Information Technology Reference
Figure 9: Wi-Fi WLAN - ad-hoc mode (IBSS).
which is done in cellular telephony systems. The 802.11 standard does not
define how roaming should be done, but defines a basic operational mode.
The moving station detects which APs are available for connection carrying
out the active or passive scanning. Depending on the received signal strength,
it decides which AP is more convenient to join. Then the station uses a
mechanism of re-association defined by the standard, by which it can cancel the
association with the old AP and join the new one. The reassociation process
consists of an information exchange between the two AP involved in the roam-
ing through the distribution system, with no overload for the radio channel.
The possibility to roam from a cell to another along with a suitable ESS
setup allows providers to supply people with wireless pervasive services in
hotels, train stations, airports, government offices and shopping centres.
A further evolution of the Wi-Fi wireless technology is the IEEE 802.16
standard (Wi-MAX), which features are closer to a wide area network
(WAN). The Wi-MAX consortium has several members, such as Intel,
Siemens, Alcatel, Fujitsu, Sumitomo Electric, as well as telecommunication
companies such as British Telecom, France Telecom and Qwest . These
ones are particularly interested in Wi-MAX, since it enables Internet service
providers to allow for a broadband Internet access at a lower cost.
3.1 Technical details
IEEE 802.11 divides the available band into channels, analogously to how
radio and TV broadcast bands are carved up but with greater channel width