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SS7 Stack



ISUP (p. 2)



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Transaction Capabilities Application Part

TCAP enables the deployment of advanced intelligent network services by supporting non-circuit related information exchange between signaling points using the SCCP connectionless service. An SSP uses TCAP to query an SCP to determine the routing number(s) associated with a dialed 800, 888, or 900 number. The SCP uses TCAP to return a response containing the routing number(s) (or an error or reject component) back to the SSP. 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. A TCAP message is comprised of a transaction portion and a component portion.

Learn about enhanced GSM roaming applications.

Transaction Portion

The transaction portion contains the package type identifier. There are seven package types:

  • Unidirectional: Transfers component(s) in one direction only (no reply expected).
  • Query with Permission: Initiates a TCAP transaction (e.g., a 1-800 query). 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. A response to an 1-800 query with permission may contain the routing number(s) associated with the 800 number.
  • 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.

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.

Component Portion

The component portion contains components. 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.

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

SS7 over IP

Telephone companies offload voice calls from public switched telephone networks (PSTNs) to voice-over-Internet Protocol (VoIP) networks because it is cheaper to carry voice traffic over Internet Protocol (IP) networks than over switched circuit networks. In the future, IP telephony networks are expected to enable innovative new multimedia services while working seamlessly with legacy telephone networks.

A VoIP network carries voice traffic cheaper than a switched circuit telephone network because IP telephony networks make better use of available bandwidth. In a public switched telephone network, for example, a dedicated 64 kilobits per second (kbps) end-to-end circuit is allocated for each call. In a VoIP network, digitized voice data is highly compressed and carried in packets over IP networks. Using the same bandwidth, a VoIP network can carry many times the number of voice calls as a switched circuit network with better voice quality. The savings realized in using VoIP networks are often passed onto users in the form of lower costs.

Learn more about reducing costs by routing SS7 over IP.


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