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2.2.2 Interfaces and Protocols

The protocols in the SS7 stack is designed to provide a reliable means of communicating, this means that all messages are to be delivered. The SS7 protocol stack can be described in four levels, notę that what is called “lev-els” in the SS7 stack is not the same as “layers” in the OSI model.

From bottom to top these are:

•    MTP Level 1 (Physical Layer)

•    MTP Level 2 (Data Link Layer)

•    MTP Level 3 (Network Layer)

•    Level 4 Application Layer

MTP 1 through 3 are sometimes bunched together to simply MTP. These provide reliable transfer of circuit-related signaling, in units referred to as Signal Units (SU), between nodes in the SS7 network. The nodes are ad-dressed through point codes (PC) and every message has both an originating PC (OPC) and destination PC (DPC).

At the top we have the application layer. The transport layer is generally considered to be a part of the application layer. The application layer consist of a lot of protocols, different depending on the application. We use SCCP, TCAP and MAP, for SMS and HLR-related messaging.

SCCP is short for Signaling Connection Control Part and is closest to MTP level 3 in the SS7 stack. SCCP encapsulates TCAP and has address-ing information stored used for sending messages between subsystems (also known as applications). SCCP supports addressing on three formats:

•    Point Codę (PC), which are used to identify the SP.

•    Subsystem Number (SSN), used to identify the application on the node.

•    Global Title (GT), can be used to identify both services and phones, MSISDNs are often GTs.

To route to the correct service you need both PC and SSN, but when the procedurę is initiated, a GT is all that is given (you enter a GT when you place a cali for instances). GTs are always a telephone number but the exact format of it can vary. The method to get the correct GT+SSN combination based on the GT is called Global Title Translation, this translation is usualfy only done on a prefix of the GT, if you place an International cali you will be routed to an node dealing with the international calls based on the country codę for instance.

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