Time Synchronization for Wireless Sensor Networks - Adaptive Signal Processing in Wireless Communications - page 372

control procedures, such as sleep/wake-up modes and dynamic routing con-
trols, are commonly considered in this regard. Time synchronization is one of
the critical components contributing to energy consumption due to the highly
energy consuming radio transmissions for achieving clock synchronization.
Indeed, the energy consumption required for time synchronization of a node
is approximately 17% of the total energy spent by a node [12]. Pottie and Kaiser
showed in [13] that the radio frequency (RF) energy required to transmit 1 bit
over 100 m (i.e., 3 J) is equivalent to the energy required to execute 3 million
instructions. Therefore, developing efficient synchronization algorithms rep-
resents an ideal mechanism for trading computational energy for reduced (RF)
communication energy. In the sequel, energy efficiency is the main concern in
designing time synchronization protocols.
Latency • : Latency in message delivery is a fundamental factor when designing

communications networks. For networks based on multihop transmissions
like wireless sensor networks, this is even more critical because the uncertainty
in message delivery significantly increases as the number of hops increases.
Besides, the effects of channel variations, mobility, and the ad hoc nature of
wireless sensor networks make this problem more complex. Efficient localiza-
tion and time synchronization protocols are necessary for reducing the latency
error and jitter.
Security and reliability

• : Network security has gained huge attention in recent

years as the networks become more accessible and vulnerable due to the devel-
opment of sophisticated spying techniques and devices. Besides, unlike wired
networks, far more frequent message losses occur in wireless networks because
of the time-varying nature of wireless channels. Therefore, a mechanism to
cope with message losses and malicious attacks in time synchronization will
be necessary for wireless sensor networks.
Network topology changes • : The performance of a time synchronization protocol

is closely related to the network topology, i.e., it varies with the density and dis-
tribution of sensors in the network. Therefore, any shift in the location or scale of
sensors incurs a network topology change, which requires at its turn a new self-
configuration. Mobility of the sensors and battery timeouts are the main reasons
for this change. Hence, for dynamic sensor networks, time synchronization pro-
tocols should be able to adapt well to frequent network topology changes.
Scalability

• : Scalability is another important factor in the design of synchro-

nization protocols. The computational complexity of synchronization algo-
rithms becomes a critical problem as the number of sensors becomes very
large. Besides, many other crucial MAC operations, such as multihop routing
and network configuration, highly depend on the network scalability as well.

13.2.3 Delay Components in Timing Message Delivery
The main role of time synchronization in a distributed network is to ensure a common
timescale for all the network nodes, and to provide the right temporal coordination
among all the nodes engaged in a collaborative and distributed interaction with the