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that its range is limited to 20-40 m and it is difficult to integrate with pre-
existent wired networks.
3.1.4 Wi-Fi
Wireless local area networks (LAN) are one of the most relevant access network
technologies available today. Their pervasive diffusion in homes, workplaces,
universities and schools, caf├ęs, airports and public areas makes them the most
readily available way to connect ubiquitous systems among them.
Among the many wireless LAN technologies developed in 1990s, the
one standard that has emerged is the IEEE 802.11, also know as Wi-Fi (for
Wireless Fidelity). A logo has been introduced by the Wi-Fi Alliance to
certify the full compatibility of devices conforming to one or more of the
Wi-Fi standards, in a way ensuring interoperability across brands and vendors.
Three main standards are available within the 802.11 standard, termed with
suffix letters a, b and g. They differs in frequency range and data rate, with
802.11g being the best trade-off between 802.11a and 802.11b, working on a
2.4-2.485 GHz frequency range with a data rate of up to 54 Mbps. Differ-
ently from the Bluetooth standard, the Wi-Fi uses a direct sequence spread
spectrum method, which offer higher resiliency to errors.
The architecture is based on a basic service set composed of a number of
access points (or base-stations) wire connected to the land network. Each
mobile device is then provided with a wireless transceiver that connects to
the access point in range through one of the Wi-Fi protocols. A typical cover-
age is in the range of 150-300 m, depending on the presence of obstacles
such as walls, trees or barriers in general.
3.2 Increasing computing power
For the past three decades, Gordon Moore's empirical observation on chip
density doubling every 18 months has proven true, with a remarkable
increase in computational power along with decreasing silicon footprint
and power consumption. This has yielded to the development of chipsets
dedicated to mobile applications that are computationally powerful and
power savvy, enabling many new applications simply unthinkable a few
years ago. Increasing computational power and battery lifetime are, of course,
key enablers for the development of disappearing systems.
3.3 Increasing memory capacity
Another key enabler is the increasing memory capacity that new gigascale
integration techniques (sub-nanometer transistor channel size) are providing.
This is more and more enabling the idea of endless memory capacity even for
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