The MSU provides the structure for transmitting circuit- and non-circuit based messages in the SS7 network. You use circuit-based messages to set up, manage, and release telephone calls. Non-circuit based messages refer to queries for additional routing information and network management data. MSUs originate from MTP3 or from an MTP3 user. MTP3 users include SCCP, ISUP, TUP, and TCAP. These user messages are transferred between two peer L4 protocols in signaling endpoints.

In the case of ISUP, the two endpoints transfer ISUP messages over the SS7 network. An MSU with a routing label carries the ISUP information. The routing label contains the point code addresses of the originating endpoint and destination endpoint.

The originating endpoint passes the ISUP information to MTP3. MTP3 expands the MTP3 message and passes the message to MTP2. MTP2 expands the MTP3 message in an MSU. At this point, the MSU is passed to MTP1 for transmission across the signaling link. The destination endpoint receives the MTP1 message and MTP2 extracts the MTP3 message. The L4 protocol or user data is identified and the message is passed to the ISUP process of the destination endpoint.

The MSU has the same fields as the FISU, with the addition of the SIO and SIF, as illustrated in Figure 4-11.

Figure 4-11. MSU Fields










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