# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 4 Topology
### Mobile Sink Based  
In this subcategory, the sink or the base station can move with the assistance of a robotic platform within the network, through the deployed nodes, in specific paths or patterns to collect data from them. In what follows in this section, two typical examples of this kind of protocols are described.  
_Mobile Sink based adaptive Immune Energy-Efficient clustering Protocol_ (MSIEEP) [134] utilizes a mobile the base station to solve the energy hole problem, to improve security, and to reduce the packet drop rate. MSIEEP uses the Adaptive Immune Algorithm (AIA) to determine the position of the mobile the base station based on three predefined paths and to calculate the optimal number of cluster heads. The first moving pattern is a rectangular amid four regions, while the second is a rectangular among eight regions and the third pattern is a straight line crossing four regions. In addition, MSIEEP allocates uniformly the cluster heads in the network and each one informs its cluster nodes of their TDMA schedule for intra-cluster communications. During inter-cluster communication, they use a unique CDMA schedule to avoid neighbor cluster interference. MSIEEP not only extends network lifetime, but also achieves low drop packet ratio and increased stability period. However, predefined routing paths limit its applications.  
_Multi-hop HEER-Sink Mobility_ (MHEER-SM) [135] is an upgraded version of HEER algorithm, which combines three operations, a fixed number of clusters, multi-hop routing for distant cluster heads, and a mechanically driven sink. MHEER-SM separates the network area in sub-areas with equal numbers of nodes that operate as clusters, performs the HEER's cluster head election process, and efficiently monitors all the areas. Furthermore, to reduce energy consumption of the distant cluster heads, MHEER-SM utilizes multi-hop routing, by forwarding data packets to nearby cluster heads until the base station gets the data. Although packets may get lost, MHEER-SM uses a Uniform Random Distribution Model to calculate dropped packets. Finally, the base station can move in a fixed start-stop pattern to collect data from static nodes directly. MHEER-SM reduces communication cost and extends network lifetime. On the other hand, nodes are deployed in a fixed way.  
A synopsis of the characteristics of the aforementioned topology routing protocols belonging to mobile sink subcategory is presented in Table 12.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline \multirow{4}{*}{MIP [132]} & It has better energy & & & & The minimum total & \\  & consumption with & & & & power consumption & \\  & small number of & & & & route is used. & \\  & sources. & & & & & \\  & It provides great & & & & & The minimum total & \\  & energy efficiency and & & & & power consumption & \\  & high delivery ratio. & & & & route is used. & \\ \hline \hline \end{tabular}
\end{table}
Table 11: Synopsis of Topology Protocols belonging to Mobile Agent subcategory.