System Architecture

A set of STA, and eventually an AP, constitutes a basic service set (BSS),
which is the basic block of the IEEE 802.11 WLAN.
The simplest BSS is constituted by two STAs that can communicate
directly. This mode of operation is often referred to as an ad hoc network
because this type of IEEE 802.11 WLAN is typically created and maintained
as needed without prior administrative arrangement for specific purposes
(such as transferring a file from one personal computer to another). This
basic type of IEEE 802.11 WLAN is called independent BSS (IBSS). infrastructure
BSS, an AP (which is a particular STA) acts as the coordinator of the
BSS.
Instead of existing independently, two or more BSS can be connected
together through some kind of backbone network that is called the distribution
system (DS). The whole interconnected WLAN (some BSSs and a DS) is
identified by the IEEE 802.11, as a single wireless network called extended
service set (ESS). The whole scenario is shown in Figure 4.2.
The association between an STA and a particular BSS is dynamic, as
discussed later; as a consequence, the set up of the system is automatic.