Strict priority plus weighted fair queuing

SP plus WFQ is a hybrid of two algorithms, and is primarily used in networks carrying voice traffic. Queuing delay and jitter (variance in delay) can readily affect the quality of a voice call. To prevent these conditions, voice traffic should be in the highest priority queue, but this risks creating queue starvation for the traffic in other queues.

Using WFQ for all queues would be fairer to lower priority traffic, but at the risk of increased voice traffic delay and jitter.

SP plus WFQ solves both problems. The highest priority queue is scheduled using strict priority, while the remaining seven lower priority queues use WFQ.

On each round, the scheduler first checks the highest priority queue. When it has a packet ready, that packet is selected for immediate transmission. When the highest priority queue is empty, the scheduler uses WFQ to select the next packet for transmission from the remaining lower priority queues.