# Review of V2X-IoT Standards and Frameworks for ITS Applications
## 2 Core ITS Standards
### Hybrid ITS Architecture  
Hybrid ITS network architecture is a combination of DSRC and C-V2X networks. ITS network consists of static devices, such as base stations, road infrastructure and dynamic devices, such as automobiles and pedestrian equipment. Such network architectures can be classified as either hierarchical or flat.  
Hierarchical hybrid ITS network architecture is based around the concept of assigning network devices to separate levels [48]. Utilization of said levels allows devices to be assigned to separate groups, restrict or grant access to certain features, and send or receive specific information. Usually, devices are classified into public transport and public services, personal transportation and other device groups. In such network architecture, C-V2X is used for communication between public transport and public services, and can also be used for access to the internet. Meanwhile, DSRC-based networks are used for communication between personal transportation and communication between personal and public transportation. A hierarchical hybrid ITS network architecture, where each device is assigned to a certain level which does not change, is called static.  
Hierarchical network architectures may also be dynamic, which means that levels, which are assigned to devices, may be updated and reassigned in response to changing network conditions. Dynamic hierarchical network architecture consists of multiple device groups [49; 50; 51; 52; 53]. Typically, devices are assigned to groups based on their position on the road. It is important to note, that every group must have a main device. The main device usually is an intermediary node or a device which has high network and computational resources. The goal of the main device is to control the connections and regulate network load. Regular devices (those, which are not main devices) can be connected to more than one main device. This enables regular devices to retransmit data between adjacent device groups. In this case, the C-V2X network is used for communication between main device and base station; communication between main devices; communication between main device and regular devices in its group. The DSRC network is used for connection between regular devices in the same group. The main advantage of this architecture is that the main device can collect information from regular devices and periodically transmit it to the base station or other data-processing device, which reduces the network load. The main disadvantage of this architecture is that the grouping process is lengthy, which limits the deployment capabilities in densely populated areas, such as city centers.  
Flat ITS network architecture does not limit the connection type for device interconnections. Connection type between devices is chosen according to the network load, quality and distance. For example, if devices use C-V2X as default, they could switch to DSRC if network conditions change. Both DSRC and C-V2X parameters are constantly updated and the best possible connection method is chosen based on a pre-determined algorithm.  
A hybrid ITS network comparison is shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source** & **Network Topology** & **Landscape** & **Use Case Scenario** & **Purpose** \\ \hline
[48] & Hierarchical & Rural & V2X & Video messages \\
[49] & Hierarchical & City & V2I & Internet \\
[54] & Hierarchical & City & V2X & Safety messages \\
[55] & Flat & City & V2V & Travel planning \\
[56] & Flat & Rural & V2I & Internet \\ \hline \hline \end{tabular}
\end{table}
Table 2: Hybrid ITS network comparison.