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How to determine the data contents of the broadcast channel(s). This is of im-
portant due to several factors as such, network bandwidth and scalability.
The mobile unit should not waste its energy in constantly monitoring the air
channel(s) to search for information—techniques should be developed to allow
the mobile user to remain in low operational power mode while waiting for
the desired information on the broadcast channel(s). This implies a disciplined
order to organize information on the air channel(s) and to hint the availability
of the desired data element. If the data element is available, the mobile unit will
be instructed to the exact location of the data element on the broadcast.
An attempt should be made to minimize the response time either by shortening
the broadcast length and/or the number of passes over the air channels.
Broadcasting is an elegant solution to the limitations of wireless environment.
Within the constraint of a limited bandwidth, the main advantage of broadcasting is
the fact that it scales as the number of users increases, and thus eliminates the need to
divide or multiplex the bandwidth among the users accessing the air channel(s). Fur-
thermore, since the information on the air could be considered “storage on the air,”
broadcasting can be considered as a solution to compensate for the limited resources
(storage) at the mobile unit.
Broadcasting information is not a new concept: Whether through a guided or un-
guided medium, the general concept is based on the encoding and transmission of the
desired signal (analog or digital) on a certain frequency—the information is supplied
by one source and read by multiple receivers. In general, data can be broadcast either
on one or several channels. A broadcast is performed in a cyclic manner. The MH
can only pull the information from the broadcast, whereas the database server is the
only entity that can push information to the broadcast. Since dependencies can exist
among the data items within the broadcast, and since data items might be replicated
within a broadcast, it is assumed that updates of data are only reflected at the fol-
lowing cycle. This constraint enforces the integrity among the data elements within
a broadcast.
3 . 1 B r o a d c a s t C o n t e n t s
The literature has addressed several methods to determine and generate data con-
tents on the broadcast channel(s). Some methods are classified as push-based tech-
niques, where the users are passive listeners and the server assumes a priori client
access patterns that does not change during the course of broadcast—i.e., strict sta-
tic client access patterns. Based on this assumption, the server repeatedly broadcast
a set of selected data items. This allows scalability and simplicity at the expense
of limitation on the application domain. Alternatively, the dynamic and adaptive
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