Common Channel Signaling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T). The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wired call setup, routing and control.
The SS7 protocol stack borrows partially from the Open Systems Interconnection (OSI) Model of a packetized digital protocol stack. OSI layers 1 to 3 are provided by the Message Transfer Part (MTP) and the Signaling Connection Control Part (SCCP) of the SS7 protocol (together referred to as the Network Service Part (NSP)). For circuit-related signaling, such as Telephone User Part (TUP) or ISDN User Part (ISUP), the User Part provides layer 7. Currently there are no protocol components that provide OSI layers 4 through 6. The Transaction Capabilities Application Part (TCAP) is the primary SCCP User in the Core Network and uses SCCP in connectionless mode. SCCP in connection-oriented mode provides the transport layer for air interface protocols such as BSSAP and RANAP. TCAP provides transaction capabilities to its Users (TC-Users), such as the Mobile Application Part, the Intelligent Network Application Part and the CAMEL Application Part. The Message Transfer Part (MTP) covers a portion of the functions of the OSI network layer including: network interface, information transfer, message handling and routing to the higher levels.
There are many switches/equipments/media gateways available today where MTP functionality is taken care of by hardware. But, with increased processing power and reduction in cost per CPU, the MTP layer can be implemented as software, thus greatly reducing the price for all SS7 solutions. There are also switches/media gateways available where MTP2 is software over SS7 hardware.
Furthermore, it is convenient to consider a media gateway as a collection of endpoints. An endpoint is a logical representation of a physical entity, such as an analog phone or a channel in a trunk. Endpoints are sources or sinks of data and can be physical or virtual. Physical endpoint creation requires hardware installation. An interface that terminates a trunk connected to a PSTN switch is an example of a physical endpoint. On the other hand, software is sufficient for creating a virtual endpoint. An audio source in an audio-content server is an example of a virtual endpoint.