A novel request scheduling algorithm was proposed for the OCS （online charging system） server. The basic idea is that one of the requests from all classes of service queues is scheduled with some priority probability which is based on the system queue-length

the request arrival rate

and the pre-assigned delay differentiation parameter. The experimental results show that

in considering request service delays

this proposed algorithm always outperforms some other traditional PDD （proportional delay differentiation） scheduling ones and the performance advantages are relatively greater when request service delays increase and vary greatly from each other. Furthermore

it can efficiently meet 6 QoS （quality of service） requirements proposed in content-based billing.