Sometimes I’m surprised by the questions at our PROFINET one-day training classes. In Detroit students stumbled over QoS (Quality of Service – a standard feature of Ethernet). This means we need to clarify that topic and how PROFINET uses it; here’s is that attempted clarification:
QoS uses a field in the Ethernet frame to identify the priority of the frame. Most Ethernet switches recognize that higher priority frames are to be sent before lower priority frames. (Even very inexpensive unmanaged switches have QoS. See PROFINET can use a Best Buy Ethernet Switch, really?)
Here are the priority levels from Wikipedia.
And here is where the QoS parameter (802.1Q) resides in an Ethernet Frame (Wikipedia):
PROFINET Real-Time (RT) frames are automatically set to 6, the next to highest priority. The PROFINET RT priority is higher than TCP/IP frames. No action is required by the user; PROFINET sets the QoS priority automatically. Here’s how it works: