The sensor nodes that are
within the broadcast regions of both transmitter nodes A are the translation
nodes. When an event occurs, a message describing the event with a timestamp
is translated by the translation nodes when the message is routed back
to the sink. Although this time synchronization service is tunable and lightweight,
there may not be translation nodes on the route path over which the
message is relayed. As a result, services may not be available on some routes.
In addition, this protocol is not suitable for medium access scheme such as
TDMA since the clocks of all the nodes in the network are not adjusted to a
common time.
Another emerging timing technique is the TDP. The TDP is used to maintain
the time throughout the network within a certain tolerance. The tolerance
229
C
Transmitters Receivers
Translation Nodes
A
B
Weilian Su
Hops
D F G C 1 2 E 3
1 2 3
Diffused Leader Nodes
Master Nodes
N
M
Fig. 5. TDP concept.
level can be adjusted based on the purpose of the sensor networks. The TDP
automatically self-configures by electing master nodes to synchronize the sensor
network. In addition, the election process is sensitive to energy requirement
as well as the quality of the clocks. The sensor network may be deployed in
unattended areas, and the TDP still synchronizes the unattended network to
a common time. It is considered as the timing technique of type (3).
The TDP concept is illustrated in Figure 5.
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