Signaling System No.7 Protocol Architecture And Sevices part 31

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Signaling System No.7 Protocol Architecture And Sevices part 31SCCP Messages and ParametersA full list and descriptions of ITU-T and ANSI SCCP messages is provided inAppendix C. This section concentrates on the core messages and parameters. Table9-3 shows the full list of SCCP messages in a chart that shows the protocolclass(es) in which the messages operate. Both ANSI [2] and ITU-T [60] haveidentical SCCP message sets.Table 9-3. The SCCP Message Types and Corresponding Protocol Class(es) Protocol ClassesSCCP Message 0 1 2 3CR (Connection Request) X XCC (Connection Confirm) X XCREF (Connection Refused) X XRLSD (Released) X XRLC (Release Complete) X XDT1 (Data Form 1) XDT2 (Data Form 2) XAK (Data Acknowledgment) XUDT (Unitdata) X XUDTS (Unitdata Service) X[1] X[1]ED (Expedited Data) XEA (Expedited Data Acknowledgment) XRSR (Reset Request) XRSC (Reset Confirm) XERR (Protocol Data Unit Error) X XIT (Inactivity Test) X XXUDT (Extended Unitdata) X XXUDTS (Extended Unitdata Service) X[1] X[1]LUDT (Long Unitdata) X XLUDTS (Long Unitdata Service) X[1] X[1][1] Type of protocol class is indeterminate (absence of protocol class parameter).Message StructureFigure 9-7 shows the format of an SCCP message. Figure 9-7. The SCCP Message Structure [View full size image]Apart from the absence of a Circuit Identification Code field (CIC) field followingthe routing label, the basic message format is the same as an ISUP message (seeChapter 8, ISDN User Part [ISUP]). As with all other MTP users, SCCPmessages are composed of three parts: a mandatory fixed part, mandatory variablepart, and an optional part. All SCCP messages contain a mandatory fixed part, butnot all of them have parameters to place in the mandatory variable or optional part.The following sections describe these three parts in more detail.Mandatory Fixed Part (MF)The mandatory fixed part consists of those parameters that must be present in themessage and that are of a fixed length. Because the parameters are of a fixed lengthand are mandatory, no length indicator is required. In addition, because theparameter types and their order is known from the SCCP message type, noparameter names are required for stating the parameter types.The mandatory fixed part contains pointers to the mandatory variable part and theoptional part of the message. A pointer to the optional part is only included if themessage type permits an optional part. If, on the other hand, the message typepermits an optional part but no optional part is included for that particular message,then a pointer field that contains all zeros is used.NOTEA pointer is simply a single- or double-octet field that contains an offset, that is, acount from the beginning of the pointer to the first octet to which it points.Mandatory Variable Part (MV)The mandatory variable part consists of those parameters that must be present inthe message and that are of a variable length. A pointer is used to indicate the startof each parameter. A length indicator precedes each parameter because theparameters are of a variable length. No parameter tags are required to state theparameter types because the parameter types and their order is explicitly definedby the SCCP message type. The parameters can occur in any order, but theassociated pointers must occur in the same order as specified by the particularmessage type.NOTEThe length indicator value excludes itself and the parameter name.Optional Part (O)The optional part consists of those parameters that are not always necessary. Eachparameter is preceded by a parameter name and a length indicator. The parametername is a unique one-octet field pattern that is used to indicate the parameter type.Because the parameter types and their order are unknown, it is required for eachparameter type.A one-oct ...