 Infrastructure Controller Enterprise Module (APIC-EM). VMware NSX can be considered an overlay SDN implementation. Here, a network layer overlays a physical network. This adds agility to the software systems to quickly bring up the network layer with much less dependence on the hardware layer. SDN is also often interchangeably used with the term Software-Defined Wide Area Network (SD-WAN). Figure 12.3 shows the high-level architecture of SDN: Application layer Applications, running on physical or virtual hosts Northbound Control layer Network controller Southbound Infrastructure layer Programmable switches Figure 12.3 - SDN architecture Bare-metal switching, which allows the use of Linux in the switches, is also often discussed in the context of SDN. Facebook (now part of Meta) also makes bare-metal switches running open OSs. Switches running Linux as the os are a good example of SDN. The actual definition of SDN depends on the vendor you ask. However, at the core of SDN is easy network programmability and network automation. The Open Networking Foundation is one of the relevant bodies in the context of SDN. Understanding MEC C-RAN As shown in Figure 12.3, in SDN architecture, the Northbound Interface (NBI) is used by business applications for appropriate communications with the controller. The Southbound Interface (SBI) facilitates communication with the devices that are part of the infrastructure layer or the data plane. In the three-tier SDN architecture in the preceding figure, the business applications are written by different vendors and third parties. Such applications are architected using the NBI to enable communication with the controllers. This helps to promote interoperability in the SDN architecture. In the prior section, we looked at the role of SDN in 5G and how it adds flexibility to the infrastructure architecture. Next, let us look at Multi-access Edge Computing (MEC) Cloud Radio Access Network (C-RAN). Understanding MEC C-RAN C-RAN is often used interchangeably with Centralized-RAN. C-RAN refe