Ethernet-APL in the ISO/OSI Model – Why the Technology Is Less New Than Many Think

Anyone dealing with Ethernet-APL for the first time often feels as though they are facing a completely new technology. Especially in the process industry, where established fieldbuses such as PROFIBUS PA or Foundation Fieldbus have shaped the standard for many years, it is easy to assume that Ethernet-APL introduces yet another standalone system with new logic and extensive training requirements.

In reality, that is not the case at all.

The key to understanding Ethernet-APL lies in the ISO/OSI model. While traditional fieldbuses define several layers of this model simultaneously — effectively creating a self-contained communication system — Ethernet-APL deliberately focuses only on the lowest layer: the physical layer. In other words, it is purely about how signals are transmitted, not about how communication itself works.

This has far-reaching implications. All higher communication layers remain unchanged. Protocols such as PROFINET, OPC UA, or HART-IP continue to operate exactly as they do in existing Ethernet networks. Ethernet-APL simply ensures that this communication can now also take place reliably over two-wire cables and under demanding conditions, such as in hazardous areas.

A useful analogy is to compare it to a road system. Ethernet-APL is the infrastructure — the road itself. The vehicles on that road are the various protocols and applications. The Ethernet-APL Field Switch acts like an intelligent intersection that controls, distributes, and monitors traffic. The important point is this: the vehicles do not change. They simply use a new and better-suited road.

This approach is exactly what makes Ethernet-APL so attractive. Instead of learning an entirely new technology from scratch, users can build on existing knowledge. The fundamental Ethernet mechanisms remain the same, as do familiar tools and engineering processes. The transition is therefore less of a disruption and more of an extension.

In practice, this means companies can implement Ethernet consistently all the way down to the field level without having to redesign their entire communication architecture. Media discontinuities and additional gateways are reduced, transparency increases, and integration becomes easier.

The real challenge is therefore often not the technology itself, but rather a shift in perspective. Anyone viewing Ethernet-APL as a “new fieldbus” will perceive unnecessary complexity. Those who understand it as a physical extension of Ethernet quickly realize that much of it is already familiar.