\begin{figure}[h!]
\centering
-\includegraphics[scale=0.9]{Figures/ch1/wsn.jpg}
+\includegraphics[scale=1.2]{Figures/ch1/wsn.jpg}
\caption{ Wireless sensor network architecture.}
\label{wsn}
\end{figure}
\item \textbf{Topology Control:} The maintenance and repair of the network topology is a challenging task due to the large number of inaccessible sensor nodes that are prone to failure. Therefore, some schemes need to be used to deal with the dynamic changing of topology and the failure of some sensor nodes due to energy depletion or malfunction.
-\item \textbf{Heterogeneity:} One essential challenge is to provide a WSN protocol that deals with different sensor node capabilities such as communication, processing, sensing, and energy. The future of WSNs will be heterogeneous with a large number of sensor nodes. These WSNs may reflect different tasks and can be integrated into one big network. Therefore, it is necessary to take the heterogeneity into consideration during the design stage of WSNs protocols.
+\item \textbf{Heterogeneity:} One essential challenge is to provide a WSN protocol that deals with different sensor node capabilities such as communication, processing, sensing, and energy. The future networks will be heterogeneous with a large number of sensor nodes. These WSNs may reflect different tasks and can be integrated into one big network. Therefore, it is necessary to take the heterogeneity into consideration during the design stage of WSNs protocols.
\item \textbf{Wireless Networking:} The networking and wireless communication represent another important challenge in WSNs. The communication range of the signals can be attenuated or faded during the signal propagation across the communication media or during passing through obstacles. The increasing distance between the sensor nodes and the sink requires increased transmission power. However, the long distance can be divided into several small distances using multi-hop communication. The multi-hop communication generates another challenge that is how to find the more energy efficient route to transmit the information from the source to the destination. The sensor nodes should cooperate to find this route and to serve as relays.
\subsection{Data-Driven Schemes}
-\indent Data-driven approaches aim to decrease the amount of data sent to the sink whilst maintaining the accuracy of sensing within an acceptable level. Therefore, removing unwanted data during the transmission and restriction the sensing tasks during data acquisition can be participating in reducing the energy consumption in WSNs.
+\indent Data-driven approaches aim to decrease the amount of data sent to the sink whilst maintaining the accuracy of sensing within an acceptable level. Therefore, removing unwanted data during the transmission and restricting the sensing tasks during data acquisition can be participating in reducing the energy consumption in WSNs.
%Several data-driven schemes have been proposed in~\cite{ref86,ref87,ref88,ref89,ref90}.
Data driven schemes are classified into two main approaches~\cite{ref59,ref22}:
\item The time during which the application requirement is satisfied~\cite{ref172}.
\end{enumerate}
-\indent According to the above definitions for network lifetime, there is no universal definition to reflect the requirements of each application and the effects of the environment. In real WSN, the network lifetime reflects a set of a particular circumstances of the environment. Accordingly, the current definitions are applicable for the WSNs that meet a particular conditions. However, many more parameters, which are affecting on the network lifetime of WSN such as~\cite{ref92}: heterogeneity, node mobility, topology changes, application characteristics, quality of service, and completeness.
+\indent According to the above definitions for network lifetime, there is no universal definition to reflect the requirements of each application and the effects of the environment. In real WSN, the network lifetime reflects a set of a particular circumstances of the environment.
+%Accordingly, the current definitions are applicable for the WSNs that meet a particular conditions.
+Many parameters are affecting the network lifetime should be taken into account~\cite{ref92}: heterogeneity, node mobility, topology changes, application characteristics, quality of service, and completeness.
The network lifetime has been defined in this dissertation as the time spent by WSN until the coverage ratio becomes less than a predetermined threshold $\alpha$.
