Wireless principles: bands and channels, RF, security protocols and interference

Why this matters

For most users, "the network" is the Wi-Fi. A help-desk analyst hears "the Wi-Fi is slow in the meeting room" or "it drops every lunchtime" far more often than any complaint about a cable. A junior network engineer is expected to read a site survey, choose channels for a new floor, pick a security mode that the oldest laptops can still join, and explain why moving an access point two metres fixed a problem nobody could find.

Wireless faults are hard because radio is invisible and shared. A microwave oven, a neighbour's access point on the same channel, a crowd of people or a nearby radar can all cut throughput without any device logging an error. Security choices made casually, such as WPA2-Personal on a band that forbids it or an open guest network that sends traffic in the clear, either break the design or leave it exposed. The four topics in this lesson are the vocabulary you need to reason about all of it.

The lesson

Wireless LANs follow the IEEE 802.11 family of standards. The Wi-Fi Alliance certifies products and gives the generations their marketing names. Recognise both:

Wi-Fi 6 and Wi-Fi 6E use the same 802.11ax amendment. The only difference is that 6E extends it into the 6 GHz band. So a laptop must support at least Wi-Fi 6E to use 6 GHz channels.

Band and channel selection

A band is a range of frequencies. A channel is a slice of a band that one access point (AP) radio uses. Choosing channels well is most of good wireless design.

The 2.4 GHz band. Channels are numbered 1 to 13 in most of the world (1 to 11 in North America), but their centres are only 5 MHz apart while each signal is about 20 MHz wide. Neighbouring channel numbers therefore overlap heavily. In North America the only set of three 20 MHz channels that do not overlap is 1, 6 and 11. Channels 4, 7 and 10 are only 15 MHz apart and still overlap. With just three clean channels, 2.4 GHz radios should stay at 20 MHz width. 802.11n does allow bonding two channels into 40 MHz in this band, but one bonded channel takes up most of it, leaving too few clean channels to separate neighbouring cells.

The 5 GHz band. Far more room: about 25 non-overlapping 20 MHz channels, depending on the country. They fall into groups called UNII bands:

• UNII-1, channels 36 to 48: no radar checks.
• UNII-2 and UNII-2 Extended, channels 52 to 64 and 100 to 144: shared with radar, so they need Dynamic Frequency Selection (DFS).
• UNII-3, channels 149 to 165.

On a DFS channel, an AP must listen for radar before using it and must leave the channel if it detects radar, which briefly disconnects its clients. An AP on channel 100 that suddenly changes channel has most likely detected radar. Channel 36 is UNII-1 and never uses DFS.

The 6 GHz band. Opened for Wi-Fi 6E and later, with up to 59 non-overlapping 20 MHz channels where the full band is allowed. It is clean because only new devices use it. It also has strict security rules, covered below.

Channel width. Bonding adjacent 20 MHz channels into 40, 80 or 160 MHz (and 320 MHz with Wi-Fi 7) raises the peak data rate of one cell, but it uses up channels. In a dense office, narrower channels and more of them usually give better total throughput. Use wide channels where the band has room, such as 5 GHz in a small office or 6 GHz.

Channel reuse. Neighbouring APs should be on different, non-overlapping channels so that each cell has its own airtime. In 2.4 GHz that means a repeating 1, 6, 11 pattern. Controller-based networks, such as those run by a Catalyst 9800 wireless controller, can assign channels and power automatically, but the principle is the same.

RF characteristics

Radio frequency (RF) signals are waves. The terms you need:

• Frequency is how many cycles per second, in hertz. 2.4 GHz is 2.4 billion cycles per second.
• Wavelength is the length of one cycle. The higher the frequency, the shorter the wavelength.
• Amplitude is the strength of the wave.

Higher frequencies attenuate faster. A 5 GHz signal loses strength over distance more quickly than a 2.4 GHz one, and walls and other obstacles absorb more of it. So an AP sending at the same power on both radios has a smaller 5 GHz cell. That is a trade-off, not a fault: smaller cells mean more of them, more channels and less contention.

Decibels. Wireless power is measured on a logarithmic scale.

• A change of +3 dB roughly doubles the power; -3 dB roughly halves it.
• A change of +10 dB multiplies power by about ten; -10 dB divides it by ten.
• dBm is power compared with 1 milliwatt. 0 dBm is 1 mW, 20 dBm is 100 mW.

So raising an AP's transmit power by 3 dB roughly doubles it, which is far more than a 3 percent rise.

Received signal strength is a negative number in dBm, and the closer to zero, the stronger. -42 dBm is very strong. -67 dBm is a common target for voice and video coverage. -88 dBm is very weak, near the point where a client cannot hold a connection. Do not be fooled by the bigger digits. Equally important is the signal-to-noise ratio (SNR), the gap between the signal and the background noise, in dB. A strong signal on a noisy channel can still perform badly.

How signals behave as they travel:

Absorption by people explains a classic complaint: a conference hall that tested well when empty loses coverage once several hundred people are seated, because the human body is mostly water. Reflection from metal fittings would be present whether the room was full or not, so it cannot explain a change that tracks attendance.

Multipath is the same signal reaching the receiver by several paths at slightly different times, after reflections. Older standards suffered from it. 802.11n and later use MIMO (multiple input, multiple output), with several antennas and spatial streams, to turn multipath into extra capacity.

Wi-Fi radios share a channel using CSMA/CA (carrier sense multiple access with collision avoidance). A radio listens before it transmits and waits if the channel is busy. Wired half-duplex Ethernet used CSMA/CD, collision detection, which radios cannot do while transmitting.

