1.2.2. Upper Stack Layers

The upper parts of the SS7 protocol stack are concerned with the actual contents of the SS7 messages and are sometimes called application layers. These include:

• ISDN User Part (ISUP), provides the signaling needed for basic ISDN circuit-mode bearer services as well as ISDN supplementary services having end-to-end significance. ISUP is the protocol that supports ISDN in the Public Switched Telephone Network. It corresponds to the transport, session, presentation, application layers and part of the network layer of the OSI model.
• Telephony User Part (TUP), an ISUP predecessor in providing telephony signaling functions. TUP has now been made obsolete by ISUP in most countries and in the international network. The TUP corresponds to the transport, session, presentation, application layers and part of the network layer of the OSI model.
• Transaction Capabilities Application Part (TCAP), provides the mechanisms for transaction-oriented (rather than connection-oriented) applications and functions. The TCAP corresponds to the application layer in the OSI model. TCAP is often used for database access by the SS7 switches but has many other applications through the network.
• Operations and Maintenance Application Part (OMAP), specifies network management functions and messages related to operations and maintenance. The OMAP corresponds to the application layer in the OSI model.
• Application Service Elements (ASEs), represent user parts that are highly application-specific, for example:
  • Intelligent Network Application Part (INAP)
  • Mobile Application Part (MAP), provides the signaling functions necessary for the mobile capabilities of voice and non-voice applications in a mobile network
  • IS41, an ANSI signaling standard used in cellular networks

For any application, all three MTP layers and at least one application layer are required. Typically, the word "user" in modules such as ISUP, TUP and so on explicitly identifies the module as a user of the transport mechanism MTP.

SS7 computer telephony applications that transport SS7 signaling over a circuit-switched network can use the ISUP (on top of the MTP layers) to control voice circuits, and sometimes TCAP to query for information or to receive commands from a SCP.

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