Explain IEEE 802.11 Architecture.
The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and competing for access to the same shared wireless medium.
A BSS may be isolated or it may connect to a backbone distribution system (DS) through an access point (AP).
The AP functions as a bridge and a relay point. In a BSS, client stations do not communicate directly with one another.
Rather, if one station in the BSS wants to communicate with another station in the same BSS, the MAC frame is first sent from the originating station to the AP, and then from the AP to the destination station.
Similarly, a MAC frame from a station in the BSS to a remote station is sent from the local station to the AP and then relayed by the AP over the DS on its way to the destination station.
The BSS generally corresponds to what is referred to as a cell in the literature. The DS can be a switch, a wired network, or a wireless network.
When all the stations in the BSS are mobile stations, with no connection to other BSSs, the BSS is called an independent BSS (IBSS).
An IBSS is typically an ad hoc network. In an IBSS, the stations all communicate directly, and no AP is involved.
A simple configuration is shown in Figure, in which each station belongs to a single BSS; that is, each station is within wireless range only of other stations within the same BSS.
It is also possible for two BSSs to overlap geographically, so that a single station could participate in more than one BSS.
Further, the association between a station and a BSS is dynamic. Stations may turn off, come within range, and go out of range.
An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system.
Typically, the distribution system is a wired backbone LAN but can be any communications network.
The extended service set appears as a single logical LAN to the logical link control (LLC) level.
Figure indicates that an access point (AP) is implemented as part of a station; the AP is the logic within a station that provides access to the DS by providing DS services in addition to acting as a station.
To integrate the IEEE 802.11 architecture with a traditional wired LAN, a portal is used. The portal logic is implemented in a device, such as a bridge or router, that is part of the wired LAN and that is attached to the DS.
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