Network Fundamentals · Topic 11 of 17
Wireless Principles
Understand Wi-Fi basics including SSIDs, 2.4 and 5 GHz bands, non-overlapping channels, RF signal terms and the encryption options used on wireless LANs.
- 5 min
- Exam objective 1.11
- The SSID is the network name. The BSSID is the AP radio's MAC address.
- A BSS is one AP and its clients. An ESS joins multiple APs under one SSID.
- 2.4 GHz has longer range and fewer channels. 5 GHz has more channels and shorter range.
What it is
A wireless LAN uses radio waves instead of cables to carry frames between clients and an access point.
The standards for this are the IEEE 802.11 family, commonly called Wi-Fi.
Radio is a shared medium. Every device in range on the same channel competes for the same airtime, and signals from neighbouring networks can interfere.
Good wireless design is largely about choosing channels, managing signal strength and securing the traffic.
How it works
A wireless network is a set of names, bands, channels and signal measurements, plus encryption on top.
Names and building blocks
- SSID (Service Set Identifier): the human-readable name of a wireless network, such as the name you pick from a list on your phone.
- BSS (Basic Service Set): one access point and the clients attached to it. Its unique identifier is the BSSID, which is the AP's radio MAC address.
- ESS (Extended Service Set): several APs sharing the same SSID, joined by a wired network, so clients can roam between them.
Frequency bands
Wi-Fi uses unlicensed bands. The two long-standing ones are:
- 2.4 GHz: travels further and passes through walls better, but has fewer channels and more interference from other devices such as microwave ovens and Bluetooth.
- 5 GHz: has many more channels and is less crowded, but has a shorter range.
Newer Wi-Fi 6E and Wi-Fi 7 equipment also use the 6 GHz band.
| Standard | Bands |
|---|---|
| 802.11b | 2.4 GHz |
| 802.11a | 5 GHz |
| 802.11g | 2.4 GHz |
| 802.11n (Wi-Fi 4) | 2.4 and 5 GHz |
| 802.11ac (Wi-Fi 5) | 5 GHz |
| 802.11ax (Wi-Fi 6) | 2.4 and 5 GHz |
Channels and overlap
A band is divided into channels. In 2.4 GHz, a standard channel is about 20 MHz wide, but channel numbers are only 5 MHz apart. Neighbouring channels therefore overlap heavily.
Only three channels do not overlap: 1, 6 and 11 in the standard channel plan. Channels above 11 are not permitted in every region.
Neighbouring APs should use these three channels, and the same channel should be avoided on APs that are close together.
In 2.4 GHz, think 1, 6 and 11. Any other combination of neighbouring channels overlaps.
In 5 GHz, there are many more non-overlapping 20 MHz channels, so planning is easier. Some channels in 5 GHz share their space with radar, and rules apply to them.
Wider channels carry more data but use up more spectrum. Widths of 20, 40, 80 and 160 MHz exist. In crowded areas, narrower channels give better stability.
RF signal basics
- Frequency is how many cycles per second, measured in hertz.
- Signal strength is measured in dBm, a logarithmic scale. A closer value to zero is stronger, so -50 dBm is a stronger signal than -80 dBm.
- Noise is unwanted energy on the channel. SNR (signal-to-noise ratio) compares the signal to the noise. A higher SNR means a cleaner link.
- Attenuation is signal loss as the wave travels, and as it passes through walls and other materials.
- Interference is energy from other transmitters on the same or nearby channel.
- Antenna type shapes the coverage, for example an omnidirectional antenna covers all around, while a directional one focuses in one direction.
Distance and obstacles reduce signal. Clients then fall to slower data rates to keep the link working.
Encryption
Because radio can be received by anyone nearby, wireless traffic is encrypted. The protocols are:
- WEP: the original, now broken and not to be used.
- WPA: an interim replacement for WEP.
- WPA2: uses AES-based encryption (CCMP). Long the standard.
- WPA3: the newest, with stronger key exchange (SAE) and better protection on open networks.
Wireless security is covered in more detail in the Security Fundamentals chapter.
What to remember for the exam
Eight facts cover wireless basics.
- The SSID is the network name. The BSSID is the AP radio's MAC address.
- A BSS is one AP and its clients. An ESS joins multiple APs under one SSID.
- 2.4 GHz has longer range and fewer channels. 5 GHz has more channels and shorter range.
- The non-overlapping 2.4 GHz channels are 1, 6 and 11.
- Signal strength is in dBm, and a number nearer to zero is stronger.
- SNR compares signal with noise. A higher value is better.
- 802.11n supports both 2.4 and 5 GHz. 802.11ac is 5 GHz only.
- WPA2 and WPA3 are the strong choices. WEP is broken.
dBm values are negative, and the number nearer to zero is stronger. -90 dBm is much weaker than -50 dBm.
Common mistakes
These five errors are common.
Choosing channels 1, 2 and 3 in 2.4 GHz. These overlap. Use 1, 6 and 11 for neighbouring APs.
Confusing SSID and BSSID. The SSID is a name that many APs can share. The BSSID is unique to one radio.
Reading dBm backwards. -90 dBm is much weaker than -50 dBm.
Assuming more power always helps. A very strong AP can drown out neighbours, and a client may hear the AP while the AP cannot hear the weaker client.
Thinking higher bands always win. 5 GHz offers speed and channels, but its signal is blocked more easily by walls and floors.
Check yourself
Work through these three prompts.
Why are only channels 1, 6 and 11 used together in 2.4 GHz?
A standard channel is about 20 MHz wide but channel numbers are only 5 MHz apart, so neighbouring channels overlap heavily. Only channels 1, 6 and 11 do not overlap.
How do 2.4 GHz and 5 GHz compare in range, channel count and interference?
2.4 GHz travels further and passes through walls better, but has fewer channels and more interference. 5 GHz has many more channels and is less crowded, but has a shorter range.
What is the difference between an SSID, a BSSID and an ESS?
The SSID is the human-readable network name, and the BSSID is the AP's radio MAC address. An ESS is several APs sharing the same SSID, joined by a wired network so clients can roam.
Topic 11 of 17 in Network Fundamentals
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