802.11 Operating Modes
The following list shows the operating modes that the IEEE 802.11 standard defines:
Ad hoc mode
In ad hoc mode, wireless clients communicate directly with each other without the use of a wireless access point (AP) or a wired network. Ad hoc mode connects wireless clients together when there is no wireless AP present, when the wireless AP rejects an association due to failed authentication, or when the wireless client is explicitly configured to use ad hoc mode.
In infrastructure mode, wireless and possibly wired clients communicate with each other through a wireless AP. The wireless client uses the wireless AP to access the resources of a wired network. The wired network can be an organization intranet or the Internet, depending on the placement of the wireless AP.
A single wireless AP supporting one or multiple wireless clients is known as a Basic Service Set (BSS). A set of two or more wireless APs connected to the same wired network is known as an Extended Service Set (ESS). An ESS is a single logical network segment, also known as a subnet, and is identified by its Service Set Identifier (SSID).
When a wireless network adapter is powered on, it begins to scan across the wireless frequencies for wireless APs and other wireless clients. Assuming that it is in infrastructure mode, after scanning, a wireless adapter chooses a wireless AP with which to associate. This selection is made automatically by using the SSID of the wireless network and the wireless AP with the best signal strength. Next, the wireless client switches to the assigned channel of the chosen wireless AP and negotiates the use of a logical wireless port. This step is known as an association.
The settings of the wireless client determine whether the wireless client prefers to associate with wireless APs or with individual wireless clients. If the signal strength of the wireless AP is too low, if the error rate is too high, or if it is instructed by the operating system, the wireless client scans for other wireless APs to determine whether a different wireless AP can provide a stronger signal to the same wireless network. If so, the wireless client switches to the channel of that wireless AP. This is known as re-association.
By switching to another wireless AP, the wireless client can distribute the load over other wireless APs, increasing the performance for other wireless clients. You can achieve wireless connectivity for large areas by placing wireless APs so that their coverage areas overlap slightly but their channels do not. As a wireless client moves its physical location, it can associate and re-associate from one wireless AP to another, and maintain a continuous connection during physical relocation.
If the coverage areas of the wireless APs within an ESS overlap, a wireless client can roam, or move from one location with a wireless AP to another location with a different wireless AP, and still maintain network layer connectivity.