Message Format

The MTP3 message consists of the SIO and SIF. As previously discussed, the SIO identifies the user or protocol type (SCCP, TUP, ISUP, or TCAP) and the version of the SS7 protocol (national or international). The SIF is divided into two parts: the routing label, and the user or L4 message. The user message contains the control information of the upper-level protocols, which are discussed in more detail in the SCCP, ISUP, and TCAP sections of this chapter.

The signaling endpoint MTP3 processes use the routing label (RL) to determine the destination address. The RL contains the Destination Point Code (DPC), Originating Point Code (OPC), and Signaling Link Selector (SLS) fields, as illustrated in Figure 4-12.

Figure 4-12. MTP3 Message Format

Figure 4-12. MTP3 Message Format

Number of Bits Used

The following list describes the fields in the RL:

• The DPC identifies the point code or address of the destination endpoint and has an address space of 14 bits.

• The OPC identifies the point code or address of the originating endpoint and has an address space of 14 bits.

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