The idea
Before any frame is exchanged, two questions need an answer. Which frequency does an AP use, and how does a host find it and join? After that, a host that moves has a third problem, because radio quality changes as it walks. This topic is those three answers.
How it works
Channels
802.11 uses the 2.4 GHz or 5 GHz bands. The AP administrator chooses the frequency for the AP. Because nothing coordinates those choices, the channel can be the same as one chosen by a neighbouring AP, and interference is then possible. The lecture notes that ISM stands for Industrial, Scientific and Medical, the unlicensed band these networks share.
The slide titled 802.11 Channels (2.4 GHz band) is a figure and its channel numbers and spacing did not extract. This page therefore gives no channel numbers.
How it works
Association
A host must associate with an AP before it can use it.
- It scans channels, listening for beacon frames. Each beacon contains the AP’s name, the Service Set Identifier (SSID), and its MAC address.
- It selects an AP to associate with.
- It may perform authentication.
- It will typically run DHCP to get an IP address in the AP’s subnet.
The lecture shows two figures, passive scanning and active scanning, as the two ways to do step 1. Both are figures and neither extracted. The text above describes scanning by listening for beacons, which is the passive method by name. The lecture’s active scanning sequence is not available here.
Aside
For the active scanning figure, check the lecture PDF directly. The textbook the lecture recommends, Kurose and Ross chapter 7, covers both, but this page does not fill the gap from it.
How it works
Advanced capability 1, rate adaptation
The base station and the mobile dynamically change the transmission rate, meaning the physical-layer modulation technique, as the mobile moves and SNR varies.
- As the node moves away from the base station, SNR decreases and BER increases.
- When BER becomes too high, switch to a lower transmission rate with a lower BER.
The lecture’s figure plots BER against SNR in dB for three modulations. As printed, they are QAM256 (8 Mbps), QAM16 (4 Mbps) and BPSK (1 Mbps), with an operating point marked on the curves. The most common algorithm named is Minstrel. Vendors also ship proprietary algorithms and the lecture mentions AI-based ones, citing a 2022 paper on improving Minstrel with shallow neural networks in IEEE 802.11ah.
Aside
The rates on the lecture’s figure are small beside the standards table, for example 8 Mbps for QAM256 against 54 Mbps for 802.11g. The figure is an illustration of the trade-off, so use it for the ordering of modulations and not for absolute rates.
How it works
Advanced capability 2, further features
The next slide lists further capabilities, under the heading 802.11 advanced capabilities (others). The slide that follows it is titled 802.11: Evolution, and its source is the Wikipedia page for IEEE 802.11be. The lecture does not name a standard on the capabilities slide.
- Up to 16 spatial streams and Multiple-Input-Multiple-Output (MIMO) protocol enhancements.
- Up to 4096-QAM (4K-QAM), so each symbol carries 12 bits rather than 10 bits.
- Multi-Access Point (AP) coordination.
- Enhanced link adaptation and retransmission protocol, for example Hybrid Automatic Repeat Request (HARQ).
- Integrating Time-Sensitive Networking (TSN) IEEE 802.1Q extensions for low-latency real-time traffic.
The lecture then shows two figure slides on long-range WiFi, 802.11ah, and on WiFi radio coverage. Both are images from the Wi-Fi Alliance page on Wi-Fi HaLow and nothing extracted from them beyond the titles.
| Step | What the host does |
|---|---|
| Discover | Scan channels and listen for beacon frames (SSID and MAC address) |
| Select | Choose an AP to associate with |
| Authenticate | May perform authentication |
| Get an address | Typically run DHCP for an IP address in the AP subnet |
Where marks get lost
Association is not the same as getting an IP address
Selecting an AP and authenticating are link-layer steps. The lecture says the host will typically then run DHCP, a separate step that gives it an address in the AP’s subnet.
In the exam
- Bands. 2.4 GHz and 5 GHz. ISM means Industrial, Scientific and Medical.
- Channel interference. The AP administrator picks the channel and may pick the same one as a neighbour.
- Association steps in order. Scan for beacons, select, maybe authenticate, DHCP.
- Beacon contents. SSID and MAC address.
- Rate adaptation. SNR down, BER up, drop to a lower rate with a lower BER. Name Minstrel.
- 4096-QAM. 12 bits per symbol against 10.
Check yourself
- AP channels are chosen by an administrator and can clash with a neighbour’s.
- A host scans for beacons, picks an AP, may authenticate and then runs DHCP.
- Rate adaptation trades rate for a lower BER as SNR falls.
- Other capabilities include MIMO with up to 16 spatial streams, 4096-QAM, multi-AP coordination, HARQ and TSN.