Low-PowerWireless Sensor Network Platforms - Signal Processing Systems - page 403

node when the current one fails is presented. A node starts acting as reference
node after a constant timeout when it has not receive any synchronization frames.
This results in possibly many new reference nodes of which only the one with the
minimum ID remains after the synchronization flooding resumes.
Delay Measurement Time Synchronization (DMTS) [ 38 ] uses one message to
synchronize a sender and all the receivers in its neighborhood. The multi-hop DMTS
algorithm uses a leader selection algorithm to select a time reference for the whole
network. The time reference is at tree level 0. The time is periodically flooded trough
the network by broadcasting it from level to level. Synchronization frames from only
lower level nodes are accepted. This is continued until leaf nodes are reached.
The Time-Diffusion synchronization Protocol (TDP) [ 49 ] consist of active and
inactive cycles. At the start of every active cycle a subset of nodes is selected
as masters who can relay synchronization data. The timing messages sent by the
masters create individual tree structures for every master. Furthermore, the protocol
includes a method for detecting outliers. TDP does no rely on external time servers
making it fully self-contained. Furthermore, the creation of new synchronization
trees in every cycle increase the fault tolerance.
Li et al. [ 28 ] propose a diffusion method where nodes achieve global syn-
chronization by spreading local synchronization information to the entire system.
The method does not rely on a single time reference which betters the robustness
of the protocol. However, any malfunctioning node affects the time accuracy of the
whole network. The authors present a solution for this by replacing a fraction of the
normal nodes by tamper-proof nodes.

8 Case Study on WSN Performance

In this section, the low power WSN performance is examined with TUTWSN
[ 27 ] . TUTWSN comprises hardware platforms built from COTS components and
communication protocols.
Two variations of the protocol are presented: low-energy and low-latency. The
design requirements for these variants are presented in Table 7 . The low-energy
TUTWSN is targeted at sensing applications requiring moderate throughput, long
network lifetime, and forwarding latencies that are in the order of seconds per
hop. The low-latency TUTWSN is targeted at localization and target tracking
applications requiring very low end-to-end delays and light throughput. It uses
a heterogeneous approach by allowing ultra low power mobile nodes, while the

Tabl e 7 Design requirements of TUTWSN low-energy and low-latency protocols
TUTWSN Network Measurement End-to-end Router Node

variant size interval latency power power Lifetime

Low-energy Thousands 30 s-15 min < 10 min Battery Battery 2 years

Low-latency Hundreds 0.5-10 s < 6 s Mains Battery 4 years