This chapter discusses the function of the signalling system in telecommunication networks. Signalling provides the ability to transfer information between customers, within networks and between customers and networks. Signalling is the life-blood, the vitalising influence, of telecommunications networks. It provides the bond that holds together the multitude of transmission links and nodes in a network to provide a cohesive entity. Without signalling, networks are inert. By providing effective signalling systems, a network is transformed into a tremendously powerful medium through which customers can communicate with each other using a range of telecommunications services. The critical nature of signalling is driving its rapid evolution. Old signalling systems that were simple mechanisms for transferring basic information are being replaced by efficient data transfer highways. The ultimate objective is to provide an unimpeded transfer of information between customers, between nodes within networks (inter-nodal or inter-exchange signalling) and between customers and exchanges (access signalling).

CCITT Signalling System No. 6 was the first CCS system to be implemented internationally. It was originally designed for use in the international network, but some flexibility is included to allow its use in national networks. It is an inter-exchange signalling system. CCITT No. 6 has been implemented widely, but it is now being superseded by CCITT No. 7. Thus, a brief description of CCITT No. 6 is given in this chapter. Details of CCITT No. 7 are given in subsequent chapters. CCITT No. 6 offers a wide range of features associated with being a CCS system, including operation in the quasi-associated mode, error detection and correction mechanisms and re routeing capabilities in fault conditions. However, its main drawback is its limited evolutionary potential. In a dynamic environment, CCITT No. 7 offers far greater flexibility and evolutionary capability, particularly due to its structured architecture.

The transfer of signalling information between signalling points is achieved by the message-transfer part (MTP) and the signalling-connection control part (SCCP). The MTP and SCCP do not understand the meaning of the messages being transferred, but it is their job to deliver the information in an uncorrupted form from one signalling point to another. For circuit-related applications (e.g. telephony), the MTP provides an adequate transfer mechanism. For non-circuit-related applications (e.g. general data transfer), a combination of the MTP and SCCP is required. The MTP comprises Levels 1 to 3 of the 4-level structure. The combination of the MTP and SCCP lies within Layers 1 to 3 of the OSI 7-layer model. Sections 5.2 to 5.5 of this chapter describe the functions of the MTP and Section 5.6 covers the SCCP.

Transaction Capabilities (TC) is a protocol that is used, in conjunction with an appropriate network-layer service (e.g. the SCCP and MTP), to provide the non-circuit-related transfer of information across networks. TC is defined by CCITT as part of CCITT No. 7. However, because it is defined in accordance with the OSI 7-layer model, TC is a general protocol that can be applied in a broad range of applications.

The DSS1 network layer (Layer 3) is responsible for establishing, maintaining and clearing circuit-switched traffic channels between customers and ISDN local exchanges. DSS1 Layer 3 can be described in terms of its formats and the procedures, defining the logical sequence of events, in meeting customer requirements. Section 9.2 describes the principles of the format technique and Section 9.3 gives examples of messages to illustrate the principles. The basic procedures for establishing circuit-switched calls are described in Section 9.4 and the procedures for clearing such calls are described in Section 9.5. DSS1 Layer 3 includes features additional to those necessary for establishing and clearing calls and these features are outlined in Section 9.6. DSS1 Layer 3 is required to provide access to packet-data facilities and the means of achieving this requirement are described in Section 9.7. Section 9.8 covers the invocation and transfer of user-to-user signalling over the access signalling channel. Finally, whilst the DSS1 network layer does not specify detailed procedures for the control of supplementary services, three generic procedures are outlined in Section 9.9.

A telecommunications network is implemented to provide services to customers. The objective is to provide features that quickly meet the needs of customers to a high level of quality and at low cost. The basic foundation of a telecommunications network comprises transmission systems and exchanges. Transmission systems provide the ability to transfer speech and data. Exchanges are used to switch transmission links for calls, thus allowing customers to share transmission links. Signalling provides the ability to transfer information between customers and networks, within networks and between customers. Signalling is the life-blood, the vitalising influence, of a network. Signalling information Hows through a network to transform it from an inert aggregate of elements to a powerful medium providing services to customers. Signalling is the bond that makes a network a cohesive force. Signalling systems can be 'channel associated' or 'common channel'. In channel-associated signalling (CAS) systems, signalling capacity is dedicated for each traffic circuit. CAS systems are optimised for use in old-technology networks (e.g. using electromechanical exchanges). In common-channel signalling (CCS) systems, signalling capacity is provided in a common pool and the capacity is allocated dynamically as required. CCS systems are optimised for use in modern-technology networks (using software-control led exchanges). This book has concentrated on describing modern CCS systems.