1.2.1. Lower Stack Layers for SS7 Over a Circuit-Switched Network

When transporting SS7 signaling over a circuit-switched network, the lowest three levels of the SS7 stack, called the Message Transfer Part (MTP), provide a reliable but connectionless (datagram or packet style) service for routing messages through the SS7 network. This service is used by the various user parts described in Section 1.2.2, Upper Stack Layers.

The MTP is subdivided into three parts as follows:

• MTP1, also called the signaling data link layer, is concerned with the physical and electrical characteristics of the signaling links. MTP1 corresponds to the physical layer of the OSI model.
• MTP2, also called the signaling link layer, is a data link control protocol that provides for the reliable sequenced delivery of data across a signaling data link. MTP2 corresponds to the data link layer of the OSI model.
• MTP3, also called the signaling network layer, provides for routing data across multiple STPs from control source to control destination. MTP3 corresponds to a part of the network layer of the OSI model.

The connectionless nature of the MTP provides a low-overhead facility tailored to the requirements of telephony. However, the MTP does not provide all the services of the corresponding OSI Network layer. To support Integrated Services Digital Network (ISDN) applications such as network management that requires expanded addressing capability and reliable message transfer, a separate module is provided:

• Signaling Connection Control Part (SCCP), defines a wide variety of network-layer services. SCCP corresponds to part of the network layer of the OSI model.

The MTP and the SCCP together form the Network Service Part (NSP). The resulting split in OSI network functions between MTP and SCCP has the advantage that the higher-overhead SCCP services can be used only when required, and the more efficient MTP services can be used in other applications.

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