# An Optimization Framework for Data Collection in Software Defined Vehicular Networks
## 3 Materials and Methods
### Proposed Data Collection Optimization Model
#### 3.3.2 Common Facts about Optimization of Data Collection
However, if a new optimization solution is found at \(t+\delta t\), node D will be assigned as an agent. Thus, when the topology changes, data loss can occur. Consider the redundant data transmission problem depicted in Figure 3b. At time \(t\), node D is assigned as an agent by the optimizer as it is an isolated node. At time \(t+\delta\), node D has moved close to nodes A, E, and C such that those nodes have become node D's neighbors. However, as node D is assigned as an agent, node D will collect data from nodes A and C as well and send it to the control plane, while node E also collects data from nodes A and C and sends it to the control plane. However, if a new solution to the optimization problem is found at time \(t+\delta t\), this problem can be avoided as the optimizer will convert node D into a broadcasting node and keep node E as the only agent. However, performing optimization for the slightest change in network entropy can cause a relatively higher channel utilization and cost, as will be proven in Section 4.3.1. Even though the best approach to avoid the above two problems is to optimize for even the slightest change in entropy (or without considering an entropy threshold at all), in order to save additional resources that are utilized additionally for an optimization step like downlink broadcasting, we can perform the optimization if and only if the heterogeneous normalized link entropy (\(H_{het}\)) of the network changes significantly compared to the last optimized network state. However, as pointed out before, there can be a loss of packets if broadcasting nodes become  
Figure 1.3 Entropy calculation for two network instances. (**a**) Vehicular network instance at time t\(-\)1; (**b**) Vehicular network instance at time t.  
isolated and a new solution to the optimization problem for data collection is not found for the new time step. Thus, the entropy threshold must be selected by inspecting the Packet Delivery Ratio (PDR) such that the entropy threshold is not too high such that packet losses will occur. Thus, there is a trade-off between packet delivery ratio (reliability) and communication resource utilization when deciding the entropy threshold value. We compare the absolute value of the entropy change with a threshold (T) to decide on whether to optimize or not. The value of T can be set in the range [0, 1], and we decide on the optimum value of T experimentally;
* Once the solution for the optimization of data collection is computed (\(z_{i},x_{i}\) are found using the optimization model), the solution should be broadcast back to all nodes, as shown in block B3 in Figure 1.4 These data can be called **broadcasting downlink metadata;**
* The transmission period (the reciprocal of data transmission frequency) should be less than or equal to the maximum time period allowed for an update (\(T_{max}\)). This constraint is given in Equation (15). This time period depends on the type of data collected according to the 3 rd Generation Partnership Project (3 GPP) standard; Figure 1.5 (15)