SS7 TCAP TRANSACTIONS
A "TCAP MSU" does not exist; if a message is destined for a TCAP application, it is carried in the SIF field of an SCCP MSU. When it receives an MSU with TCAP data, SCCP sends the data to the TCAP router. The TCAP router distributes the data to the correct subsystem (e.g., TCAP service application) running at the signaling point.
The TCAP portion of an SCCP MSU comprises 2 sub-layers:
- Transaction Sublayer (TSL) - Manages the message. Used for identifying the type of message, executing the message, etc.
- Component Sublayer (CSL) - Contains actual content (e.g., instructions for a database or returned information for the requesting application.)
Many common uses of TCAP involve accessing databases that reside at remote SCPs. One example is a database query to determine the secondary number to which a call should be forwarded. This type of remote "end-to-end signaling" (i.e., non-circuit-related signaling between signaling end points) uses SCCP's connectionless classes of service.
When TCAP queries a database, it is called a "transaction." Upon receiving an outbound TCAP message for a database query, SCCP will package it and have MTP Level 3 send it out as an independent message. All messages sent (or received) as part of this transaction are also sent independently; use of the same path between end points will not be required.
Note: In the North American network, use of SCCP's connection-oriented services for TCAP is described, but is not implemented. Connectionless services are what U.S. networks use for communication between TCAP applications.
How do signaling points involved in a TCAP transaction know which messages correspond if a circuit is not dedicated until its completion?
A transaction ID is provided in the Transaction Sublayer (TSL) of a TCAP message. This serves as a reference so that the receiving signaling point can associate a message with a transaction in progress. However, the transaction ID is significant only to the TCAP application that initiates a transaction. The remote point assisting in the transaction merely uses the ID as a reference in all returned responses.
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