-In~\cite{DESK}, the author design a novel distributed heuristic, called Distributed Energy-efficient Scheduling for k-coverage (DESK), which ensures that the energy consumption among the sensors is balanced and the lifetime maximized while the coverage requirement is maintained. This heuristic works in rounds, requires only one-hop neighbor information, and each sensor decides its status (active or sleep) based on the perimeter coverage model from~\cite{ref133}. Figure~\ref{desk} shows the DESK network time line.
+In~\cite{DESK}, the author design a novel distributed heuristic, called Distributed Energy-efficient Scheduling for k-coverage (DESK), which ensures that the energy consumption among the sensors is balanced and the lifetime maximized while the coverage requirement is satisfied. This heuristic works in rounds, requires only one-hop neighbor information, and each sensor decides its status (active or sleep) based on the perimeter coverage model from~\cite{ref133}.
+
+DESK is based on the result from \cite{ref133}, where two rules named k-Unit-disk Coverage (k-UC) and k-Non-unit-disk Coverage (k-NC) for
+uniform and non-uniform sensing range sensor networks, respectively, are proposed. For the disk sensing range, the whole area is k-covered if and only if the perimeter of sensing regions of all sensors are k-covered. The k-NC is the generalization of k-UC. The coverage level of perimeter of a sensor $s_i$ is determined by calculating the angle corresponding to the arch that each of its neighbors covers its perimeter. Figure~\ref{figp}a illuminates such arches whilst figure~\ref{p2}b shows the angles corresponding with those arches, which were posted into the range [0,2$ \pi $]. According to figure ~\ref{figp}b, the coverage level of sensor $s_i$ can be calculated via traversing the range from 0 to 2$ \pi $.
+
+
+
+\begin{figure}[h!]
+ \centering
+ \begin{tabular}{@{}cr@{}}
+ \includegraphics[scale=0.475]{Figures/ch2/P2.jpg} & \raisebox{2.75cm}{(a)} \\
+ \includegraphics[scale=0.475]{Figures/ch2/P1.jpg} & \raisebox{2.75cm}{(b)}
+ \end{tabular}
+ \caption{Determining the perimeter-coverage of $s_i$’s perimeter.}
+ \label{figp}
+\end{figure}
+
+
+
+
+Figure~\ref{desk} shows the DESK network time line.
