Section 5.1: Wireless LAN Technology Overview

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.

5.1.1: 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.

An ad hoc or independent basic service set (IBSS) network is created when individual client devices form a self-contained network without the use of an access point. These networks do not involve any pre-planning or site survey, so they are usually small and only last long enough for the communication of whatever information needs to be shared. Unlike the case of an ESS, the clients are directly connected to each other, which creates only a single BSS that has no interface to a wired LAN (i.e., no distribution system that is essential to tying BSSs to create an ESS). There is no standards-based limit as to the number of devices that can be in an IBSS. But because every device is a client, often, certain members of the IBSS cannot talk to each other because of the hidden node issue. In spite of this, there is no mechanism for a relay function in an IBSS.

Because no access point is in an IBSS, timing is controlled in a distributed manner. The client that starts the IBSS sets the beacon interval to create a set of target beacon transmission times (TBTT).

5.1.2: 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.

5.1.3: 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. 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.