network protocols handbook.pdf

315Protocols GuideSS7 / C7 ProtocolProtocol NameTCAP: Transaction CapabilitiesApplication PartProtocol DescriptionTransaction Capabilities Application Part (TCAP), a protocol inthe SS7 protocol suite, enables the deployment of advancedintelligent network services by supporting non-circuit relatedinformation exchange between signaling points using the SignallingConnection Control Part (SCCP) connectionless service.TCAP also supports remote control—ability to invoke features inanother remote network switch.An SSP uses TCAP to query an SCP to determine the routingnumber(s) associated with a dialed 800, 888, or 900 number.The SCP uses TCAP to return a response containing the routingnumber(s) (or an error or reject component) back to the SSP.Calling card calls are also validated using TCAP query and responsemessages. When a mobile subscriber roams into a newmobile switching center (MSC) area, the integrated visitor locationregister requests service profile information from the subscriber’shome location register (HLR) using mobile applicationpart (MAP) information carried within TCAP messages.TCAP messages are contained within the SCCP portion of anMSU. A TCAP message is comprised of a transaction portionand a component portion.Protocol StructureThe TCAP transaction portion contains the package type identifierwith the following package types:• Unidirectional: Transfers component(s) in one directiononly (no reply expected).• Query with Permission: Initiates a TCAP transaction(e.g., a 1-800 query). The destination node may end thetransaction.• Query without Permission: Initiates a TCAP transaction.The destination node may not end the transaction.• Response: Ends the TCAP transaction. A response toa 1-800 query with permission may contain the routingnumber(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 TCAPtransaction. The destination node may not end thetransaction.• Abort: Terminates a transaction due to an abnormal situation.The transaction portion also contains the Originating TransactionID and Responding Transaction ID fields, which associatethe TCAP transaction with a specific application at the originatingand destination signaling points respectively.The TCAP component portion contains components as follows:• Invoke (Last): Invokes an operation. For example, aQuery with Permission transaction may include an Invoke(Last) component to request SCP translation of adialed 800 number. The component is the “last” componentin the query.• Invoke (Not Last): Similar to the Invoke (Last) componentexcept that the component is followed by one ormore components.• Return Result (Last): Returns the result of an invokedoperation. The component is the “last” component in theresponse.• Return Result (Not Last): Similar to the Return Result(Last) component except that the component is followedby one or more components.• Return Error: Reports the unsuccessful completion of aninvoked operation.• Reject: Indicates that an incorrect package type or componentwas received.Components include parameters which contain application-specificdata carried unexamined by TCAP.The TCAP header structure:InformationElementTagLengthContentsTag Length ContentsFigure 2-30: TCAP Protocol StructureTagLengthContents• Information Element - An information element is firstinterpreted according to its position within the syntaxof the message. Each information element within aTCAP message has the same structure. An informationelement consists of three fields: Tag, Length andContents.• Tag - The Tag distinguishes one information elementfrom another and governs the interpretation of theContents. It may be one or more octets in length. TheTag 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 intendedto convey.