TCAP: SS7 Transaction Capabilities Application Part

Transaction Capabilities Application Part (TCAP), a protocol in the SS7 protocol suite, enables the deployment of advanced intelligent network services by supporting non-circuit related information exchange between signaling points using the Signalling Connection Control Part (SCCP) connectionless service. TCAP also supports remote control - ability to invoke features in another remote network switch.

An SSP uses TCAP to query an SCP to determine the routing number(s) associated with a dialed 800, 888, or 900 number. The SCP uses TCAP to return a response containing the routing number(s) (or an error or reject component) back to the SSP. Calling card calls are also validated using TCAP query and response messages. When a mobile subscriber roams into a new mobile switching center (MSC) area, the integrated visitor location register requests service profile information from the subscriber's home location register (HLR) using mobile application part (MAP) information carried within TCAP messages.

The TCAP transaction portion contains the package type identifier with the following package types:

• Unidirectional: Transfers component(s) in one direction only (no reply expected).
• Query with Permission: Initiates a TCAP transaction (e.g., a 1-800 query). The destination node may end the transaction.
• Query without Permission: Initiates a TCAP transaction. The destination node may not end the transaction.
• Response: Ends the TCAP transaction. A response to an 1-800 query with permission may contain the routing number(s) associated with the 800 number.
• Conversation with Permission: Continues a TCAP transaction. The destination node may end the transaction.
• Conversation without Permission: Continues a TCAP transaction. The destination node may not end the transaction.
• Abort: Terminates a transaction due to an abnormal situation.

The transaction portion also contains the Originating Transaction ID and Responding Transaction ID fields which associate the TCAP transaction with a specific application at the originating and destination signaling points respectively.

The TCAP component portion contains componentsas follows:

• Invoke (Last): Invokes an operation. For example, a Query with Permission transaction may include an Invoke (Last) component to request SCP translation of a dialed 800 number. The component is the "last" component in the query.
• Invoke (Not Last): Similar to the Invoke (Last) component except that the component is followed by one or more components.
• Return Result (Last): Returns the result of an invoked operation. The component is the "last" component in the response.
• Return Result (Not Last): Similar to the Return Result (Last) component except that the component is followed by one or more components.
• Return Error: Reports the unsuccessful completion of an invoked operation.
• Reject: Indicates that an incorrect package type or component was received.

Components include parameterswhich contain application-specific data carried unexamined by TCAP.

The TCAP header structure: 

• Information Element - An information element is first interpreted according to its position within the syntax of the message. Each information element within a TCAP message has the same structure. An information element consists of three fields: Tag, Length and Contents.
• Tag - The Tag distinguishes one information element from another and governs the interpretation of the Contents. It may be one or more octets in length. The Tag is composed of Class, Form and Tag codes.
• Length - Specifies the length of the Contents.
• Contents - Contains the substance of the element, containing the primary information the element is intended to convey.

http://www.itu.int/itudoc/itu-t/rec/q/q500-999/q773_24880.html : Q.773: TCAP Specification