# Software-Defined Small Cell-Linked Vehicular Networks: Architecture and Evaluation
## 2 Related Works
### diSRsVN  
diSRsVN is based on an integration of two pertinent network concepts: SDN and small cells [25]. Thus, the small cell centralizes the intelligence of the small cells cluster and vehicles. The diSRsVN allows (i) partitioning the network in a small cells cluster according to the criteria that consist of linking up cellar base station with RSU to form a small cell; (ii) selecting the head cluster vehicle in each small cell linked-up RSU; and (iii) centralizing the global view of the network by connecting adjacent small cell- linked \begin{table}
\begin{tabular}{l l} \hline \hline
**Components** & **Description** \\ \hline \(smallcell-typeRSU\) & \(\text{RSUs can operate as it is set up with 5G small cells.}\) \\ \(SDNOpenFlowController\) & \(\text{SDN openflow holds overall (global view) network of the proposed architecture}\) \\ \(OpenFlowSwitch\) & \(\text{delivering V2X rules to SD-onboard vehicle module}\) \\ \(smallcell-typeRSUEnabledSDN\) & \(\text{delivering V2X rules to small cell RSU node}\) \\ \(Small-celltypeRSUController\) & \(\text{SDNCs control and forward routing rules into the network related to small cell-type}\) \\ \(SDNV2 XnetworkController\) & \(\text{RSU of the proposed architecture}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Definition of functional components of the diSRsVN. Figure 2: The diSRsVN architecture with small cells linked to RSU. RSU-enabled SDN deployed at each small cell-linked RSU device. Therefore, deploying a small cell-linked RSU device and using a distributed small cell-linked RSU controller provides better performance in handling the traffic load, and enhances scalability with efficient data dissemination in the 5G V2X system. Vehicles and road users are included in the data plane. V2X services also include V2N, V2I, V2V, and V2P links. Furthermore, the V2X onboard access unit is regarded as an SDN device, much like the data plane in the SDN approach. Functions of the SD onboard wireless communication are discussed later in this section (Section 3.2). In the V2X access layer of the data plane, the SD-onboard wireless vehicle primarily collects data, forwards it, queues it, and sends it into the small cell-linked RSU node. Furthermore, the forwarding devices, such as the OpenFlow V2X network switch and a small cell-linked RSU-enabled SDN, are included in the forwarding layer. The control plane decisions are not entirely made by a single SDN controller centralized element; adjacent SDN controllers in the control layer could collaborate to make decisions. In fact, this collaborative decision is provided through mobility management, and quality of service (QoS) support provided at the SDN OpenFlow controller. The SDN OpenFlow controller should achieve a fast recovery after failures between adjacent controllers. A logically distributed control plane, distributed small cell RSU controller and SDN V2X network controller(s), achieve optimal performance and robustness against failures. The SDN V2X network controller has the role to select an in-vehicle wireless access communication radio to connect the small cell-linked RSU. As shown in Figure 2, diSRsVN adapts multi-cell device-to-device (D2D) communication assisted with densely deployed small cells [27] to support efficient communication between distributed small cell linked-up RSU. Each small cell linked-up RSU represents the first level of cluster called software-defined small cells (SD-small cells), where the distributed small cell RSU controller centralizes the topology state of vehicles driving in the same direction. It forms a cluster head called SD-small cell head (SD-SmCH). We assume that SD-smCH initiates a transient software controller from the small cell RSU controller as long as it has the longest travel time until it leaves the coverage of small cell RSU. The transient software controller of the SD-SmCH includes flow entries of the routing paths in case SD-smCH fails to remain in the small cells.