networks//network theory//protocol stack

A protocol stack is the set of layers one communication passes through, each layer answering a different question, so naming the link a device uses (Ethernet, say) tells little about what it communicates.

A protocol stack is the set of layers one communication passes through, each layer answering a different question, so naming the link a device uses (Ethernet, say) tells little about what it communicates.

One message passes through several layers.

ApplicationModbus, OPC UA, HTTPS, MQTT; what the data means

TransportTCP or UDP; ordering, delivery, ports

NetworkIP; addressing and routing

LinkEthernet, Wi-Fi, RS-485 framing; how neighbors share a medium

PhysicalCopper, 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. So one LAN, a single local link with its gateway out, can carry Modbus, OPC UA and HTTPS at once, and none of them is the network.

A digital sensor need not be on Ethernet: it 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.