Multicast Virtual Scheduling Fundamentals
Multicast virtual scheduling is a user-level scheduling. When a downstream device functions as the multicast replication point, the BRAS copies the multicast traffic of all users rather than the specified user to the downstream device. As a result, the multicast traffic does not enter the user queue along with the unicast traffic on the BRAS for traffic scheduling. Unicast traffic is still forwarded using the maximum bandwidth of the user, leaving insufficient bandwidth to forward the multicast traffic of the user. When a user joins a multicast group, the BRAS needs to deduct the bandwidth of the multicast traffic from the unicast bandwidth of the user, to ensure bandwidth for the multicast traffic of the user. In this manner, the total bandwidth of the user remains unchanged, but bandwidth is ensured for the multicast traffic of the user. When the user 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 user is implemented, ensuring the QoS for the BTV service and the user.
As shown in Figure 1, the maximum bandwidth for traffic from the DSLAM to the user is 3 Mbit/s. Assume that the user 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 user is 5 Mbit/s, 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 have the QoS capability, packets are discarded randomly. As a result, multicast traffic is discarded and the quality of the multicast program cannot be ensured. 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 user through the user's VLAN to the BRAS. After receiving the IGMP Report message, the BRAS reduces the bandwidth for the user's unicast traffic to 1 Mbit/s, leaving the remaining 2 Mbit/s for the user's multicast traffic. In this manner, quality service is ensured for the requested multicast program.
