Depending on the actual technologies used, these di?®erences may be small or
large.
The case where the communication ranges are only slightly di?®erent is
shown in the left-hand side of Figure 5. Here, RD and RP denote the range
of the data and paging radio respectively. When RD < RP , a sleeping device
can always be reached with a page. The problem occurs in the case that
RD > RP , which is shown in the figure. In this case, node A should not be
allowed to turn o?® its main radio completely, since it cannot be reached with
a page. A possible solution is to send an explicit ???go to sleep??? message to A
via the paging device, such that the node only turns o?® its radio if it is indeed
within paging range [18]. If node mobility is present, these pages have to be
repeated periodically to also account for the situation where a node, such as
B, is initially within paging range but then moves out of it while sleeping [18].
On the other hand, if the communication ranges of the data radio and the
paging radio are vastly di?®erent, using this approach by itself would severely
limit the possible benefits of low-power paging. One solution would be to
duplicate the paging infrastructure, as shown on the right hand side of Figure
5 [17]. This again has to be combined with the periodic paging updates
discussed earlier. To be applicable to less infrastructure-oriented networks, a
type of paging backbone would need to be established, possibly with nodes
specifically dedicated for this functionality.
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