Transaction Capabilities Application Part
| SS7 protocols by OSI layer | |
| Application | TCAP, CAP, ISUP, ... |
|---|---|
| Network | MTP Level 3 + SCCP |
| Data link | MTP Level 2 |
| Physical | MTP Level 1 |
Transaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the same machines, using Transaction IDs to differentiate these, similar to the way ports facilitate multiplexing connections between the same IP addresses on the Internet.
TCAP uses ASN.1 BER encoding, as well as the protocols it encapsulates, namely MAP in mobile phone networks or INAP in Intelligent Networks.
TCAP messages are sent over the wire between machines. TCAP primitives are sent between the application and the local TCAP stack. All TCAP messages are primitives but there are primitives that are not messages. In other words, some are only transferred inside the local machine. A TCAP primitive is made up of one or more TCAP components.
An ITU-T TCAP primitive may be one of the following types:
A Begin primitive has an Originating Transaction ID (up to 4 bytes). Continues have an Originating Transaction ID and a Destination Transaction ID. Ends and Aborts only have a Destination Transaction ID. Each primitive has both an optional component and (optional) dialogue portions. The component portion for the unidirectional primitive is mandatory.
The dialogue portion carries dialogue or unidialogue control PDUs. For MAP and INAP, dialogue PDU is used which performs establishment and release of dialogues for the application context provided in the primitives. Following primitives are defined for the dialogue PDU:
Each ITU-T TCAP component may be one of the following types:
Invoke components have a signed 7 bit InvokeID which is present in all the other components to identify which invoke they relate to.
TCAP is based on the OSI defined , Remote Operations Services Element protocol.
The transaction ID is a TCAP reference for a set of TCAP operations that are performed within a single dialog. When machine A starts a TCAP dialog with another machine B, the machine A sends a Begin message to machine B. This Begin message contains an Originating Transaction ID, which is the Transaction ID reference for A. When the machine B replies to A with a Continue message it includes A's Transaction ID as the Destination Transaction ID. Furthermore B includes its own Transaction ID as the Originating Transaction ID.
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