The Signaling Connection Control Part (SCCP) is defined in ITU-T Recommendations Q.711-Q.716 [58–63] and for North American markets in ANSI T1.112 [2]. SCCP sits on top of Message Transfer Part 3 (MTP3) in the SS7 protocol stack. The SCCP provides additional network layer functions to provide transfer of noncircuit-related (NCR) signaling information, application management procedures and alternative and more flexible methods of routing.
As shown in Figure 9-1, the combination of the MTP, and the SCCP is termed the Network Service Part (NSP). The NSP follows the principles of the OSI reference model, as defined in Recommendation X.200 [99]; as such, it provides a subset of the Layer 3 services, which are defined in Recommendation X.213 [100].
SCCP was developed after the MTP, and together with the MTP3, it provides the capabilities corresponding to Layer 3 of the OSI reference model.
Because SCCP is OSI Layer 3 compliant, in theory it can be transmitted over any OSI-compliant network.
Because the MTP was originally designed to transfer call-control messages coming from the Telephony User Part (TUP), it was, therefore, designed to transfer only circuit-related signaling. In combination with the MTP, the SCCP can transfer messages that are not circuit-related. These messages are used to support services such as toll-free calling, Local Number Portability (LNP) and Completion of Calls to Busy Subscribers (CCBS) in Intelligent Networks and mobility, roaming, and SMS in cellular networks.
SCCP provides the following additional capabilities over the MTP:
- Enhances MTP to meet OSI Layer 3
- Powerful and flexible routing mechanisms
- Enhanced transfer capability, including segmentation/reassembly when message is too large to fit into one Message Signal Unit (MSU)
- Connectionless and connection-oriented data transfer services
- Management and addressing of subsystems (primarily database-driven applications)
SCCP is used extensively in cellular networks. Base Station Subsystem Mobile Application Part (BSSMAP) and Direct Transfer Application Part (DTAP) use it to transfer radio-related messages in Global System for Mobile communication (GSM). In conjunction with Transfer Capabilities Application Part (TCAP), SCCP is also used throughout the GSM Network Switching Subsystem (NSS) to transport Mobile Application Part (MAP) signaling between the core GSM components to enable subscriber mobility and text messaging (SMS), among other items. For example, when the Visitor Location Register (VLR) queries the Home Location Register (HLR) to obtain the subscriber's profile, SCCP is responsible for transferring both the query and the response back to the VLR. For more information about GSM, see Chapter 13, "GSM and ANSI-41 Mobile Application Part (MAP)."
Cellular intelligent network protocols, Wireless Intelligent Network (WIN), and Customizable Applications for Mobile Enhanced Logic (CAMEL) also use SCCP with TCAP (see Chapter 10, "Transaction Capabilities Application Part [TCAP]") to provide intelligent network functionality in a cellular environment. Figure 9-2 shows a typical cellular protocol stack, as found at a GSM-MSC.
Fixed-line networks primarily use SCCP for intelligent network applications and advanced supplementary services. Fixed-line intelligent networks use Advanced Intelligent Network (AIN) within North America and Intelligent Network Application Protocol (INAP) outside of North America (see Chapter 11, "Intelligent Networks [IN]"). AIN/INAP both use SCCP's transport, application management, and enhanced routing functionalities. Two example supplementary services that require the use of SCCP include CCBS and Completion of Calls on No Reply (CCNR).
This chapter looks at the functions of SCCP in some detail, beginning with an outline of the SCCP architecture and then moving onto protocol classes, connectionless and connection-oriented procedures, SCCP management functions, and most importantly, SCCP routing, including the use of global titles.
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