The idea
A wireless LAN needs two things the wired one never did. It needs a way to describe who is talking to whom when there is no cable to say so, and it needs a way to cope with a user walking out of one coverage area and into another.
IEEE 802.11, sold as WiFi, covers the physical and data link layers. It answers the first need with the basic service set and the second with the extended service set.
How it works
The standards table
| IEEE 802.11 standard | Year | Max data rate | Range | Frequency |
|---|---|---|---|---|
| 802.11b | 1999 | 11 Mbps | 30 m | 2.4 Ghz |
| 802.11g | 2003 | 54 Mbps | 30m | 2.4 Ghz |
| 802.11n (WiFi 4) | 2009 | 600 | 70m | 2.4, 5 Ghz |
| 802.11ac (WiFi 5) | 2013 | 3.47Gpbs | 70m | 5 Ghz |
| 802.11ax (WiFi 6) | 2020 (exp.) | 14 Gbps | 70m | 2.4, 5 Ghz |
| 802.11af | 2014 | 35 – 560 Mbps | 1 Km | unused TV bands (54-790 MHz) |
| 802.11ah | 2017 | 347Mbps | 1 Km | 900 Mhz |
Aside
Three entries on that slide look wrong, and the table above keeps them as printed because the exam marks the lecture. The 802.11n rate is printed as 600 with no unit, and the usual reading is 600 Mbps. The 802.11ac rate is printed 3.47Gpbs, which is a typo for Gbps. The 802.11ax year is printed 2020 (exp.), meaning expected at the time the slide was written. The slide also says “The three 802.11 standards use the same medium access protocol” directly under a table of seven, so read “three” as a leftover from an older version of the slide. The 802.11ac, 802.11n and 802.11ax range of 70m and the 802.11b and 802.11g range of 30 m are as printed.
All of them use the same medium access protocol, CSMA/CA, and the same frame structure for link-layer frames. All allow both infrastructure mode and ad hoc mode.
How it works
Basic service set
A BSS is the building block of a wireless LAN. There are two kinds.
- A BSS without an access point is an ad hoc network.
- A BSS with an access point is an infrastructure network.
Ad hoc mode. There are no access points. Nodes can only transmit to other nodes within link coverage. They organise themselves into a network and route among themselves.
Infrastructure mode. The access point (AP) connects mobiles into the wired network. A handoff happens when a mobile changes the AP that provides its connection into the wired network.
How it works
Extended service set
An ESS is made up of two or more BSSs with APs, and all of its BSSs are connected via a distribution system. A mobile station can belong to more than one BSS.
Mobility between one BSS and another, and between one ESS and another, is possible. The lecture is explicit that service continuity is not guaranteed by IEEE 802.11.
The slide that shows four BSSs sharing one infrastructure is a figure, and only its labels extracted.
| Ad hoc BSS | Infrastructure BSS | |
|---|---|---|
| Access point | None | One |
| Who can a node reach | Only nodes within its own link coverage | The wired network, through the AP |
| Routing | Nodes route among themselves | The AP connects mobiles into the wired network |
| Handoff | Not described | A mobile changes AP |
Where marks get lost
BSS and ESS are not the same size of thing
An ESS is a set of two or more BSSs joined by a distribution system, so a single AP and its stations is a BSS, not an ESS. A question asking which unit contains an AP is asking about the BSS.
In the exam
- Reproduce the standards table as printed. Year, rate, range and band for each row. The lecture’s values include the odd ones, such as 802.11af at 35 to 560 Mbps in unused TV bands.
- BSS with an AP is infrastructure. BSS without an AP is ad hoc. That one sentence is the usual question.
- ESS is two or more BSSs with APs connected by a distribution system.
- Handoff is a mobile changing AP.
- No guarantee of service continuity between BSSs or ESSs.
- Shared by every standard in the table: CSMA/CA, one frame structure, both modes.
Check yourself
- 802.11 covers the physical and data link layers and uses CSMA/CA throughout.
- A BSS has an AP or it does not. With one it is an infrastructure network and without one it is ad hoc.
- An ESS joins BSSs through a distribution system.
- Mobility between BSSs is possible and continuity is not guaranteed.