Download

484 SIGNALING CONNECTION CONTROL PART15.4 CONNECTION-ORIENTED SCCPIn this mode of operation, a (virtual) connection is set up before data transferbetween two SCCP users takes place. We distinguish two connection types.Temporary connections are established and released at the start and end of a transaction.Permanent connections are long-term connections that can be set up andreleased only by administrative or maintenance personnel.Connection-oriented service is the preferred way for transactions that involve thetransfer of large amounts of data, which puts a momentary heavy load on theinvolved subsystems and the signaling network. Transactions of this nature arenot call-related and can be deferred, say, for several minutes. Connection-orientedservice gives the called subsystem an opportunity—at the time it receives a connectionrequest—to determine whether it can handle the transaction at this time. If yes,it accepts the request. If not, it refuses. This avoids cluttering up the network withmessages that cannot be processed anyway.ITU-T has defined two classes of connection-oriented service [4,7]: basicconnection-oriented service (class 2) and connection-oriented service with flowcontrol (class 3).When reading this section, it is helpful to look up the message contents(Table 15.2-2), and the parameter descriptions (Section 15.2.3).15.4.1 PrimitivesThe primitives and connection-oriented service are shown in Fig. 15.1-2. TheN-connect and N-disconnect primitives are used to establish and release connections.The N-data primitives are used for the transfer of user data during theconnection.In addition to the familiar request and indication primitives, SCCP also usesresponse and confirmation primitives. They play a role in the establishment of aconnection.15.4.2 Connection-Oriented Class 2 ServiceFigure 15.4-1 shows the SCCP messages and primitives during a temporary connectionwith class 2 service. The transaction involves subsystem P at signaling point A,and subsystem Q at signaling point B.Establishing the Connection. Subsystem P initiates the transaction and passesan N-connect request to SCCP-A. The SCCP sends a Connection Request messageto SCCP-B, which passes an N-connect indication to subsystem Q. The subsystemdecides to accept the connection request and passes an N-connect response toSCCP-B, which now sends a Connection Confirm message. SCCP-A informs subsystemP that the connection request has been accepted, with an N-connectconfirmation.