# Transaction Capabilities Application Part

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.

## Overview

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:
| Unidirectional | A single primitive with no subsequent primitives. Sometimes referred to as a Notice. |
| Begin | Start a dialog, further primitives will follow. |
| Continue | Send a subsequent primitive on an existing dialog, further primitives will follow. |
| End | The last primitive on an existing dialog, Close an existing dialog. |
| Abort | An error has caused the dialog to close. |
| Cancel | The invoke timer has expired without a response being received (this is a primitive but not a message) |
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:
| AARQ | Dialogue request. For MAP and INAP, AARQ is sent in the Begin primitive with the Invoke component in general, with the application context of MAP/INAP operation's package. |
| AARE | Dialogue response. Sent in response to AARQ in either Continue or End primitives. |
| ABRT | Dialogue abort. |
Each ITU-T TCAP component may be one of the following types:
| Invoke | A new operation is being requested, this may or may not solicit a response |
| Return Result Last | A final response to an Invoke |
| Return Result Not Last | A response to an Invoke, further responses will be sent |
| Return Error | An error occurred |
| Reject | Component is rejected for some reason like duplicate invocation, unrecognized linked id, unrecognized operation or mistyped argument |
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 ROSE, Remote Operations Services Element protocol.

## Transaction ID

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, A sends a Begin message to B. This Begin message contains an Originating Transaction ID, which is the Transaction ID reference for A. When 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.
As the TCAP dialog goes on, each Continue message includes the Transaction ID of the destination machine as the Destination Transaction ID and the Transaction ID of the originating machine as the Originating Transaction ID. When either machine wants to close the dialog, it sends an End message or an Abort message to the other machine. This message contains the Destination Transaction ID only.

## Invoke ID

Invoke ID is a TCAP reference for a specific TCAP operation and must be unique within a dialog.

## Decoded TCAP Message

This is a MO-SMS sent by a MAP layer and the hex stream is taken from TCAP layer.
According to tag length values, this can be decoded as below.