# Multi-Layer Problems and Solutions in VANETs: A Review
## 2 Multi-Layer Challenges in VANETs
### Proposed Solutions
#### 2.2.3 Routing Layer  
1 Clustering Routing Protocols for VANETs: In [49] a comprehensive study of the routing protocols in vehicular networks is done. According to this study there are five broad categories of the routing protocols that can be applied to VANETs as shown in Table 1 with their subcategories. Topology Based Protocols: There are 2 types of topology based protocols. Both use the link information to forward the traffic from one node to the next node in the network. * Reactive: The reactive protocols reduce the congestion and overhead in network. It is an on-demand protocol and works on the requirement of the network. The main problem with this protocol is the flooding of the network due to new route discovery messages. * Proactive: Uses the routing table to store routes. The advantage of using this protocol is low latency. But keeping the paths updated is a problem due to the fast movement of vehicles in VANETs. A proactive approach in VANETs means a lot of overhead traffic to keep the paths updated, and even so, many times, when you need the path, it is not available any more. Geographical (Positional) Based Protocols: These protocols use the geographical or positional information of the nodes along with its neighbors using GPS and maps. These protocols are further divided into three sub-categories: DTN (Delay tolerant networks), Non-DTN and a Hybrid (DTN+Non DTN). DTN is an opportunistic routing protocol due to end to end disconnection. It is suited for applications with high latency, low data rate, and long queuing delay. Geographic based routing in VANETs have been investigated in [50] in terms of segment vehicle, link quality, and degree of \begin{table}
\begin{tabular}{c c c} \hline \hline
**Challenges** & **Solutions** & \\ \hline Synchronization & Hybrid Synchronization Algorithms & [30] \\ \hline Congestion & Congestion Detection and Control Algorithms: & [31; 32; 33; 34; 35; 36; 37; 48] \\  & Proactive, Reactive, and Hybrid & \\ \hline \multirow{2}{*}{Contention versus Scheduling} & Self-Organized Time Division Multiple Access & \\  & (STDMA) & [38; 39; 40; 41; 42; 43] \\ \hline \multirow{2}{*}{Hidden/Exposed Node} & Using Request to Send (RTS)/Clear to Send (CTS) & [44] \\  & packets & \\ \hline Contention Window Adjustment & Dynamic Adjustment of the Contention Window Size & [13; 37; 45; 46] \\ \hline Stability/Scalability & Using Clustering based MAC protocols & [47] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Medium Access Control (MAC) Layer. connectivity. It uses a geographic distance routing protocol with improvement in results in terms of lower link disconnection and end-to-end delay with higher throughput compared to other state of the art protocols. Location error resilient geographical routing protocol (LER-GR) [51] has been used to improve the accuracy of the location of the vehicle which has been ignored in the past. An error calculation and location prediction with correction has been utilized to predict the location of the neighboring vehicles, which is then used for the routing purpose. Opportunistic Routing Protocols: Assisted Data Delivery (VADD) [52] is one such protocol in which a moving vehicle carries the packet and forwards it to the first available neighbor. VADD has many different types as shown in Table 3. A Trajectory-Driven Opportunistic protocol for vehicular-cyber physical systems (VCPS) in [53], aims to lower the routing overhead compared to other opportunistic protocols. 2 Congestion Control Routing: In [33], a congestion control based re-routing algorithm is proposed. It compares the shortest path AOHDV (Adhoc On-Demand Multipath Distance Vector) routing protocol with the proposed LIEMRO (Low Interference Energy-Efficient Multipath Routing protocol) optimal routing protocol. The result shows that the proposed solution has a better path selection capability and it reduces the overhead in the network.