel width guidance of 20 MHz on 2.4 GHz, 80 MHz on 5 GHz and 160 or 320 MHz on 6 GHz, for preferring channels with less interference, and for reverting to defaults when connection quality degrades.
Ubiquiti, Creating UniFi WiFi SSIDs, for attaching a wireless network to a VLAN, the requirement that upstream switch ports allow that VLAN, and the default security of WPA2 and WPA3 with optional protected management frames.
Ubiquiti, WiFi Troubleshooting Guide.
Ubiquiti, Optimizing WiFi Connectivity and Reducing Latency.
Wi-Fi Alliance, Wi-Fi security, for WPA2, WPA3 and protected management frames.
Kodi A. Cochran, CompTIA Network+ (N10-009) Certification Companion (Apress, 2026), chapter 3, wireless standards and site surveys, and chapter 11, wireless troubleshooting.