WLAN Topologies

802.11 networks are flexible by design. You have the option of deploying three types of WLAN topologies:

• Independent basic service sets (IBSSs)
• Basic service sets (BSSs)
• Extended service sets (ESSs)

A service set is a logical grouping of devices. WLANs provide network access by broadcasting a signal across a wireless radio frequency (RF) carrier. A receiving station can be within range of a number of transmitters. The transmitter prefaces its transmissions with a service set identifier (SSID). The receiver uses the SSID to filter through the received signals and locate the one it wants to listen to.

IBSS
An IBSS consists of a group of 802.11 stations communicating directly with one another. An IBSS is also referred to as an ad-hoc network because it is essentially a simple peer-to-peer WLAN. Figure 2-1 illustrates how two stations equipped with 802.11 network interface cards (NICs) can form an IBSS and communicate directly with one another.

BSS
A BSS is a group of 802.11 stations communicating with one another. A BSS requires a specialized station known as an access point (AP). The AP is the central point of communications for all stations in a BSS. The client stations do not communicate directly other client stations. Rather, they communicate with the AP, and the AP forwards the frames to the destination stations. The AP might be equipped with an uplink port that connects the BSS to a wired network (for example, an Ethernet uplink). Because of this requirement, a BSS is also referred to as an infrastructure BSS. Figure 2-2 illustrates a typical infrastructure BSS.

ESS
Multiple infrastructure BSSs can be connected via their uplink interfaces. In the world of 802.11, the uplink interface connects the BSS to the distribution system (DS). The collection of BSSs interconnected via the DS is known as the ESS. Figure 2-3 shows a practical implementation of an ESS. The uplink to the DS does not have to be via a wired connection. The 802.11 specification leaves the potential for this link to be wireless. For the most part, DS uplinks are wired Ethernet.