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 TCP 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). A Continue primitive has an Originating Transaction ID and a Destination Transaction ID. End and Abort primitives 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.