on November 11, 2009 by denpras in SS7, Comments (0)

SS7 : Transaction Capabilities Application Part (TCAP)

We continue to learn SS7 layer TCAP after learning MTP2, MTP3, and SCCP, now we discuss about TCAP. TCAP needed to be learnt because our need to undertstand deep about SMS flow which use non circuit related.

As mentioned Transaction Capabilities Applications Part (TCAP) supports the exchange of non-circuit related data between applications across the SS7 network using the SCCP connectionless service as a transport.

In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming.

The SCP uses TCAP to return a response containing the routing number(s) (or an error or reject component) back to the SSP.
Please see about SS7 architecture if this too further to discuss

Calling card calls are also validated using TCAP query and response messages. When a mobile subscriber roams into a new mobile switching center (MSC) area, the integrated visitor location register requests service profile information from the subscriber’s home location register (HLR) using mobile application part (MAP) information carried within TCAP messages.

TCAP messages are contained within the SCCP portion of an MSU ( Message Signal Unit ). A TCAP message is comprised of a transaction portion and a component portion.

Transaction Portion

The transaction portion also contains the Originating Transaction ID and Responding Transaction ID fields which associate the TCAP transaction with a specific application at the originating and destination signaling points respectively.
The transaction portion contains the package type identifier which have types :

* Unidirectional: Transfers component(s) in one direction only which no reply expected.
* Query with Permission: Initiates a TCAP transaction .
The destination node may end the transaction.
* Query without Permission: Initiates a TCAP transaction. The destination node may not end the transaction.
* Response: Ends the TCAP transaction.
* Conversation with Permission: Continues a TCAP transaction. The destination node may end the transaction.
* Conversation without Permission: Continues a TCAP transaction. The destination node may not end the transaction.
* Abort: Terminates a transaction due to an abnormal situation.

Component Portion

The component portion contains components.

Components include parameters which contain application-specific data carried unexamined by TCAP.

There are six kinds of components:

* Invoke (Last): Invokes an operation. For example, a Query with Permission transaction may include an Invoke (Last) component to request SCP translation of a dialed 800 number. The component is the “last” component in the query.
* Invoke (Not Last): Similar to the Invoke (Last) component except that the component is followed by one or more components.
* Return Result (Last): Returns the result of an invoked operation. The component is the “last” component in the response.
* Return Result (Not Last): Similar to the Return Result (Last) component except that the component is followed by one or more components.
* Return Error: Reports the unsuccessful completion of an invoked operation.
* Reject: Indicates that an incorrect package type or component was received.

TCAP traffic on telephony signaling networks has increased in recent years because of an increase in services such as LNP, Calling Name Delivery, and Short Messaging Service (SMS), which rely on TCAP communication. This upward trend is likely to continue as IN services are more widely deployed, thereby making TCAP an increasingly important component in the role of network services.