# Software-Defined Small Cell-Linked Vehicular Networks: Architecture and Evaluation
## 4 Implementation of the V2x Data Dissemination over diSRsVN and Its Results
### Influence of the SDN Technology  
To indicate the SDN approach's impact on diSRsVN, we evaluate the performance of the proposed architecture by taking into account the presence of a distributed small cell RSU controller, central SDN controller, and without an SDN controller. In the simulation, as the number of vehicles increases, so does the PDR for both the diSRsVN architecture aided by a single central controller and the no-SDN-based architecture. The use of distributed small cell RSU controllers decreases the PDR of the diSRsVN as the density of the vehicle increases. In the case of using distributed small cell RSU controllers, it seems that a higher number of packets are exchanged between vehicles. The SDN controller may select another small cell RSU controller distinct from the one previously chosen and located in proximity to the SD-SmCH to disseminate the vehicle's speed information. This reduces the packet delivery ratio. In the case of cooperative warning query dissemination from ROs, the proposed scheme is aided by the central SDN controller, which provides stable route information based on network performance as measured by the number of small cell RSU. It is not necessary to rely on the distributed small cell controller because the V2X controller has the routing rules for the destination vehicles. Therefore, it does not need to forward the dissemination request to the source node, which may influence the V2X controller to support the best small cell RSU. This reduces the number of messages exchanged between the small cell RSU and the vehicle. It also reduces packet collision in the SDN controller. This is due to the computation of the global view of the network incurring a lower packet delivery ratio. A number of packet losses are incurred when the proposed architecture partially relays distributed small cell controllers to reach the small cell RSUs.With a vehicle density of 80 (Figure 4c), the distributed small cell RSU controller has a packet delivery ratio of 95.18, with the use of the central controller, it measures 96.15, and with a PDR value of 90.68 without using an SDN controller. Figure 4a shows the average throughput. On an expanding number of nodes, the average throughput of the proposed diSRsVN architecture assisted by the distributed small cell RSU controller increases for all scenarios. As the number of vehicles increases, the connectivity is stable between cluster members; consequently, the diSRsVN architecture fastens the transmission packets. However, the use of a central SDN controller provides stable average throughput to generate routes to the small cell RSU nodes than no-SDN-based. The diSRsVN uses the topology discovery message when the distributed small cell RSU controller suggests a list of different small cell positions and according to the selection of the cluster head. In terms of vehicle density of 80, the average throughput of diSRsVN assisted by the distributed small cell RSU controller is 101.57 Kbps, whereas 93.24 Kbps and89.46 Kbps are the average throughputs of diSRsVN assisted by a central SDN controller and without an SDN controller, respectively. Figure 4b shows the average E2E delay versus vehicle density. In the proposed architecture, the E2E delay increases as the number of vehicles increases. In general, as the number of vehicles increases, connectivity remains stable. The results indicate that as the number of dissemination requests increases proportionally to the number of vehicles, the SDN controller receives an important number of flow routing updates according to the number of small cell RSUs available to forward dissemination responses to source nodes. In this paper, the topology discovery process increases the E2E delay as the number of nodes increases. Hence, diSRsVN with distributed small cell RSU controllers has less delay because they assist in selecting reliable routing paths to deliver the packets as the knowledge of the topology discovery process is completed and flow tables are populated to update the routing rules.