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15.5 SCCP MANAGEMENT 489message with Par.9 ¼ 0. It then passes the reassembled subsystem data to the calledsubsystem, in one N-data indication.Class 3 Services. These include the class 2 services and flow control. This protectsthe called subsystem in a connection against being overloaded with incomingDT2 messages [3,7]. The calling SCCP assigns cyclically increasing (...0, 1, ...,126, 127, 0, ...) send sequence numbers [P(S)] to consecutive outgoing DT2 (dataform 2) messages of a connection. The called SCCP sends Data Acknowledgmentmessages, which include a receive sequence number [P(R)] that represents thehighest numbered (mod 128) accepted DT2. Parameters P(S) and P(R) are part ofPar.10, which is included in DT2 and acknowledgment messages.The calling SCCP may send additional DT2s as long as P(S) P(R) þ W, whereP(R) is the receive sequence number in the most recently received acknowledgmentmessage, and window W represents a fixed number of DT2 messages. The value ofW is negotiated during the establishment of a class 3 connection. In this way, thecalled SCCP controls the message flow.15.5 SCCP MANAGEMENTThe purpose of SCCP management (SCMG) is to maintain—if possible—the transferof SCCP messages when failures occur in the signaling network and/or subsystems,and to inform SCCP users to stop sending messages that cannot be delivered. Ifa failed subsystem has a duplicate, SCMG requests its SCCP to reroute messages tothe backup subsystem [6].15.5.1 SCMG InterfacesSCMG has interfaces with the subsystems, the MTP, and the SCCP connectionlesscontrol at its signaling points—see Fig. 15.1-2.N-Primitives are the interface with the local subsystems. The subsystems passinformation, to SCMG in N-requests and N-responses, and SCMG passes informationto the subsystems in N-indications and N-confirmations.The interface with SCCP connectionless control allows SCMG to send andreceive SCCP Unitdata messages to/from SCMGs in other signaling points.The interface with the local MTP consists of MTP-pause, MTP-resume, andMTP-status indications (Section 8.9.1).15.5.2 N-Primitives and Their ParametersPrimitives. The N-primitives (Fig. 15.1-2) are outlined below:N-State Primitives. These primitives indicate a change in status of a subsystem.A subsystem indicates a status change to its SCMG in an N-state request, and