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The SS7 stack within the IN Signaling Points consists of four levels

The physical level (Layer one) provides the physical interface, which is typically T1/E1. The data link level of the SS7 protocol stack (MTP2) provides the SS7 network with error detection/correction and sequenced delivery of all SS7 message packets. This layer does not provide the routing, but the mechanisms to ensure the reliable transfer of the data over the network. The network level (MTP3) provides 3 functions, routing, message discrimination and distribution and they all depend on the services of level two. The User Parts represents level four of the SS7 stack and consists of several different protocols. The Telephone User Part (TUP) or ISDN User Part (ISUP) protocols are used for basic telephone call connection and disconnection. The Transaction Capabilities Application Part (TCAP) protocol is used to access network databases supporting the functions required to connect to an external database, perform a query to the database or retrieve information. The Operations, Maintenance and Administration Part (OMAP) is an application entity which uses the services of TCAP. OMAP is used to provide communications control functions throughout the network via a remote operations center terminal. The Mobile Application Part (MAP) provides a mechanism by which cellular subscriber information may be passed from one cellular network to another.