WLAN Technology Basics · Topic 1 of 3
WLAN Concepts, Radio Basics and 802.11 Standards
Learn WLAN basics for H12-811: STA, AP, AC, SSID, BSSID, VAP, 2.4 GHz and 5 GHz channels, MIMO and OFDMA, 802.11 standards and Wi-Fi generations.
- 6 min
- STA is the terminal, AP is the radio bridge, AC manages Fit APs.
- SSID is the network name; BSSID is a MAC address; a VAP gives one radio several SSIDs.
- 2.4 GHz has three non-overlapping channels (1, 6, 11); 5 GHz has many more.
What it is
A wireless local area network (WLAN) connects terminals to a network over radio waves instead of cables. Its standards come from the IEEE 802.11 working group. Wi-Fi is the Wi-Fi Alliance brand for products certified to those standards.
A WLAN gives users mobility and is easy to extend where cabling is hard. The wired network still carries the traffic behind the APs.
How it works
Basic WLAN concepts
| Term | Meaning |
|---|---|
| STA (station) | A terminal with a wireless NIC, such as a laptop or phone |
| AP (access point) | Bridges STAs on the radio side to the wired network |
| AC (access controller) | Centrally manages and configures Fit APs |
| SSID | The network name STAs see and select, up to 32 characters |
| BSS (basic service set) | One AP radio and the STAs associated with it |
| BSSID | The identifier of a BSS, the MAC address of the AP radio or VAP |
| ESS (extended service set) | Several BSSs that share one SSID, so a STA can roam between APs |
| VAP (virtual AP) | A logical AP on one radio; each VAP has its own SSID and BSSID |
VAPs let one physical AP offer several SSIDs, such as staff and guest networks.
Wired side basics
APs are usually powered by PoE (Power over Ethernet) from the access switch. IEEE 802.3af supplies up to 15.4 W per port at the switch and 802.3at (PoE+) up to 30 W, which suits most enterprise APs.
Between a Fit AP and its AC, management and data run over CAPWAP tunnels, explained on the next page.
Key WLAN technologies: radio basics
| Band | Characteristics |
|---|---|
| 2.4 GHz | Longer range and better wall penetration, but crowded; only 3 non-overlapping 20 MHz channels: 1, 6 and 11 |
| 5 GHz | Many more non-overlapping channels and less interference, but shorter range |
| 6 GHz | Added with Wi-Fi 6E and used by Wi-Fi 7; wide, clean spectrum |
Neighboring APs should use non-overlapping channels to avoid co-channel and adjacent-channel interference. Wider channels (40, 80, 160 MHz, and 320 MHz in Wi-Fi 7) carry more data but leave fewer channels to reuse.
Other technologies behind 802.11 performance:
- OFDM splits a channel into many subcarriers. OFDMA (Wi-Fi 6) assigns groups of subcarriers to different users at the same time.
- MIMO uses several antennas to send several spatial streams. MU-MIMO sends to several users at once.
- QAM modulation carries more bits per symbol: 64-QAM in 802.11n, 256-QAM in 802.11ac, 1024-QAM in 802.11ax, 4096-QAM in 802.11be.
- CSMA/CA: radio is shared and half-duplex, and a STA cannot detect a collision while sending. It listens before sending, waits a random backoff and expects an ACK for each unicast frame.
Introduction to 802.11 standards
| Standard | Wi-Fi generation | Band | Maximum rate |
|---|---|---|---|
| 802.11 (1997) | 2.4 GHz | 2 Mbps | |
| 802.11b (1999) | 2.4 GHz | 11 Mbps | |
| 802.11a (1999) | 5 GHz | 54 Mbps | |
| 802.11g (2003) | 2.4 GHz | 54 Mbps | |
| 802.11n (2009) | Wi-Fi 4 | 2.4 GHz and 5 GHz | 600 Mbps |
| 802.11ac | Wi-Fi 5 | 5 GHz | 6.9 Gbps (Wave 2) |
| 802.11ax | Wi-Fi 6 (Wi-Fi 6E adds 6 GHz) | 2.4 GHz and 5 GHz | 9.6 Gbps |
| 802.11be | Wi-Fi 7 | 2.4, 5 and 6 GHz | Much higher than Wi-Fi 6 |
The Wi-Fi Alliance introduced the generation names with Wi-Fi 6. 802.11n, 802.11ac and 802.11ax were named Wi-Fi 4, 5 and 6; older standards have no official generation number.
| Generation | Headline features |
|---|---|
| Wi-Fi 4 (802.11n) | MIMO with up to 4 streams, 40 MHz channels, dual band |
| Wi-Fi 5 (802.11ac) | 5 GHz only, 80 and 160 MHz channels, 256-QAM, downlink MU-MIMO |
| Wi-Fi 6 (802.11ax) | OFDMA, uplink and downlink MU-MIMO, 1024-QAM, BSS coloring, Target Wake Time |
| Wi-Fi 7 (802.11be) | 320 MHz channels, 4096-QAM, Multi-Link Operation (MLO) |
A worked example
An office deploys three APs in a row on 2.4 GHz. If all three use channel 1, their STAs contend for the same air and throughput drops. The engineer sets channels 1, 6 and 11 on the three APs so their cells do not overlap.
Each AP also broadcasts two SSIDs, "Staff" and "Guest". That is two VAPs per radio, each with its own BSSID. A laptop walking past the three APs stays on "Staff", which is one ESS, and roams from BSS to BSS.
How the exam asks about this
- "Which Wi-Fi generation corresponds to IEEE 802.11ax?" → Wi-Fi 6.
- "Which of the following 2.4 GHz channels do not overlap?" → 1, 6 and 11.
- "Which statement about the BSSID is true?" → It identifies a BSS and is the MAC address of the AP radio or VAP.
What to remember for the exam
- STA is the terminal, AP is the radio bridge, AC manages Fit APs.
- SSID is the network name; BSSID is a MAC address; a VAP gives one radio several SSIDs.
- 2.4 GHz has three non-overlapping channels (1, 6, 11); 5 GHz has many more.
- Wi-Fi 4 is 802.11n, Wi-Fi 5 is 802.11ac, Wi-Fi 6 is 802.11ax, Wi-Fi 7 is 802.11be.
- 802.11a and 802.11ac are 5 GHz only; 802.11b and 802.11g are 2.4 GHz only.
802.11ac (Wi-Fi 5) works only in 5 GHz. A dual-band Wi-Fi 5 AP serves 2.4 GHz clients with 802.11n on the other radio.
Common mistakes
Mixing up SSID and BSSID. The SSID is the human-readable name and can be shared by many APs. The BSSID is a MAC address unique to one radio or VAP.
Assuming collisions are detected on radio. Ethernet used CSMA/CD. WLAN uses CSMA/CA, avoiding collisions with listen-before-talk, random backoff and ACKs.
Thinking wider channels are always better. Wider channels raise one link's rate but leave fewer non-overlapping channels, which increases interference in a dense deployment.
Check yourself
What does a VAP allow an AP to do?
It lets one physical radio offer several SSIDs, each as a separate virtual AP with its own BSSID.
Which standards operate only in the 5 GHz band?
802.11a and 802.11ac (Wi-Fi 5).
What does OFDMA add in Wi-Fi 6?
It divides a channel's subcarriers among several users so they can be served at the same time.
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