An 802.11 network is one access point and the stations that have chosen it, and the choosing is a protocol in its own right.
Words you will meet
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BSS (basic service set) basic service set One access point and the wireless stations associated with it — 802.11’s building block. introduced in ch. 7 — one access point and the stations associated with it. The building block.
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AP (access point) access point 802.11's word for a base station: the central device of a basic service set, which relays between its associated stations and the wired network behind it. introduced in ch. 7 — 802.11’s word for a base station.
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SSID (Service Set Identifier) Service Set Identifier The network name an access point announces in its beacon frames. introduced in ch. 7 — the network name an administrator gives an AP, announced in its beacons.
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Association association Simple Choosing an access point and creating a virtual wire to it. Precise The process by which a wireless station selects one access point and establishes a link-layer relationship with it. Only the associated AP sends data frames to the device, and the device sends into the network only through that AP. The 802.11 standard requires association but does not specify how to choose among the available APs. introduced in ch. 7 — open in glossary — choosing an AP and creating a virtual wire to it.
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Beacon frame — the periodic announcement an AP is required to send.
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Passive and active scanning — listen for beacons, or broadcast a probe and collect replies.
Why this matters
Everything in chapters 1 to 6 assumed a host was already on a network. Plug in the cable and you are on it; the only question left is what address you get.
Radio has no cable, so being on a network becomes a decision — and a decision needs a protocol. This page is that protocol, and it happens before DHCP (Dynamic Host Configuration Protocol) Dynamic Host Configuration Protocol Gives a host its IP address and the address of its local DNS server when it joins a network. Built properly in §4.3.2. introduced in ch. 2 , before ARP (Address Resolution Protocol) Address Resolution Protocol Translates an IP address into the MAC address of an interface on the same subnet. It works only within one subnet, which is the whole point. introduced in ch. 6 , before anything section 6.7 covered.
The standards
802.11 — also known as WiFi (IEEE 802.11 wireless LAN) IEEE 802.11 wireless LAN Wireless LAN access; a user must be within a few tens of metres of the access point. WiFi is a trade name, not an abbreviation. introduced in ch. 1 — is the class of wireless
LAN (Local Area Network) Local Area Network A network covering one building, campus or home. introduced in ch. 1standards that won. The b, g, n, ac and ax generations are for local networks of 70 m or less; af and ah trade rate for reach.
| Standard | Year | Max data rate | Range | Frequency |
|---|---|---|---|---|
Cells marked ⓘ have an explanation — click to read it.
Sortable. Sort by range and the table splits cleanly in two: five WLAN standards at 30–70 m, and two long-range ones aimed at sensors and metering.
They share a great deal. The same frame format, the same medium access protocol ( CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) Carrier Sense Multiple Access with Collision Avoidance 802.11’s MAC protocol. It avoids collisions rather than detecting them, because a radio adapter cannot listen while it transmits. introduced in ch. 7 , next section), and backward compatibility — a device that speaks only 802.11g still works with an 802.11ax base station.
The differences are physical. There are two frequency ranges. 2.4 GHz is unlicensed, and therefore shared with cordless phones and microwave ovens. 5 GHz reaches less far for a given power, and suffers more from multipath. The n, ac and ax standards use MIMO (Multiple Input Multiple Output) Multiple Input Multiple Output Using several antennas at each end to raise the rate over one channel. introduced in ch. 7 : two or more antennas at each end, sending different signals. And ac and ax can beamform, aiming a transmission at a particular receiver.
Read Table 7.1’s rates as “at best, in a laboratory”
The book says so itself. These are rates for an idealized environment — a receiver close to the base station, with no interference. It calls that “a scenario that we’re unlikely to experience in practice”.
Its own comment is the right one to remember: your wireless data rate may vary.
The architecture
Drag any device to rearrange the picture. Hover a link to see its rate, delay and length.
A BSS is one access point and the stations associated with it. Every wireless link is dashed and pink; the links from the APs back to the switch are ordinary wired Ethernet. In a typical home the AP and the router are one box.
Read this diagram as text
- station — wireless link — AP 1
- station — wireless link — AP 1
- station — wireless link — AP 2
- station — wireless link — AP 2
- AP 1 — wired link — switch or router
- AP 2 — wired link — switch or router
- switch or router — wired link — Internet
The basic service set basic service set Simple One access point and the stations associated with it. Precise The fundamental building block of the 802.11 architecture: one or more wireless stations together with a central base station, known in 802.11 as an access point. Each station and each AP interface has a 6-byte MAC address administered by the IEEE, exactly as in Ethernet. introduced in ch. 7 — open in glossary is the building block: one access point and the stations that have associated with it. Each station has a 6-byte MAC (Media Access Control) Media Access Control The link-layer address burned into a network interface. Before a host has an IP address it is the only identity it has, which is why DHCP keys on it. Chapter 6 covers it properly. introduced in ch. 4 address in its adapter’s firmware, exactly as in section 6.4.1, administered by the IEEE (Institute of Electrical and Electronics Engineers) Institute of Electrical and Electronics Engineers The body whose 802 committee standardises Ethernet and WiFi. introduced in ch. 1 and globally unique. So does each AP’s wireless interface.
Wireless LANs built around APs are infrastructure wireless LANs, where the infrastructure is the APs plus the wired Ethernet joining them. 802.11 stations can also form an ad hoc network, with no central control and no connection to the outside world. Think of laptops in a conference room or a train, finding each other with no AP present.
Channels
An administrator installing an AP assigns it two things: an SSID ssid Simple The network name an access point announces. Precise The one- or two-word Service Set Identifier a network administrator assigns to an access point. It is carried in every beacon frame along with the AP's MAC address, and it is what appears in the list of networks a device offers you. introduced in ch. 7 — open in glossary , which is the name you see in a list of networks, and a channel number.
