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In-Depth Information
for the uplink communication (from clients to servers). To resolve the previ-
ous constraints, one approach consists of the use of broadcasting to dissemi-
nate information of general interest. The use of broadcast saves bandwidth
by reducing the need of point-to-point communications among the MUs
and the BSs and saves MU battery power because it reduces uplink commu-
nications, which are more expensive in terms of energy than downlink
communications.
10.5.2.2 Disconnections
Wireless communications are very susceptible to disconnections, an impor-
tant aspect to keep in mind in the design of an architecture to support mobile
computing. Disconnections can be classified into two types: forced disconnec-
tions, which are usually accidental and unavoidable (e.g., the disconnection
that occurs when the user enters an out-of-coverage area), and voluntary dis-
connections, in which the user decides to disconnect the unit with the goal of
saving energy. Voluntary disconnections can also occur when abrupt changes
in the signal are detected or when the power level in the batteries is low. In
those cases, the unit takes the necessary measures to change to disconnected
mode in a stable way and without the risk of losing data. The undesirable
effects of the disconnections can be mitigated using caching techniques
[19, 20]. With caching techniques, the user can continue working even in a
disconnection state, which will help avoid the unnecessary use of the wireless
communication.
10.5.3
Portability of Mobile Elements
Although mobile computers exist that present different capabilities, in gen-
eral their limitations are related mostly to their size and battery life.
10.5.3.1 Limitations on Size and Capabilities of Mobile Computers
The design of portable computers implies that they must be small and light-
weight, and consume little energy. That means the computers generally have
more limited functionalities than FHs, mainly in aspects such as computa-
tion power, storage capacity, screen size and graphic resolution, and auton-
omy. Among the solutions that try to overcome such limitations are those
that adapt the images for their visualization by reducing their size, definition,
or colors, or by using filters. Filters are programs used to process every mes-
sage coming or going to the MUs. They can abolish the message, delay it,
reorganize it into segments, or transform it [21].
