## 2 Classifications of Routing Protocols: Overview  
In this section, an overview of routing protocols types designed for ad hoc networks is briefly presented, including the classification type, a brief description and their pros and cons. The routing of wireless ad hoc networks is still challenging owing to the absence of a centralized authority and the unpredictability of network topology. Besides that, the characteristics of self-organization, mobility, medium type and node deployment present further difficulties in the design of an ad hoc network routing protocol [31; 32]. Therefore, routing protocols for ad hoc networks are categorized into various classes; the two most well-known are (a) topology-based and (b) geographic-based routing protocols.  
In fact, topology-based routing protocols, like the Ad hoc On-Demand Distance Vector (AODV), are also known as traditional routing protocols because they store link information in the routing table to forward packets from source to destination nodes. Onthe other hand, geographic-based routing algorithms, like GPSR, are designed to cope with some of the constraints of topology-based routing by using extra strategies, where the sender utilizes a location service to figure out the destination's position. Furthermore, the information is obtained through a beaconing procedure. There are also other routing protocol classifications such as hybrid-, hierarchical-, multicast-, Geo-cast-, and cluster-based routing protocols. Table 1 briefly summarizes the main routing categories [33; 34; 35; 36; 37].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Routing Protocol** & **Categories** & **Description** & **Advantages** & & **Drawbacks** \\ \hline \multirow{6}{*}{Topology-based} & \multirow{3}{*}{Reactive: AODV, DSR, ACOR, DYMO} & On-demand routing; routes are founded when required by establishing route requests across the network. & \multirow{3}{*}{\(\bullet\)} & \multirow{3}{*}{Adaptable to high dynamic topologies Support multicasting} & \(\bullet\) & High latency. \\  & & The routes kept updating in a table & \multirow{3}{*}{\(\bullet\)} & \multirow{3}{*}{Loop-free Established route up to date} & \(\bullet\) & Excessive flooding. \\  & & even though there is no demand for a & & & \(\bullet\) & Not reliable shortest path due to \\  & & route (table-driven) & & & \\ \hline \multirow{6}{*}{Geographic-based} & \multirow{3}{*}{GPSR, LAR, MORA, VADD, from GPS equipment.} & \multirow{3}{*}{The path establishment is based on the node’s location information from GPS equipment.} & \multirow{3}{*}{\(\bullet\)} & Efficient in rapid networks & \\  & & The path establishment & & \(\bullet\) & Low energy consumption & \\  & & is based on the node’s consumption & & & \\  & & from GPS equipment. & \multirow{3}{*}{\(\bullet\)} & Low overhead & \\  & & & Low latency & & \\  & & & Scalability & & \\ \hline \multirow{6}{*}{Hierarchical-based} & \multirow{3}{*}{HSR, HDVG} & \multirow{3}{*}{The nodes are organized into hierarchical groups.} & \multirow{3}{*}{\(\bullet\)} & \multirow{3}{*}{Low overhead} & \multirow{3}{*}{\(\bullet\)} & Complexity in maintenance of the cluster head. \\  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & A node establishes route to a single or multiple destinations simultaneously. & \multirow{3}{*}{\(\bullet\)} & \multirow{3}{*}{Low Processing Cost} & \(\bullet\) & Low reliability. \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & Send messages to a single or multiple & & & & \\ \cline{1-1} Geo-cast-based & \multirow{3}{*}{DGR, IVG} & \multirow{3}{*}{\(\bullet\)} & \multirow{3}{*}{Low overhead} & \multirow{3}{*}{\(\bullet\)} & Multicast routing and position information \\  & & location (combine & & & \\ \cline{1-1}  & & geographic and multicast routing). & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Routing categories in Mobile Ad hoc Networks.