Time Synchronization for Wireless Sensor Networks - Adaptive Signal Processing in Wireless Communications - page 394

D

B

P

E

C

A

FIgure 13.7 Time-diffusion synchronization protocol.

procedure. It is shown analytically in [32] that the TDP enables the clocks in the whole
network to converge to a unique value.
In the first phase, master nodes are elected in the sensor field. The election criteria
include the quality of clock and the energy resources of a particular node. Referring to
Figure 13.7, assume that node P is elected to be the master node (here we illustrate the
concept with one master node, while in more complicated networks, more than one mas-
ter node might be possible). Node P then sends a number of time-stamped messages to its
neighbors. Once the neighbors receive the messages, they self-determine if they would
become diffused leader nodes, based on the results of the last round peer evaluation pro-
cedure (the third phase). In Figure 13.7, nodes A, B, and C are the elected diffused leader
nodes. The elected diffused leader nodes respond to the master node, thus enabling the
master node to measure the average and the standard deviation of the round-trip delay
from its neighbors. At the same time, the diffused leader nodes start sending messages to
their own neighbors to measure the mean and standard deviation of round-trip delay to
their neighbors. The process is repeated until all the nodes have been covered.
In the second phase, the time information from the master node will be diffused
(with the help of diffused leader nodes) to all the nodes in the network. The diffusion
procedure takes place according to the following sequence of events. First, the master
node sends a time-stamped message containing the standard deviation of the round-trip
delay to its neighbors. Before transmission, the time stamp of the message is adjusted
with half of the measured average round-trip delay (from the first phase) to account for
the message delivery delay to its neighbors. Once the diffused leader nodes receive the
time-stamped message, they set their clock according to the received time stamp and
then broadcast their own time-stamped messages, containing their measured standard
derivations of the round-trip times to their neighbors. Again, before transmission of
the messages, the time stamps have to be adjusted with half of the measured average