Image Processing Reference
In-Depth Information
TABLE .
Three Types of Timing Techniques
Type
Description
. Relies on fixed time servers
The nodes are synchronized to time servers that
to synchronize the network
are readily available. These time servers are
expected to be robust and highly precise.
. Translates time throughout
The time is translated hop-by-hop from the
the network
source to the sink. In essence, it is a time
translation service.
. Self-organizes to synchronize
The protocol does not depend on specialized time
the network
servers. It automatically organizes and determines
the master nodes as the temporary time servers.
between two nodes may be different in the forward and return paths. In addition, the jitters may
vary significantly due to frequent node failures, since the messages are relayed hop-by-hop between
thetwonodes.hesynchronizationprotocolsinthefollowingsectionfocusonsynchronizingnodes
hop-by-hop, so the propagation time and variation do not play too much effect on the error of the
synchronized clocks. Although the sensor nodes are densely deployed and they can take advantage of
the close distance, the medium and software access times may contribute the most in the nondeter-
ministicofthepathdelayduringaonehopsynchronization.hewaytoprovidetimesynchronization
for sensor networks may be different for different applications. The current timing techniques that
are available for different applications are described in the following section.
5.5 Time Synchronization Protocols for Sensor Networks
There are three types of timing techniques as shown in Table ., and each of these types has to
address the design challenges and factors affecting time synchronization as mentioned in Sections
. and ., respectively. In addition, the timing techniques have to address the mapping between
the sensor network time and the Internet time, e.g., universal coordinated time. In the following,
examples of these types of timing techniques are described, namely the Network Time Protocol
(NTP) [], Timing-sync Protocol for Sensor Networks (TPSN) [], H-sensor Broadcast Synchro-
nization (HBS) [], Time Synchronization for High Latency (TSHL) [], Reference-Broadcast
Synchronization (RBS) [], Adaptive Clock Synchronization [], Time-Diffusion Synchroniza-
tion Protocol (TDP) [], Rate-Based Diffusion Algorithm [], and Adaptive-Rate Synchronization
Protocol (ARSP) [].
In Internet, the NTP is used to discipline the frequency of each node's oscillator. he accuracy of
the NTP synchronization is in the order of milliseconds []. It may be useful to use NTP to disciple the
oscillators of the sensor nodes, but the connection to the time servers may not be possible because of
frequent sensor node failures. In addition, disciplining all the sensor nodes in the sensor ield maybe a
problem due to interference from the environment and large variation of delay between diferent parts
of the sensor field. The interference can temporarily disjoint the sensor field into multiple smaller
fields causing undisciplined clocks among these smaller fields he NTP protocol may be considered
as type () of the timing techniques. In addition, it has to be refined in order to address the design
challenges presented by the sensor networks.
Asofnow,theNTPisverycomputationalintensiveandrequiresaprecisetimeservertosynchro-
nize the nodes in the network. In addition, it does not take into account of 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 suffer 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.
