, the sequence numbers of the group will change. Although the standard defines that new members are added at the end of the VCG, removing one or more members can occur at any point. However, any changes at the source will be detected at the sink, which will consequently change the MST value of the affected members, and this will be relayed back to the source. To acknowledge to the source that a sequence change has been detected at the sink, the Resequence Acknowledgment (RS-Ack) bit is toggled. Before any further changes are com- mitted to the VCG (or before any new MST information is evaluated), the source waits until the RS-Ack is received to acknowledge the change. As an example of the process between two LCAS interfaces, consider the addition of a member to a VCG with reference to Figure 11.15. When the provisioning of the bandwidth from source to sink through the SDH network is complete, an ADD command from the Chapter 11 Next Generation Synchronous Digital Hierarchy Network Management SQ = 0, CTRL = NORM System SQ = 1, CTRL = NORM 1 ADD SQ = 2, CTRL = EOS 2 CTRL = ADD CRC OK? MST = OK Delay OK? 5 RS-Ack SQ = 0, CTRL = NORM Capacity Available? SQ = 1, CTRL = NORM Source SQ = 2, CTRL = NORM SQ = 3, CTRL = EOS Fig. 11.15 Example of the LCAS process for adding a new member to a VCG. network management system to the LCAS interface is initiated. The source sends a CTRL = ADD in the control packet to the sink node to notify it of the impending action. The sink node then checks that (a) the CRC of the control packet is correct; (b) the differential delay of the new member relative to existing members is within bounds; and (c) capacity is available to accommodate the new member. If any of these fail, the member is rejected. Otherwise, the sink node sends MST = OK to indicate the success of the end-to-end member connection. The source then makes the new member the last in the sequence using the control packet CTRL = EOS, and the penultimate member sends a CTRL = NORM message. When a confirmation RS-Ack toggle (0 1 or