Image Processing Reference
In-Depth Information
Master nodes
Hops
M
C
D
E
F
G
1
3
2
3
2
1
N
Diffused leader nodes
FIGURE .
TDP concept.
Another emerging timing technique is the TDP. The TDP is used to maintain the time
throughout the network within a certain tolerance. he tolerance level can be adjusted based on the
purpose of the sensor networks. he 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. he sensor network may be deployed in unattended areas, and the
TDP still synchronizes the unattended network to a common time. It is considered as a type () of
the timing techniques.
..The TDP concept is illustrated in Figure .. ..The elected master nodes are nodes C and G. First, the
master nodes send a message to their neighbors to measure the round-trip times. Once the neigh-
bors receive the message, they self-determine if they should become diffuse leader nodes. he ones
elected to become diffuse leader nodes reply to the master nodes and start sending a message to
measure the round-trip to their neighbors. As shown in Figure ., nodes M, N, and D are the dif-
fused leader nodes of node C. Once the replies are received by the master nodes, the round-trip
time and the standard deviation of the round-trip time are calculated. The one-way delay from the
master nodes to the neighbor nodes is half of the measured round-trip time. Afterwards, the master
nodes send a time-stamped message containing the standard deviation to the neighbor nodes. The
time in the time-stamped message is adjusted with the one-way delay. Once the diffuse leader nodes
receive the time-stamped message, they broadcast the time-stamped message after adjusting the time,
which is in the message, with their measured one-way delay and inserting their standard deviation of
the round-trip time. his diffusion process continues for
n
times, where
n
is the number of hops from
the master nodes. From Figure ., the time is diffused three hops from the master nodes C and G.
he nodes D, E, and F are the diffused leader nodes that diffuse the time-stamped messages originated
from the master nodes.
For the nodes that have received more than one time-stamped messages originated from different
master nodes, they use the standard deviations carried in the time-stamped messages as weighted
ratio of their time contribution to the new time. In essence, the nodes weight the times diffused by
the master nodes to obtain a new time for them. his process is to provide a smooth time variation
betweenthenodesinthenetwork.hesmoothtransitionisimportantforsomeapplicationssuchas
target tracking and speed estimating.
The master nodes are autonomously elected, so the network is robust to failures. Although some
of the nodes may die, there are still other nodes in the network that can self-determine to become
master nodes. his feature also enables the network to become server-less if necessary and to reach
an equilibrium time. In addition, the master and diffusion leader nodes are self-determined based
on their own energy level. Also, the TDP is lightweight, but it may not be as tunable as the RBS.