802.11 defines 11 channels in the 2.4 GHz band, and they overlap. Two channels are non-overlapping only if they are separated by four or more.
Channels are drawn evenly spaced and equally wide, which is schematic — the point is which of them overlap, not their exact shape in the spectrum.
Read all steps as text
- Eleven channels in 85 MHz — 802.11 defines 11 channels in the 2.4 GHz band, and they overlap. Two channels are non-overlapping only if they are separated by four or more.
- 1, 6 and 11 — the only non-overlapping set of three — Separated by five each, so none of the three interferes with the others. It is the only such trio in the band.
- Which is why an administrator installs three APs — Put three access points in the same place, give them channels 1, 6 and 11, and wire each to the switch. The aggregate rate is three times a single AP’s, because the three do not interfere.
The band’s width is given twice, and the two do not agree
Page 543 says 802.11 operates in 2.4–2.485 GHz. Page 545 says 2.4 GHz to 2.4835 GHz, and calls it an 85 MHz band.
Those cannot all be right. 2.4835 − 2.4 is 83.5 MHz, not 85. The figure that matches “85 MHz” is the 2.485 on page 543.
Nothing on this page depends on which is correct, because the channel argument works either way. But if you are checking your arithmetic against the book and it comes out 1.5 MHz short, this is why.
Finding an AP, and joining it
A WiFi jungle is the book’s name for a place where your device hears strong signals from several APs at once. The example is a New York café, where one AP belongs to the café and the others to flats upstairs. Each is on its own channel and its own IP (Internet Protocol) Internet Protocol The network-layer protocol that defines the datagram format and addressing every Internet device must use. introduced in ch. 1 subnet.
To get on the Internet your device must join exactly one subnet, and so must associate with exactly one AP.
In plain words
Associating creates a virtual wire. Only the associated AP will send data frames to your device, and your device sends into the Internet only through that AP.
It is exactly the relationship a cable gives you — chosen, rather than plugged in.
Listen. Every AP beacons periodically, so a device that scans all 11 channels will hear whichever ones are in range.
Click any arrow to see what that message says and why it is sent.
Three runs. The first two are the book’s Figure 7.9; the third is what the text says happens immediately afterwards, and it is the moment this chapter hands back to chapter 4.
Read this diagram as text
- AP 1 sends beacon — SSID + MAC address to H1. The 802.11 standard requires an AP to send beacon frames periodically. Each carries the AP’s SSID and its MAC address.
- AP 2 sends beacon — SSID + MAC address to H1. A second AP, on a different channel and probably a different IP subnet. In a café there may be five.
- H1 sends association request to AP 1. The device has chosen. The standard does NOT say how — that algorithm is left to whoever wrote the device’s firmware.
- AP 1 sends association response to H1. Now there is a virtual wire between the two: only this AP will send data frames to the device, and the device sends only through it.
Lifelines, left to right: AP 1 (access point), H1 (host), AP 2 (access point).
An AP is required to beacon periodically, and each beacon carries the AP’s SSID and MAC address. Your device scans all 11 channels listening for them — that is passive scanning. Or it broadcasts a probe request and collects probe responses — active scanning.
Then it picks one. And here the standard stops:
Nobody specifies how to choose
The 802.11 standard does not say which AP to associate with. That algorithm is left to whoever wrote the firmware in your device.
Typically it picks the strongest beacon. That is a reasonable heuristic and not a good one. A strong AP may already be loaded with other devices all sharing its bandwidth, while a slightly weaker one sits idle. Alternative selection methods have been proposed and none is standard.
So two identical phones in the same café may associate with different APs and get very different service, and neither is doing anything wrong.
After selecting, the device sends an association request and the AP replies with an association response. That second handshake is genuinely needed in the active case. An AP answering a probe has no idea which of the responding APs the device will choose. It is the same reason a DHCP client must formally request from one of several offering servers.
Then, still with no address, the device sends DHCP discover through its new AP to get an address on that AP’s subnet. From then on the rest of the world sees it as an ordinary host on that subnet.
It may also have to authenticate first — by MAC address, as many companies do, or with a username and password, as many cafés do. Either way the AP usually relays to a separate authentication server using RADIUS or DIAMETER, so one server can serve many APs and the sensitive decisions stay in one place.
Two slips in Figure 7.9
Both checked against the printed page.
1 · The coverage areas are labelled “BBS 1” and “BBS 2” — four times, twice in each half of the figure. It is BSS, the basic service set, defined two pages earlier.
2 · Both halves are captioned “a.” The figure reads “a. Passive scanning” and then “a. Active scanning”. The second should be b. — and the body text does refer to “Figure 7.9a” and “Figure 7.9b”, so a reader following the text’s own pointer to 7.9b will not find one.
Check yourself
Check yourself
0 of 6 answered1.What is a basic service set?
2.What do 802.11b, g, n, ac and ax have in common?
3.predictAn administrator wants three times a single AP's rate in one room. What should they do?
4.What is the difference between passive and active scanning?
5.predictTwo identical phones sit at the same café table and associate with different APs, getting very different speeds. Is one of them malfunctioning?
6.predictA device has just completed its association response. Can it browse the Web?
What to remember
- A BSS is one AP plus its associated stations. All of b, g, n, ac and ax share one frame format, one MAC protocol, and backward compatibility — only the physical layer differs.
- 11 channels, overlapping. 1, 6 and 11 are the only non-overlapping trio, so three co-located APs give three times the rate.
- The standard does not say which AP to choose. Most firmware takes the strongest signal, which ignores how loaded it is. Association is a virtual wire, and it comes before DHCP.