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methods (pull-based methods) assume the clients' access patterns are either un-
known in advance or changes relatively frequently during the course of broadcast.
Finally, hybrid schemes assume a backchannel for users to explicitly request data
items that are not included in the standard broadcast cycle. As a result, the push
mode and on-demand data are interleaved on a broadcast cycle. As noted in Sec-
tion 1.1 , on-demand-based methods (pull-based methods) are beyond the scope of
this work. Within the scope of push-based methods and hybrid methods, parame-
ters such as the data type and data size can be used to further distinguish different
solutions.
3.1.1 Push-Based Schemes
3.1.1.1 Flat Broadcast. Flat broadcasting of data is the most basic way
to distribute public data. All data within a database is statically and sequentially
broadcast to the users of the network—server broadcasts the union of the data of
interest to a user population. The users retrieve data from the air channels when it is
available. There are no requests made by the users to the server database, and hence,
data is made available to the users through the push method. Consequently, the wait
time for data is relatively long and proportional to the half of the broadcast length on
the average—data is not scheduled on the broadcast. However, flat broadcast does
ensure that all users will get the data they require eventually as all data has equal
priority of being broadcast [1] .
3.1.1.2 Broadcast Disks. Broadcast disks are similar to flat broadcast in
that they broadcast all of the information in the database [2,12,64] . Broadcast disks
improve upon flat broadcasting by attempting to reduce the waiting time for retriev-
ing popular data. Instead of using one spinning disk to broadcast all of the data,
broadcast disks distribute data among multiple disks with varying sizes and spin-
ning speeds. As a result, pages available on faster spinning disks get mapped more
frequently than those available on slower disks. Figure 2 illustrates this concept. As-
sume the list of ten pages to be broadcast ( Fig. 2 (a)). Further assume that these pages
are partitioned and distributed among three disks where disk 1 is twice faster than
disk 2 , and disk 2 is twice faster than disk 3 ( Fig. 2 (b)). Figure 2 (c) shows the allo-
cation of the aforementioned database pages on a broadcast channel. Note that the
broadcast cycle now is composed of smaller chunks (minor cycle).
The disks create a “memory hierarchy” for the data items. Data items move up
and down in the memory hierarchy based on their frequency of access. The effect
of varying the disk spinning speeds and number of disks used on the response time
at the MHs was investigated and a threshold, above which increasing the number of
faster disks degraded the overall response time, was found.
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