## 3 Non-AI Consistency Solutions for PDLC SDN Controllers
### Static Consistency Synchronization Techniques  
First, we examine some notable industry solutions for PDLC SDN controllers, such as HyperFlow [11], Onix [22], ONOS [24], ElstiCon [25], and Orion [26] (See Table 1). Each proposed solution impacts several distributed SDN controller challenges, including scalability, reliability, interoperability, consistency, synchronization overhead, Technological Readiness Level (TRL), East-Weast communication protocol, and implementation programming language [2, 4, 15, 27, 28, 29]. The logical classification used in this paper was adopted from [6, 9]. We consider the Technological Readiness Level (TRL) scale. It was recommended by the European Commission and published in [30]. It serves as a measure of the maturity of a certain technology. The TRL of technology must be taken into account due to the sophistication of the mechanisms we are dealing with in this study in order to minimize development efforts as much as possible [6]. Second, several works are presented by academia to solve the inconsistency problem in PDLC SDN controllers in WANs. Some work uses static consistency. Table 2 compares some of the notable static synchronization techniques for PDLC SDN consistency in academia. It tabulates these solutions based on scalability, robustness (reliability), consistency, synchronization approach, synchronization overhead, interoperability, East-Weast communication protocol, and implementation programming language. Many recent articles have examined the architecture of a large-scale or global SDN controller with a static consistency synchronization approach, including [11, 31, 32, 33]. Each SDN-distributed controller platform has its own communication method. This is a major drawback of these controllers since it makes it challenging for the SDN network to share data between various domains [1]. [PERSON], [PERSON], and [PERSON] [34] introduce the Distributed SDN control plane Framework (DSF). It is a modular design for constructing an East-West interface [34]. The proposed work synchronizes topologies by utilizing Data Distribution Service (DDS), a standardized data-centric Real-Time Publish-Subscribe (RTPS) approach. The local network state changes published by the controllers into the domain data space with a predefined structure of topic and Quality of Service (QOS) attributes [34]. The researchers have justified the aim of using a data-centric RTPS approach in the SDN control plane domain. This approach allows many communicating participants to interact in real-time and in a synchronized manner, providing secure end-to-end data connectivity. This is in contrast to the broker-based or client-server models which can cause extra processing delay, introduce a single point of failure, if a failure occurs, and limit concurrent read/write [34]. The problem with DSF is that every time an update is made, they broadcast it to every other controller using the active replication mechanism. Consequently, the broadcasting strategy lowers the network's performance [5]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline
**Discluded** & **Year** & **Discluded** & **Connectivity** & **Scalability** & **Robustness** & **Consistency** & **Space. ** & **Space. ** & **Integrability** & **TBL** & **East-West Camera. ** & **Programming** \\  
In ref. [1], [PERSON], [PERSON], and [PERSON] introduced WECAN: an efficient West-West-West-East control associated network for large-scale SDN systems. WECAN proposes to provide a scalable network control layer interoperable with an East-West interface. This work aims to solve the communication problem between different controllers [1]. The substantial WECAN component is the decision layer with several servers known as decision elements that are directly connected to the network. Recalling the original SDN layers, WECAN's decision layer is a newly developed layer between the SDN controller layer and the application layer. All network control choices are made by the decision layer.