Information Technology Reference
Figure 8: Wi-Fi WLAN - Infrastructure mode, multiple cells (ESS).
distribution system, is known as extended service set (ESS) (Figure 8). An
ESS is seen as a single 802 network by higher levels of OSI model.
There is a different operational mode for Wi-Fi-based WLANs in which
the access point is not needed. This mode is called ad-hoc, which allows
direct connection to computers. In ad-hoc mode, wireless client machines
connect to one another in order to form a peer-to-peer network, i.e. a network
in which each machine acts as both a client and an access point at the same
time. The setup formed by the stations is called the independent basic service
set (IBSS) (Figure 9).
To access an existing BSS, each station needs to acquire synchronization
information from the corresponding AP. This information can be acquired in
any of the following ways:
• Passive scanning . In this case, the station waits for a Beacon Frame from
the AP. A beacon frame is periodically transmitted by the AP, and it
contains information about the timing of data transmission.
• Active scanning . In this case, the station looks for an AP by transmitting a
Probe Request Frame , and then it waits for a Probe Response Frame
coming from an AP.
Both methods are suitable and the choice between one or the other is
carried out according to the energy or performance constraints.
The process that allows a station to move from a cell (or BSS) to another
with no loss of connection is called Roaming . This function is similar to that