TCAP (Transaction Capabilities Application Part)

Questionnaire on TCAP:

1.) Why use TCAP, what is the sublayers of TCAP.
2.) define the ITU and ANSI TCAP message type?
3.) Define the components portion.
4.) define dialogue portion?
5.) define the Error handling on TCAP.

Answers:

1.) TCAP is used for Non-Circuit related signaling, to retrieve the information from the database in form of dialogue of question and answer. TCAP does not have any transport facility so it use SCCP transport to transfer the message.

TCAP message is composed of following two main section:

transaction Sublayer: A translation is set of related TCAP messages that are exchanged between the network nodes. The transaction portion identifies the messages that belong to the same transaction identifier (TRID).

component Sublayer: The messages' component portion contains the actual instruction i.e. "opeartion" that are being to sent to remote application.

2.)  ITU TCAP Message:

- Unidirectional (TIDpresent: No TID): Sent in one direction and expects no reply
- Begin (TIDpresent: OTID): Start a transaction
- End (TIDpresent: DTID): Ends the Transaction
- Continue (TIDpresent: OTID, DTID): Continues an established transaction
-Abort (TIDpresent: DTID): Sent to notify the transaction is aborted due to some cause like protocol error.

ANSI TCAP Message:

- Unidirectional (TIDpresent: No TID): Sent in one direction and expects no reply
- Query with permission (TIDpresent: OTID): start the transaction , giving the permission to receiving node to end the transaction
- Query without permission (TIDpresent: OTID):  start the transaction , No permission to receiving node to end the transaction
- Conversation with permission( TIDpresent: OTID, DTID)): continues a transaction , giving the permission to receiving node to end the transaction
- Conversation without permission( TIDpresent: OTID, DTID)): continues a transaction , No permission to receiving node to end the transaction
-Abort (TIDpresent: DTID): Sent to notify the transaction is aborted due to some cause like protocol error.

 3.) Components are a means of invoking an operation at a remote node. A TCAP message can contain several components, thereby invoking several operations simultaneously.

Following four Operational Protocol Data Units (OPDUs):

- Invoke - Requests an operation to be performed
- Return Result - Reports the successful completion of a requested operation

it is further divided in
1. Return result last   - The Return Result Last indicates that an operation’s final result has been returned.
2. Return result not Last. -  The Return Result Not Last indicates that further results will be returned
- Return Error -  Reports the unsuccessful completion of a requested operation
- Reject -  Reports a protocol violation, such as an incorrect or badly formed OPDU

The contents of the  components include the following information:
• Component Type
• Component ID-  message can contain several components. Each Invoke Component is coded with a numeric Invoke ID, which must be unique for each operation in progress because the ID is used to correlate the exchange of components for a particular operation.
• Operation Code (Invoke Component only)/ Error/Problem code (Return error/reject)
• Parameters -  Components can have parameters associated with them. The parameters are the data that is necessary to carry out the operation requested by the component Operation Code. For example, a component containing a “Play Announcement” Operation Code also contains an announcement parameter. The announcement parameter typically provides the announcement ID so the correct recording is played to the listener.

4.) Dialogue Portion :It establishes a flow of information within a particular context for a transaction. Information, such as the protocol version and application context, is used to ensure that two nodes interpret the component sublayer’s contents in the same manner using an agreed upon set of element definitions.

ITU Dialogue:

There are two categories of dialogues:
- structured - A structured dialogue requires a reply.
- unstructured. An unstructured dialogue is one in which no reply is expected. This type of dialogue uses a Unidirectional message type at the transaction layer.

The following are four types of APDU (Application protocol date unit):
• Dialogue Request - The Dialogue Request consists of an Application Context Name and, optionally, Protocol Version and User Information. It is used to request dialogue information from another node
• Dialogue Response - The Dialogue Response is sent as a reply to a Dialogue Request.
• Dialogue Abort
• Dialogue Unidirectional - The Unidirectional Dialogue consists of an Application Context Name and optional Protocol Version and User Information. It is used to convey dialogue information in one direction, for which no reply is expected.

5.) TCAP errors falls into three catogory:

- Protocol Errors: Protocol Errors are the result of TCAP messages being incorrectly formed, containing illegal values, or being received when not expected. example of an error would be receiving a responding Transaction ID for a nonexistent transaction.
- Application Errors: Application Errors are anomalies within the application procedure.example is a missing customer record error, which is an error that is used to indicate that a database lookup failed to find the requested information.
- End-User Errors - The End-User Error is similar to the Application Error in that it is an anomaly of the application procedure. However, as indicated by the name, the anomaly is the result of some variance from the normal actions by the user. The user might take an action, such as abandoning the call prematurely.