TRENDS IN DATABASE TECHNOLOGY - Database Design and Development: An Essential Guide for IT Professionals - page 668

are many more applications of mobile computing including emergency response ser-
vices, sales call assistance, and travel information services.
The enormous progress in portable computing devices and tremendous
improvements in wireless communications have created a new paradigm of
remote access to databases. However, a wide array of issues must be addressed
before the new type of remote database access can be fully successful. We will
explore the nature of mobile computing, its special needs, and the pertinent data-
base requirements.

Nature of Mobile Computing Mobile computing is not the same as distributed
computing. Mobile users keep moving, and along with them moves the computing
equipment, whereas in distributed computing, although the computing devices are
dispersed, they stay in fixed locations.
The following points highlight the basic nature and considerations of mobile
computing.

• Allows users to communicate with one another and perform their functions
while on the move.
• Machines accessing the database do not have fixed locations or constant
network addresses.
• Machines tend not to stay connected with the database server; they are off-line
for long periods. Maintaining transaction consistency becomes difficult.
• Because the battery power of remote units is usually low, receiving informa-
tion through separate, individual queries is not efficient.
• Most information received by remote units is through broadcasts from the data-
base, on fixed or changeable schedules. Broadcasts are an efficient method
because they can reach a large number of remote units.
• Query results sent to remote units are often dependent on the location of the
user, not necessarily the current location. For example, a travel information
system providing hotel and other such services must be based not on the current
location of the user but on future locations.
• Downloaded data are maintained in local files to be used while working off-
line. Local files need to be kept synchronized with the source data.

Mobile Computing Model and Architecture Because of its nature, a mobile
computing model must be based on a distributed architecture. Figure 20-26 presents
a typical architecture.
Examine the details shown in the figure and note the descriptions mentioned
below.

Fixed host A computing device at a fixed location.

Mobile unit Also called a mobile host, provides remote computing services.

Support station Connects a set of fixed hosts and support stations. Support sta-
tions are also known as base stations. Base stations have wireless connections with
the mobile units they support.