OPC UA (IEC 62541) is an industrial communication standard that carries not just values but a model of the equipment producing them. Where Modbus gives you a number at an address, an OPC UA server exposes an address space of typed nodes with names, data types, engineering units, timestamps and quality flags, and you can browse it at runtime without documentation.
OPC UA telemetry, in practice, means reading values from that address space — either by polling reads, or by creating a subscription with monitored items so the server pushes changes with a dead-band you specify. Subscriptions are what make it viable at scale: a thousand tags at 100 ms is a lot of polling and very little actual change.
It also brings security as a first-class concern rather than an afterthought — X.509 certificates, message signing and encryption, and user authentication are part of the specification. That is genuinely better than Modbus, and it is also the part that consumes the first week of an integration.
Here is what OPC UA does not solve, and it is the thing people are most surprised by: self-describing is not the same as standardised. Two vendors model the same machine differently. One exposes spindle speed under an object called Spindle, another under Channel/1/Axis/S1, a third reuses the controller's internal variable names. Browsing tells you which nodes exist. It does not tell you that node A on machine 1 and node B on machine 2 are the same measurement. That mapping is still work, and it is the work Forge does.
They solve different problems and increasingly run together. OPC UA models the asset so you can interrogate it; MQTT moves messages cheaply through a broker. OPC UA PubSub over MQTT exists precisely because the industry stopped treating this as a choice.
No. It standardises how a model is expressed, not what anything is called. Companion specifications narrow this for some domains, but adoption is uneven and vendors extend freely.
TCP 4840 for the binary transport, which is what almost everything uses. An HTTPS binding exists in the specification and is rare in the field.