What is IEEE802.1Qbu?

802.1Qbu is one of the IEEE 802.1 standards related to Ethernet networking.
This standard allows high-priority Ethernet FRAMES to be inserted during transmission of low-priority Ethernet FRAMES, thereby improving QOS (Quality of Service).

802.1Qbv (Enhancements for Scheduled Traffic) controls when and from which queue an Ethernet FRAMES is sent by setting up a protection window according to the schedule in the gate control list.
However, if an Ethernet FRAMES is currently being sent at the end of the scheduled period, the next scheduled Ethernet FRAMES cannot be sent from the queue until that frame is finished. This is shown in Figure 1.
In cases where latency requirements are strict, such as time-sensitive video traffic, 802.1Qbu functionality is being added to 802.1Qbv functionality.

The start timing of transmission from Output Queue y is affected by the end timing of transmission from Output Queue x and is delayed from the start timing of the protection window period.

If Output Queue y has a higher priority than Output Queue x and such a delay in the start of transmission is not permitted, the 802.1Qbu (preemption) function is applied, and as shown in Figure 2 below, even if an Ethernet FRAMES is currently being transmitted from Output Queue x, which has a lower priority, the transmission is temporarily suspended and the transmission of the Ethernet FRAMES from Output Queue y, which has a higher priority, is started.

Figure 3 was created to more easily visualize the difference between having and not having the 802.1Qbu (Preemption) function, as shown in Figures 1 and 2.
The 802.1Qbu feature allows transmission from a high-priority Output Queue to begin at the scheduled time.