WLAN Extended service set - WLAN ESS

WLAN Extended Service Set (ESS) links BSSes together with the backbone area. Backbone is usually Ethernet.

WLAN networks offer that stations in different BSSes can communicate among. When backbone area is one broadcast domain - one VLAN, 802.11 networks can offer link-layer mobility.

Distribution system is the key of ESS. It provides mobility by connecting access points. DS is responsible for tracking location of stations. Router on the picture just sends frame to destination by MAC address. Distribution system delivers frames to the right access point, and access point delivers frame to specific station.

Inter-access point communication - method to manage associations among access points. Station first associate to the AP1 and then AP1 sends packet - I “have” that station, to other access points (AP2, AP3 and AP4).

IAPP (Inter-Access Point Protocol) is recommendation described by IEEE 802.11F. It provides multivendor access points communication.

Wireless medium also can be a distribution system. Wireless Distribution or Wireless Bridge allows you to connect two distinct locations. You could connect to your neighbor network with two Wireless Bridges.

802.11 networks have overlapping Network boundaries. This fuzziness is even desirable. If we make closer AP1 to AP3 (or increase their powers), BSS1 and BSS3 overlaps. So, when station moves from BSS1 to BSS3, it will not lose network even if AP2 fails.

Looking above, 802.11 networks look like Wireless Ethernet. If you are familiar with Ethernet, you will have no problems with WLAN.

From user’s prospective, WLAN is the same as Ethernet. Network administrators have to know that frame delivery is unreliable, comparing to wired networks. Because that, 802.11 standard requires a numbers of additional services and more complex framing.