## 5 Protocol Comparison
* _Number of sinks_: The network lifetime can be improved by increasing the number of sinks up to a specific point. When the number of sinks exceeds that point, the network lifetime is constant. The reason behind this phenomenon is that each sink becomes at most 1-hop away from a sensor node. All multi-sink approaches in Table 1 support more than one sink through the \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Protocol**} & \multicolumn{10}{c}{**Metrics of Comparison**} \\ \cline{2-13}  & \multicolumn{3}{c}{**Multi-**} & \multicolumn{1}{c}{**Mobile**} & \multicolumn{1}{c}{**Multi-**} & \multirow{2}{*}{**Power**} & \multicolumn{1}{c}{**Sensor**} & \multicolumn{1}{c}{**Sink**} & \multirow{2}{*}{**Movement**} & \multirow{2}{*}{**Location**} & \multirow{2}{*}{**Number**} & \multirow{2}{*}{**Network**} & \multirow{2}{*}{**Data**} & \multirow{2}{*}{**Application**} & \multirow{2}{*}{**Sink**} \\  & **Sink** & **Sink** & **Path** & & & & & **Pattern** & & & **of Sinks** & **Structure** & **Aggregation** & **Type** & **Speed** \\ \hline
**MSDD [5]** & Yes & No & Yes & No & No & N/A & No & K (\(k\geq 2\)) & Flat & Yes & Query- & N/A \\  & Yes & No & No & No & N/A & No & K (\(k\geq 2\)) & Flat & No & Event \& N/A \\
**GLOBAL** & Yes & No & No & No & N/A & No & K (\(k\geq 2\)) & Flat & No & Time-Driven & \\
**[6]** & & & & & & & & & & & \\
**MSLBR [7]** & Yes & No & Yes & No & N/A & No & K (\(k\geq 2\)) & Flat & No & Time-Driven & N/A \\
**[8]** & Yes & Yes & No & No & Random & Yes _Only_ & K (\(k\geq 1\)) & Flat & No & Time-Driven & Const \\  & & & & & & & _Sinks_ & & & & \\
**[9]** & Poss. & Yes & No & No & No & Fixed & Yes & K (\(k\geq 1\)) & Flat & No & Time-Driven & Const \\
**[10]** & Yes & Yes & No & No & Controlled \& Yes _Only_ & K (\(k\geq 1\)) & Flat & No & Time-Driven & N/S \\  & & & & & & & Random & _Sinks_ & & & \\
**MSRP [11]** & No & Yes & No & Poss. & No & Controlled & Yes _Only_ & 1 & Hierarch- & Yes & N/S & Adaptive \\  & & & & & & & _Sink_ & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of multi-sink and mobile sink mechanisms. network while in some mobile sink algorithms [8; 9; 10], one or more sinks can be deployed in sensor field. * _Network structure_: Routing algorithms in WSNs are usually classified into three group as follows: Flat (data-centric), hierarchical and geographic (location-based). Most of the protocols in Table 1 are flat-based and only one protocol [11] use hierarchical structure accompany with mobile sink mechanism to prolong network lifetime. * _Data aggregation_: This technique can enhance the network lifetime by reducing the number of data packets transmitted in the network. Data aggregation mostly is employed in hierarchical protocols [11] where the cluster heads proceed to gather data from cluster members before they act to send them to the sink node. * _Application Type_: This factor shows that which kind of mechanisms will be employed to send data to the sink. In time-driven method, the data are sent to the sink continuously by all or special groups of sensor nodes that caused fast energy depletion through the network. In event-driven strategy [6], on the other hand, only the data about an interested event will be forwarded to the sink while in the query-based method [5], the data should be transmitted according to the sink's request. Most of the algorithms [8; 9; 10] that support sink mobility are used for time-driven applications. * _Sink speed_: In mobile WSNs, the sink speed is an important factor. A sink can move from one place to another by using a constant speed [8]. Some approaches [9] use a move/stop mechanism in which, the sink node moves to a new place and stops in that position for a specific period of time in order to collect data from k-hop neighbors and after that moves to another place, and so on. Sometimes the sink speed is adaptive [11] based on the number of congested areas that should be visited for data gathering.