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Figure 4. Effect of non-uniform broadcast
Figure 5. Effect of move Speed
to get the data objects that are on the slower disks.
CPIX and ACP adapt to the change of broadcast
better because they both consider the data avail-
ability on the broadcast channel. DGCoca makes
cache decision based on access distribution thus
it is not adaptive to the broadcast program.
Effect of Transmission Range
The aim of this set of experiments is to study the
effect of mobile peers' transmission range on the
schemes' performance. As shown in Figure 6,
all schemes' performance improves with longer
transmission range. This is due to the fact that
a mobile peer may contact with more neighbors
with longer transmission range.
One interesting observation here is that the
performance gap between the schemes decreases
with the increase of transmission range. When a
mobile peer can communicate with more peers,
the data availability in its neighborhood improves
significantly. In this case, sharing of cache contents
leads to performance improvement, and gain from
clever cooperative cache management will be less
obvious. Increasing the density of mobile peers or
introducing multi-hop communication will have
a similar effect.
Effect of Moving Speed
The effect of the mobile peers' move speed on
the schemes' performance is studied by varying
the mobile peers' speed from 1 to 20m/s. Figure
5 shows the results.
We observe that when the mobile peers move
with a faster speed: (1) the response time of
DGCoca increases dramatically, (2) the response
times of CPIX and ACP increase very slowly. The
reason for (1) is that the mobile peer groups that
DGCoca tries to form are not stable when mobile
peers move fast; a mobile peer may not even be
able to find stable neighbors. Recall that in DG-
Coca, information about current neighbors' cache
contents is used to make local caching decisions.
When the mobile peers move fast, the reliance
on current neighbors have negative effect. The
reason for (2) is that in CPIX and ACP a mobile
peer does not rely on a specific set of neighbors,
but rather consider the overall availability from
the external sources (include neighbors and the
broadcast channel).
Effect of Cache Size
The effect of cache size is studied by varying a
mobile peer's cache size from 1 data object to
half of its access range. Figure 7 shows that with
the increase of cache size, the schemes all have
better response time. This is as expected because
increasing the cache size effectively improves the
data availability. With larger cache space more
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