EPOCH DURATION defines the collected data reporting interval, which is
the amount of time between two consecutive samplings. This time interval is called
an epoch. Sensors executing the query transmit the data that matches the SQL query
condition to the base station at each time interval defined by the EPOCH DURATION.
In the collection phase, sensors report the sensed data back to the base station
along the routing tree created by the underlying routing protocol. Instead of aggregating
packets at the base station, TAG aggregates packets in-network whenever possible
to reduce the number of transmissions. In order to reduce the number of transmissions
as much as possible, nodes should wait until they have received all packets
from their children before they transmit. As nodes send their sensed data once in each
epoch, TAG has to separate the transmission time slot for nodes at different levels of
the tree, i.e., for nodes that have a different number of hops to the base station, such
that the transmission time slots are scheduled from the tree leaves to the base station,
as shown in Figure 3.
If we assume that the tree depth is d, we can divide an epoch time into (EPOCH
DURATION)/d intervals such that nodes at ith level transmit during ith interval.
While nodes are not receiving or transmitting, they can turn off their radio and sensing
devices to reduce the idle power consumption. However, if the tree depth is high,
or the network sampling rate is high, the length of an interval may not be long enough
338 Kai-Wei Fan, Sha Liu, and Prasun Sinha
Fig.
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