Strict priority plus deficit weighted round robin queuing

Strict priority plus deficit weighted round robin is a hybrid of two methods, 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.

Using strict priority for all queues would be good for voice traffic, but increases the risk of lower priority queue starvation. DWRR for all queues would be fairer to the lower priority traffic, but at the risk of increased voice traffic delay and jitter.

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

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 DWRR to select the next packet for transmission from the remaining lower priority queues.