Broadcasting a Means to Disseminate Public Data in a Wireless Environment—Issues and Solutions - Advances in Computers - page 63

The retrieval protocols proposed in Section 3.6 attempt primarily to reduce the
conflicts in each broadcast pass; however, when the number of potential conflicts
increases considerably, some conflicts become unavoidable causing an increase in
the number of passes and hence an increase in the response time. The increase in
the number of passes makes the retrieval scheme inadequate when the percentage of
requested data elements is large. The simulation results also showed that when the
percentage of requested data elements approaches 100%, the response time reduces.
This shows the validity of the proposed scheduling protocols since, when a rela-
tively large number of data elements are requested, they generate the same retrieval
sequence as the Row Scan method would.
7.2.3.2 Channel Switching Frequency. In general, with respect to the
bottom up retrieval scheme as the number of requested data elements increases, up
to a threshold value, one should observe more frequent channel switches. This is
due to the increase in the number of conflicts. As the number of conflicts increases,
the bottom up retrieval scheme attempts to reduce their effect on the response time
through the use of channel switches. The number of channel switches reaches a
maximum and begins to decrease when retrieving more than 50% of the broad-
cast. This decrease occurs because of the heuristic rule employed that attempts
“to maximize the number of data elements retrieved during each broadcast cycle.”
As the density of requested objects increases, the bottom up retrieval scheme does
not have to switch channels as often to retrieve the maximum number of requests
( Fig. 30 ).
The Top down retrieval scheme compromises the response time to minimize the
number of channel switches. Figure 31 depicts the channel switching frequency
F IG . 30. Channel switching frequency ( Bottom Up Retrieval Scheme ).