# Software Defined Networking Architecture for Energy Transaction in Smart Microgrid Systems
## 4 SDN Architecture Development for a Transactive Energy Network
_Improved security in the SDN architecture_  
The universal network visibility of an SDN architecture is a security vulnerability open for intrusion and malicious activity. An encrypted data path will hide the information to improve the security of the SDN architecture. It is worth mentioning that a centralized SDN design has two design concerns,  
* Control traffic at the SDN controller for an extensive, geographically distributed network can be high compared to the data traffic of the TESMS application. * The number of network nodes between SDN nodes at the controller and the remote site can influence the reliability of the data path from the remote sites. SDN nodes send information regarding hosts (users) to the controller. The controller then sends control information regarding the data flow for the host to the SDN Node. Therefore, the control traffic will also increase with increased hosts or users on the SDN network. An SDN network's usage and utilization performance is linearly related to the number of nodes. Therefore, decentralizing the network by \(n\)-times controllers will reduce the usage and utilization at each controller by factor \(n\). A decentralized controller placement approach also reduces the number of network nodes between the SDN controller and the SDN node. From literature reviews [36], the Internet infrastructure consists of multiple network elements with diverse routes and technology types. Decentralizing the SDN network will reduce the number of circuits between the nodes by reducing the number of failure points in the system that will influence the network's reliability. The availability of each circuit represents the failure probability of each circuit. The availability parameter of a link in % represents the uptime. As previously mentioned, [PERSON] et al. [29] stated that the reliability (\(R(t)\)) of a single device or link could be expressed as a function of the failure rate (\(\lambda(t)\)) over time,  
\[R_{(t)}=exp\left[-\int_{0}^{t}\lambda(u)\mathrm{d}u\right] \tag{1}\]  
If the failure rate \(\lambda\) is constant, then the expression for availability reduces to,  
\[R_{(t)}=\mathrm{e}^{-\lambda t} \tag{2}\]  
The total link availability is the sum of all the circuit failures connected in series. This result is because failures do not occur simultaneously. Over time \(t\), the total failures accumulate. The failure rates of each circuit are added together to calculate the total failure rate. \[\lambda_{t}=\lambda_{1}+\lambda_{2}+\lambda_{3}+\cdots+\lambda_{n} \tag{3}\]  
The Equation can be written as follows if all the circuits have the same failure rate. \[\lambda_{t}=\lambda\cdot n \tag{4}\]However, the failure rate \(\lambda\) is the difference between the total time (\(t\)) = 100% and availability \(A\). Therefore, \(\lambda\) can be replaced by (\(1-A\)) to provide the total availability,  
\[A_{T}=\mathrm{e}^{-n(1-A)} \tag{5}\]  
As the number of circuits increases, the total link availability reduces. Decentralizing the SDN network and placing controllers per region can improve link availability. From Equation (5), the total link availability is calculated by using the number of circuits, each with the same availability parameter. For the reduction in the number of circuits (\(x\)) from the initial number of circuits (\(n\)) to the reduced number of circuits (\(na\)), the availability improvement (\(A_{i}\)) is the difference between the availability after the reduction (\(A_{A}\)) and the availability before the reduction (\(A_{B}\)). The improvement can therefore be calculated as follows,  
\[A_{i} =A_{A}-A_{B} \tag{6}\] \[A_{i} =\mathrm{e}^{-na(1-A)}-\mathrm{e}^{-n(1-A)}\]  
where,  
\[na=n-x \tag{7}\]  
Therefore,  
Figure 6:0 Figure 6:1 Figure 6:2  
For redundant links from the same SDN node, the total link availability is a product of all the circuit failures that are connected in parallel. This result is because the failures of all the circuits do not occur simultaneously. The failure rate of each circuit is multiplied by each other.