Security protocols

Wireless security has moved through several generations. Know which are obsolete and which are current.

Each of WPA, WPA2 and WPA3 comes in two modes:

• Personal uses a pre-shared key (PSK), the passphrase everyone types.
• Enterprise uses 802.1X with each user's own credentials. There are three roles. The client is the supplicant. The AP or wireless LAN controller is the authenticator, which relays the EAP exchange but does not decide. An authentication server, usually RADIUS, checks the credentials and decides whether access is granted. No PSK is stored on the AP in Enterprise mode.

What WPA3 changes. In WPA2-Personal, the encryption keys are derived directly from the PSK. Anyone who captures the four-way handshake can try passwords offline at high speed. WPA3-Personal replaces this with Simultaneous Authentication of Equals (SAE), a password-authenticated key exchange that resists offline dictionary attacks. WPA3 also makes Protected Management Frames (PMF, 802.11w) mandatory. PMF protects management frames such as deauthentication and disassociation, so an attacker can no longer forge them to knock clients off the network. PMF does not stop raw radio jamming, and it is SAE, not PMF, that defeats offline password guessing.

WPA3 transition mode lets WPA2 and WPA3 clients share one SSID during a migration. It admits WPA2 clients, never WEP ones. Moving from WPA2-Personal to WPA3-Personal does not require a RADIUS server; only Enterprise mode does.

Wi-Fi Enhanced Open uses Opportunistic Wireless Encryption (OWE). Clients join without a password, but each one gets its own encryption keys. It suits a café or a guest network. A plain open network with a captive portal also asks for no password, but its traffic crosses the air unencrypted.

6 GHz rules. The Wi-Fi Alliance requires WPA3 or Enhanced Open, with PMF, on every SSID in the 6 GHz band. WPA2-Personal cannot be offered there, so a plan to copy a WPA2 SSID from 5 GHz onto 6 GHz fails. WPA3-Personal with SAE is allowed, so 802.1X is not the only option.

On a Catalyst 9800 controller, a WPA3-Personal WLAN looks like this:

WLC1(config)#wlan STAFF 2 STAFF
WLC1(config-wlan)#no security wpa akm dot1x
WLC1(config-wlan)#no security wpa wpa2
WLC1(config-wlan)#security wpa wpa3
WLC1(config-wlan)#security wpa psk set-key ascii 0 Example-Passphrase-2027
WLC1(config-wlan)#security wpa akm sae
WLC1(config-wlan)#security pmf mandatory
WLC1(config-wlan)#no shutdown

The lines map directly to the theory: WPA2 off, WPA3 on, SAE as the key management, PMF mandatory.

Cause of interference

Interference is anything that makes a receiver hear something other than the frame it wants. There are four kinds to tell apart.

Co-channel interference (contention). Two nearby APs on the same channel hear each other. Because of CSMA/CA, every radio waits while any other radio on that channel transmits, so the two cells share one channel's airtime. Each AP may show a strong signal to its own clients, yet throughput in both drops at busy times. The fix is better channel planning or lower transmit power, so that fewer cells hear each other.

Adjacent-channel interference. APs on overlapping but different channels, such as 2.4 GHz channels 1 and 3, corrupt each other's frames rather than politely taking turns. This is usually worse than co-channel contention, and it is why only 1, 6 and 11 are used.

Non-802.11 interference. Devices that are not Wi-Fi but transmit in the same band:

• Microwave ovens leak energy around 2.45 GHz. A 2.4 GHz WLAN that stalls at lunchtime while 5 GHz clients in the same room carry on is the textbook case.
• Bluetooth devices frequency-hop across the 2.4 GHz band. Bluetooth does not use 5 GHz.
• Cordless phones, baby monitors, wireless video senders and some industrial equipment.

Such devices ignore CSMA/CA, so they simply transmit over Wi-Fi traffic. A spectrum analyser, or the spectrum features built into many APs, finds them.

Radar. Weather and military radar use parts of the 5 GHz band, which is why those channels need DFS. Radar never affects 2.4 GHz. When it appears on a DFS channel, the AP must move.

Physical changes cause problems that look like interference: a new metal partition, a full room, a moved AP. Check them before assuming a rogue transmitter.

What the exam is really asking

• Non-overlapping 2.4 GHz channels are 1, 6 and 11. Keep 2.4 GHz at 20 MHz.
• DFS applies to 5 GHz UNII-2 channels (52 to 144), not channel 36 and never 2.4 GHz. A sudden channel change on channel 100 most likely means radar.
• Wi-Fi 6 versus Wi-Fi 6E. Same 802.11ax; only 6E uses 6 GHz.
• Higher frequency, smaller cell. 5 GHz attenuates faster and is absorbed more.
• Decibels. +3 dB doubles, +10 dB is tenfold. In dBm, closer to zero is stronger.
• Absorption versus reflection. People and water absorb; metal reflects.
• Ciphers. WEP and TKIP use RC4. WPA2 uses CCMP with AES. GCMP-256 is WPA3-Enterprise 192-bit mode.
• SAE versus PMF. SAE stops offline password guessing. PMF stops forged deauthentication.
• 802.1X roles. Supplicant is the client, authenticator is the AP or controller, the RADIUS server decides.
• Enhanced Open versus captive portal. Both have no password; only OWE encrypts.
• 6 GHz security. WPA3 or Enhanced Open only.
• Co-channel versus non-802.11 interference. Same-channel APs share airtime all the time; a microwave or Bluetooth hits 2.4 GHz only, and the timing gives it away.