Basic Service Set (BSS)
The basic service set (BSS) is the basic building block of an IEEE 802.11 LAN. Figure shows a BSS with three stations that are members of the BSS, in addition to the access point (AP). The BSS covers a single RF area, or cell, as indicated by the circle. As a station moves further from the AP, its data rate will decrease. When it moves out of its BSS, it can no longer communicate with other members of the BSS. A BSS uses infrastructure mode, a mode that needs an AP. All stations communicate by way of the AP, and do not communicate directly. A BSS has one service set ID (SSID).

Independent BSS (IBSS)
The independent basic service set (IBSS) is the most basic type of IEEE 802.11 LAN. A minimum IEEE 802.11 LAN consists of only two stations. In this mode of operation, IEEE 802.11 stations communicate directly. Because this type of IEEE 802.11 LAN is often formed without pre-planning for only as long as the WLAN is needed, it is often referred to as an ad hoc network.

Because an IBSS consists of STAs that are directly connected, it is also called a peer-to-peer network. There is, by definition, only one BSS and there is no Distribution System (DS). An IBSS with four stations is shown in Figure . An IBSS may have an arbitrary number of members. In order to communicate outside of the IBSS, one of the STAs must be acting as a gateway or router.

Distribution System (DS)
Physical limitations determine the station-to-station distances that may be supported. For some networks this distance is sufficient. For other networks, increased coverage is required. Instead of existing independently, a BSS may also form a component of an extended service set (ESS). An ESS is built from multiple BSSs that are connected through APs. The APs are connected to a common DS as shown in Figure . The DS can be either wired or wireless, LAN or WAN. The IEEE 802.11 WLAN architecture is specified independently of the physical characteristics of the DS.

The DS enables mobile device support by providing the services necessary to handle address to destination mapping and seamless integration of multiple BSSs. Data moves between a BSS and the DS through an AP. Note that all APs are also STAs, which makes them addressable entities.

Extended service set (ESS)
An extended service set (ESS) is defined as two or more BSSs connected by a common DS as illustrated in Figure . This allows for the creation of a wireless network of arbitrary size and complexity. As with a BSS, all packets in an ESS must go through one of the APs.

A key concept is that the ESS network appears the same to the LLC layer as an IBSS or a single BSS network. Stations within an ESS may communicate directly and mobile stations can move from one BSS to another within the same ESS transparently to LLC.

Roaming
Roaming is the process or ability of a wireless client to move from one cell or BSS to another without losing connectivity to the network. Access points hand the client off from one to another and are invisible to the client. The IEEE 802.11 standard does not define how roaming should be performed, but does define the basic building blocks, which include active and passive scanning and a re-association process. Re-association with an AP must occur when a STA roams from one AP to another.
  

Interactive Activity Interactive Activity: WLAN Logical Architecture: Acronym Recognition In this activity, demonstrate familiarity with WLAN Acronyms by dragging them to the corresponding targets.