A queuing model used in Packet Scheduling that, like Priority Queuing, sort Packets arriving in the queue into priority classes.
The round robin approach transmits a packet from each class in a circular manner. Thus it will start from class A, then class B, then class C, etc. until it has transmitted a packet from each class. It then starts over at transmits a packet from class A, then class B, and so on, going back to the starting point, class A, each time.
If round robin is implemented with a work-conserving queuing discipline, then it will never allow the link to remain idle, when there are packets of any class queued for transmission. Such a round robin will thus skip any classes that have no packets queued.
Example
Here packets 1,2, and 4 belong to class A (dark blue) and packets 3 and 5 belong to class B (light blue).