# Enhancing OLSR Protocol by an Advanced Greedy Forwarding Mechanism for VANET in Smart Cities
## 2 Routing Protocols in VANET
Hybrid protocols combine and use the advantage of the reactive and proactive protocol. As with the ZRP protocol (Zone Routing Protocol) [14], the principle is to divide the network into two areas, to minimize the number of re-broadcast nodes in each area. Information is exchanged proactively inside the area and reactively outside. Advantages:
* No hierarchies eliminates definitively the congestion. Disadvantages:
* Delay increases proportionally to the network size. * Difficult to define zones. As for the geographic protocol, the principal mechanism is the Greedy Perimeter Stateless Routing (GPSR) [15], which is a routing protocol based on the position node, so we assume that each node already knows its neighbors in a table saving all positions. Each node sends a \"beacon\" message containing its ID with the actual position. The node is deleted from the table if we do not receive any beacon message after a while. The GPSR protocol uses greedy forwarding or the perimeter forwarding mechanism, according to the traffic density, to transmit the packet to the destination. Advantages:
* The source node needs only to save the position of one neighbor node to send a packet to the destination. * The decision to send a packet is made dynamically. Disadvantages:
* Fake routes can exist in the neighborhood table if mobility is high due to latency. * The packet header is never updated by the intermediate nodes in the transfer process, hence the risk of the packet being lost due to mobility. In Table 1, we summarize the comparison between the protocols described previously: \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocols** & **Features** & **Routing Type** & **Metrics** & **Transmission Type** & **PROS** & **CONS** \\ \hline \multirow{3}{*}{AODV} & Route discovery on demand & \multirow{3}{*}{Reactive} & Fast and short path (Number of hops) & \multirow{3}{*}{Multihop} & Reduce the need for memory, & \multirow{3}{*}{Excessive consumption of bandwidth} \\  & “Topology-based” & & & & Can be used for large network. & Important end-to-end delay \\ \hline \multirow{3}{*}{DSR} & Route discovery a,d & \multirow{3}{*}{Reactive} & Position, number & \multirow{3}{*}{Multihop} & No need for memory. & Higher end-to-end delay \\  & maintenance route & & of nodes & & DSR creates multiple routes & Overhead is directly \\  & “Topology-based” & & & to destination & & proportional to the number \\ \hline \multirow{3}{*}{OLSR} & Proactive link-state & \multirow{3}{*}{Proactive} & \multirow{3}{*}{Status of the link} & \multirow{3}{*}{Multihop} & Reduces the overload. & \multirow{3}{*}{Unsued paths take up an important part of the \(\text{Low end-to-end delay}\).} & \multirow{3}{*}{Unsued paths take up an important part of the \(\text{Low end-to-end delay}\).} \\  & routing protocol & & & & & \\  & “Topology-based” & & & & & \\ \hline \multirow{3}{*}{PSR} & It adopts the & \multirow{3}{*}{Proactive} & \multirow{3}{*}{Distance} & \multirow{3}{*}{Multihop} & Reduces the bandwidth & No control for failure link. \\  & “Topology-based” & & & & & & \\ \hline \multirow{3}{*}{ZRP} & Divide the network in two & \multirow{3}{*}{Hybrid} & \multirow{3}{*}{Short path} & \multirow{3}{*}{Multihop} & No hierarchies eliminates & End-to-end Delay “EDED” \\  & areas. Information exchanged & & & & & & & \\ \cline{1-1}  & proactively inside the area and & & & & & & \\ \cline{1-1}  & “Restoning-based” & & & & & & \\ \hline \multirow{3}{*}{GPSR} & Uses greedy forwarding or & \multirow{3}{*}{Reactive} & \multirow{3}{*}{position} & \multirow{3}{*}{Greedy} & Transmission decision is & End-to-end delay is higher. \\  & mechanism, according to the & traffic density. & & & & & \\ \cline{1-1}  & “Position-based” & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of routing protocols.