Multicast virtual scheduling is a subscriber-level scheduling. When a downstream device functions as the multicast replication point, the BRAS copies the multicast traffic of all subscribers rather than the specified subscriber to the downstream device. As a result, the multicast traffic of the subscriber does not enter the subscriber queue along with the unicast traffic on the BRAS for traffic scheduling. Unicast traffic of the subscriber is still forwarded using the maximum bandwidth of the subscriber, leaving insufficient bandwidth to forward the multicast traffic of the subscriber. When a subscriber joins a multicast group, the BRAS needs to deduct the bandwidth of the multicast traffic from the unicast bandwidth of the subscriber, to ensure bandwidth for the multicast traffic of the subscriber. In this manner, the total bandwidth of the subscriber remains unchanged, but bandwidth is ensured for the multicast traffic of the subscriber. When the subscriber leaves the multicast group, the BRAS releases the multicast bandwidth. In this process, coordinated bandwidth allocation for both the unicast traffic and multicast traffic of a subscriber is implemented, ensuring the QoS for the BTV service and the subscriber.

As shown in Figure 11-2, the maximum bandwidth for traffic from the DSLAM to the subscriber is 3 Mbit/s. Assume that the subscriber uses up the 3 Mbit/s of bandwidth for unicast traffic service, and then demands a multicast program which requires 2 Mbit/s of bandwidth. In this case, the total traffic required by the subscriber is 5 Mbit/s, much higher than the allowed 3 Mbit/s bandwidth. As a result, the link between the DSLAM and LAN Switch is congested, and packets begin to be dropped. Because the DSLAM does not provide QoS treatment, packets are randomly discarded. As a result, multicast traffic is discarded, and the subscriber cannot have quality service for the requested multicast program. To ensure quality service for the requested multicast program, the BRAS needs to be configured to dynamically adjust the bandwidth for unicast traffic according to the bandwidth for multicast traffic. The DSLAM sends the IGMP Report message of the subscriber through the subscriber's VLAN to the BRAS. After receiving the IGMP Report message, the BRAS reduces the bandwidth for the subscriber's unicast traffic to 1 Mbit/s, leaving the remaining 2 Mbit/s for the subscriber's multicast traffic. In this manner, quality service is ensured for the requested multicast program.

Figure 11-2 Diagram of the multicast virtual scheduling