A highly readable and lucid introduction to the complex subject of signalling enabling the reader to understand detailed signalling specifications and international standards recommendations.
Inspec keywords: expert systems; telecommunication networks; telecommunication computing; database management systems; telecommunication signalling; cost accounting
Other keywords: common-channel signalling; telecommunications life-blood; CCITT signalling system No. 6; telecommunications experts; signalling experts; interworking; remote databases; digital subscriber signalling system No. 1; CCITT signalling system No. 7; specialist subject; glossary; cost-effective services; gaining access; channel-associated signalling systems; chapter summary
Subjects: Communications computing; Information and communication theory; Telecommunication applications; Administration and management
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).
Lack of international standards for channel-associated signalling (CAS) systems in national networks has resulted in a large variety of implementations. However, CAS systems can be divided into six categories. This chapter describes the principles behind the six categories of CAS systems.
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.
Modern signalling systems are very complex and they need to continue to evolve to match more demanding requirements of customers and networks. It is essential that modern signalling systems are flexible enough to handle new services and changes to existing services. To achieve this flexibility, modern CCS (computer communications software) systems are specified in a structured way, thus allowing the specification to evolve in a controlled manner. A structured specification also allows a disciplined approach to design and development, thus easing the process of implementation. The term 'architecture' is used to describe the structured approach to the specification of CCS systems. The architecture of CCS systems is the key to their flexibility and evolutionary capability.
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.
This chapter describes the user part of CCITT No. 7, which utilises the message-transfer part (MTP) to transfer signalling messages through the signalling network. Whereas the MTP provides a comprehensive transfer mechanism, including a dynamic routeing capability, the MTP cannot interpret the meaning of the Level-4 messages being transferred. It is the user part that defines the meaning of the messages that are being transferred and determines the sequence in which messages are sent. It is the user part that also interacts with the call control function within an exchange to establish an overall controlling mechanism for calls. Three user parts have been specified for CCITT No. 7: the telephone user part (TUP) ι, the ISDN user part (ISUP) and the data user part (DUP). All three are defined primarily to control the establishment and release of traffic circuits and they are, therefore, circuit-related in design. The data user part (DUP) was defined in the early development of CCITT No. 7 to control the establishment and release of circuit-switched data calls. The implementation of the DUP is not extensi e, with only a few network operators having implemented dedicated circuit-switched-data networks. Requirements for data networks will in future be handled by the ISUP, with the result that the DUP is unlikely to be used extensively in telecommunications networks. The DUP is therefore not described in this book.
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.
This chapter describes the functions of the physical and data-link layers of digital subscriber signalling system No. 1 (DSS1). Customers need a flexible communications system that can provide increasingly-sophisticated services. In response to this need, network operators throughout the world are implementing integrated services digital networks (ISDNs) that provide customer-to-customer connections using digital transmission links and software-controlled exchanges. The DSS1 is defined to meet the demanding requirement of providing flexible signalling for customers and is a step towards the objective of providing an unimpeded signalling-transfer capability.
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.
This chapter discusses the interworking of CCS systems.
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.