Frame Preemption

Interspersing Express Traffic /Frame Preemption (IEEE P802.3br/IEEE P802.1Qbu)


The use of scheduled traffic imposes limitations when starting a transmission.  As shown below, if a frame begins transmission just before the start of a reserved time period, it can extend transmissions outside its allocated window, potentially interfering with more critical traffic. Therefore, a guard band equal in size to the largest possible interfering frame is required before the window starts.










Obviously, a larger guard band equates to a less efficient use of network bandwidth. Frame preemption addresses this issue by breaking the interfering frame into smaller “fragments”.  Thus, the guard band need only be as large as the largest possible interfering fragment instead of the largest possible interfering frame.

Preemption can have another benefit. Industrial applications, such as machine control, are typically built in long line configurations. For these installations, to minimize wiring cost and complexity, typical installation uses “daisy chain” where each node has (2) external switched ports and an internal port that goes to the end-node. In these cases, the accumulated latency per-hop becomes problematic. Preemption mitigates this problem by minimizing the frame delay at each hop.