A typical example in which H-QoS is used is where a transport provider uses a 7705 SAR as a PE device and sells 100 Mb/s of fixed bandwidth for point-to-point Internet access, and offers premium treatment to 10% of the traffic. A customer can mark up to 10% of their critical traffic such that it is classified into high-priority queues and serviced prior to low-priority traffic.
Without H-QoS, there is no way to enforce a limit to ensure that the customer does not exceed the leased 100 Mb/s bandwidth, as illustrated in the following two scenarios.
If a queue hosting high-priority traffic is serviced at 10 Mb/s and the low-priority queue is serviced at 90 Mb/s, then at a moment when the customer transmits less than 10 Mb/s of high-priority traffic, the customer bandwidth requirement is not met (the transport provider transports less traffic than the contracted rate).
If the scheduling rate for the high-priority queue is set to 10 Mb/s and the rate for low-priority traffic is set to 100 Mb/s, then when the customer transmits both high- and low-priority traffic, the aggregate amount of bandwidth consumed by customer traffic exceeds the contracted rate of 100 Mb/s and the transport provider transports more traffic than the contracted rate.
The second-tier shaper—that is, the per-SAP aggregate shaper—is used to limit the traffic at a configured rate on a per-SAP basis. The per-queue rates and behavior are not affected when the aggregate shaper is enabled. That is, as long as the aggregate rate is not reached then there are no changes to the behavior. If the aggregate rate limit is reached, then the per-SAP aggregate shaper throttles the traffic at the configured aggregate rate while preserving the 16-priority scheduling priorities that are used on shaped SAPs.