networks//network theory//protocol stack

"It communicates over Ethernet" says much less than it sounds, because a communication is a stack of layers and each layer answers a different question.

"It communicates over Ethernet" says much less than it sounds, because a communication is a stack of layers and each layer answers a different question.

Application meaning: Modbus, OPC UA, HTTPS, MQTT. What the data means.

Transport: TCP or UDP. Ordering, delivery, ports.

Network: IP. Addressing and routing.

Link: Ethernet, Wi-Fi, RS-485 framing. How neighbors share a medium.

Physical: copper, fiber, radio, 4G. How bits become signals.

Ethernet is a link technology, not an automation protocol; TCP/IP transports and addresses data but does not define what an industrial value means. The meaning comes from the top layer: Modbus as one small vocabulary, OPC UA as a rich one.

The same top layer can ride different stacks. OPC UA over TCP/IP over 4G over radio at a remote pipeline station; HTTPS over TCP/IP over Ethernet over fiber in the office. Fiber is a medium, radio is a medium, 4G is a cellular access technology that uses radio, Ethernet is a link technology that can use copper or fiber, IP is the network protocol, TCP and UDP are transports, HTTPS is HTTP over TLS. They are not alternatives at one level; they stack.

"Digital is not Ethernet": a digital sensor can speak IO-Link, PROFIBUS, Modbus RTU or CAN with no Ethernet anywhere (fieldbus). And Modbus TCP is the same register vocabulary as Modbus RTU with a different stack underneath.