# Connected Vehicles: Technology Review, State of the Art, Challenges and Opportunities
## 2 Enabling Technologies
### Hybrid Architecture  
In the aforementioned subsections, the benefits and drawbacks of DSRC and cellular technology in regards to the vehicular environment were discussed. Recent efforts [54; 55; 56] have been directed towards the integration of both enabling technologies to develop and improve vehicular wireless communication by exploiting the complementary aspects of DSRC and cellular technologies. This is because V2X applications and services demand effective and efficient usage of diverse access wireless communication technologies [57]. Inter-working between DSRC and cellular technology, which forms a hybrid solution, is considered promising, as it utilizes the benefits of both enabling technologies. For instance, in situations where transmitted data between vehicles are shattered and V2V multi-hop communication fails, cellular technologies may perform as a backup solution to relay transmitted information. Moreover, vehicles may reconnect to the Internet access network of cellular technology in case of Internet connection loss with the RSUs. Figure 2 illustrates examples of safety applications where the integration between DSRC and cellular technology will be beneficial. Additionally, the integration between DSRC and cellular technology can also be utilized to ease effective clustering techniques. In clustering techniques, nearby vehicles are grouped into clusters, where the sustainability and handling of network resources of each cluster is achieved by a cluster head (CH). Moreover, remaining vehicles within the cluster are referred to as cluster members (CMs) and may be part of several adjacent clusters. CH may choose some vehicles to perform as a gateway to ease the communication between neighboring clusters. In a cluster-based manner, CHs and gateways form a dynamic hierarchy that allows two-way data transmission from and to cellular networks; for instance, a multimedia file is downloaded by CH from a cellular network and then DSRC technology is utilized to disseminate it to its CMs in the cluster [28]. The main benefits of clustering techniques are efficiently decreasing the size of the network and the delay in each cluster. However, the rapid change in the mobility of vehicles affects the cluster steadiness as a result of its effect on the splitting and merging of the cluster, which imposes considerable cluster control overheads on the network and hence necessitating the need for effective and minimal overhead techniques. Clustering stability is a challenge that should be considered when implementing a hybrid architecture of DSRC-cellular networks that depend on a cluster approach [28; 58]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Features** & **DSRC** & **5G-Cellular Technology** & **Wi-Fi 6** \\ \hline Latency & Low latency and high reliability in less-congested areas [49]. & Latency is considered stable in comparison to DSRC. & 2–3 ms [50]. \\  & & & Expected \textless{}1 ms. & \\ \hline Communication Range & 300–1000 m. & Up to 2000 m. & 50 m indoors and 300 m outdoors [51]. \\ \hline Benefits & Established and deployed technology. Have an allocated 5.9 GHz spectrum. Perform well in harsh weather conditions [52]. & Maximum download data rate can reach up to 4.5 Gb/s [53]. Covers a large area and is suitable for long-range applications. & Higher spectral efficiency/channel capacity. Broader outdoor coverage. \\ \hline Challenges & Short-term V2I interconnection. Low scalability in dense traffic environments. Unfairness in acquiring resources. & High communication cost. No established dedicated standard. Interference at low-level altitude. Centralized architecture of cellular networks. & Lower coverage range. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between DSRC, 5G-cellular, and Wi-Fi 6 technologies. Another important aspect of the hybrid architecture of DSRC and cellular technology is the strategic paradigm used for network selection between both enabling technologies.