In addition, it does not
take into account the energy consumption required for time synchronization.
As a result, the NTP does not satisfy the energy aware, server-less, and lightweight
design challenges of the sensor networks. Although the NTP can be
robust, it may su?®er large propagation delay when sending timing messages
to the time servers. In addition, the nodes are synchronized in a hierarchical
manner, and some time servers in the middle of the hierarchy may fail causing
unsynchronized nodes in the network. Once these nodes fail, it is hard to
reconfigure the network since the hierarchy is manually configured.
227
Weilian Su
Synchronization pulse
2
g
4
g
3
g
1
A
B
Acknowledgement
g
Fig. 3. Two-way message handshake.
Another time synchronization technique that adopts some concepts from
NTP is TPSN. The TPSN requires the root node to synchronize all or part of
the nodes in the sensor field. The root node synchronizes the nodes in a hierarchical
way. Before synchronization, the root node constructs the hierarchy by
broadcasting a level discovery packet. The first level of the hierarchy is level 0,
which is where the root node resides. The nodes receiving the level discovery
packet from the root node are the nodes belonging to level 1. The nodes in
level 1 broadcast their level discovery packet after receiving from level 0, and
neighbor nodes receiving the level discovery packet for the first time are the
level 2 nodes.
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