# Multi-Layer Problems and Solutions in VANETs: A Review
## 2 Multi-Layer Challenges in VANETs
### Proposed Solutions
#### 2.2.3 Routing Layer
An improved GPSR (Greedy Parameter Stateless Routing) Protocol which controls Network Congestion in [54] is shown to reduce the delay and packet loss rate compared to the traditional GPSR Protocol. 3 Block Head Cluster based Routing: Routing can be performed by dividing the network in block head (BH) and block nodes (BN) [55]. It is similar to the clustered based network mentioned before. The BH and BN form a mesh topology. The BH sends continuous hello messages to the members in the block and updates status in the table. In this approach the AODV protocol has been modified to form a BM-AODV in which the nodes update their status to the BH. The results show that this strategy outperforms the conventional AODV protocol in terms of throughput and end to end delay. 4 Cross Layer Routing: Conventional Routing protocols have several retransmissions due to unsuccessful delivery of packets which can lead to congestion. Interference can be one other limiting factor. This can increase the delay in the network, making the routing scheme very inefficient. Using parameters from different layers (PHY and MAC) we can design efficient routing protocols by utilizing those parameters in decision making. This is called the Cross Layer design. Several research papers highlight the cross layer design for MANETs but there is limited work on cross layer approach in VANETs. [PERSON], et al. [56] propose a scheme where these parameters have been used to make routing decision in VANETs. Using parameters from the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Sub Category** & **Protocols** \\ \hline \multirow{2}{*}{Topology Based Routing Protocols} & Reactive & DSR, TORA, AODV \\ \cline{2-3}  & Proactive & FSR \\ \hline \multirow{4}{*}{Geographical or Position based Routing Protocols} & DTN & DTN-VADD, GeOpps \\ \cline{2-3}  & & CBF \\ \cline{2-3}  & & PBR-DV, GPSR, GRANT, STBR, \\ \cline{2-3}  & & EAR, LOUVRE, GPCR, GyrAR, \\ \cline{2-3}  & & STAR, GPSrj + TO - GO \\ \cline{2-3}  & Hybrid & GeoDTN + NAV \\ \hline \multicolumn{2}{c}{Opportunistic Protocols} & VADD & D-VADD, L-VADD, H-VADD \\ \hline \multicolumn{2}{c}{Interference Aware Protocols} & - & DUAR, SIR-ADDV, MIMO DV \\ \hline \multicolumn{2}{c}{Information Dissemination Protocols} & - & UMB \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Routing Protocols. physical layer (data rate, Signal to Interference and Noise Ratio, channel gain etc) and buffer space and retransmission count from the MAC layer, the upper layers can be optimized [24]. Nodes in the network can send feedback to the source node and on the basis of this feedback; the routing can be made more efficient. Some of the cross layer protocols include the SBRS-OLSR (Signal Strength Assessment based Route Selection- Optimized Link State Routing), MP2R (Mobility Prediction Progressive Routing), R-AOMDV (Adhoc On-demand Multipath Distance Vector with retransmission counts metric), and PROMPT (position-based delay-aware communication protocol) [49]. The selection of parameters from different layers is still in open issue in the cross layer design. Figure 5 shows the cross layer design for routing purpose. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Challenges** & **Solutions** & \\ \hline \multirow{4}{*}{Type of Protocol Used} & Topology Based & \\  & Geographical & [25; 50; 51; 52; 53; 57; 58] \\  & Assisted Data Delivery & [25; 50; 51; 52; 53; 57] \\  & Dynamic Vehicle Routing Protocols & \\ \hline \multirow{2}{*}{Congestion Control} & Modified protocols to control the congestion & [54; 59] \\ \cline{2-3}  & Block Head and Block Nodes based & \\ \cline{2-3}  & clustering protocols & [55] \\ \hline \multirow{2}{*}{Cross Layer Design} & Routing can be optimized by feedback from & \\ \cline{2-3}  & Physical and MAC Layer & [24; 56] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Routing Layer. Figure 5: The Cross Layer Approach to Routing.