Major changes are occurring in society and the evolution of the Information Age will have a significant impact upon working methods and social behaviour. A key response of networks to these changes is to separate the functions of network and service provision. The aim is to provide a flexible and secure network platform that can support a vast range of services. Signalling provides the ability to transfer control information and it is signalling that is at the heart of the Information Age revolution. This book provides a foundation for the principles of signalling in telecommunications and IP networks. Effective signalling turns inert elements of equipment into a living, cohesive and powerful network that is the lifeblood of meeting the expectations of its customers.

When defining modern signalling systems it is essential to adopt a structured approach for three main reasons: The complexity of modern signalling systems would make a specification exceedingly cumbersome if produced in a monolithic form. Signalling systems need to continue to evolve to match increasingly demanding requirements of customers and networks. A structured approach provides flexibility to handle new services and changes to existing services. A structured specification 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 requirements for an effective archi tecture are outlined in Section 3.2.

The Signalling Connection Control Part (SCCP) defines the functions, additional to the MTP, to meet the Layer 3/4 boundary requirements of the OSI 7-Layer Model. Thus, the combination of the MTP and SCCP provides a Network Service for higher layers. The SCCP is used in non-circuit-related applications.

Networks are evolving to meet the need for more uninhibited information flow. Part of this evolution is a trend towards the use of more non-circuit-related signalling. Transaction Capabilities (TC)ι is a protocol that provides a non-circuit related capability to transfer information between nodes. TC uses the Network Service (Layers 1-3) defined by the OSI Model (explained in Chapter 3), e.g. as provided by the SCCP and MTP.

The intelligent network (IN) is a significant step in providing flexibility in networks. Aspects of call control are centralised in a single functional node, the Service Control Point (SCP). The SCP takes the role of providing call control for a defined set of services, overcoming many of the limitations of traditional networks. The approach of centralising call control intelligence means that the introduction of a new service requires the modification of only a single node. This can be performed very quickly and with minimum risk to the rest of the network. In addition, the central node has available a wide range of information to provide advanced supplementary services. Signalling within the IN is by means of the Intelligent Network Application Part (INAP).

It is essential to provide a modern access signalling capability to take full advantage of the provision of ISDNs. Digital Subscriber Signalling System no.1 (DSS1) has been defined to provide such a capability. DSS1 is defined in terms of a 3-layer model. Layer 1 of DSS1 defines the physical, electrical and functional characteristics of the transmission link between the user and the local node. Two forms of access are defined. The basic access is formed of two B channels (each of 64 kbit/s) and a D channel of 16 kbit/s. The D channel is the signalling channel. The primary access is formed of 30 B channels and a D channel of 64 kbit/s. Layer 2 of DSS1 is responsible for transferring information between the user and the local node. Information is transferred in frames. Frames consist of a number of fields. Flags delimit the frame. The address field identifies the receiver of a command frame or the sender of a response frame. The control field indicates the type of frame being transmitted (I-format, S-format or U-format). The information field contains the information supplied by Layer 3 to be transferred. The check bits are used to detect errors during transmission of the frame.

Communications are a critical component in the competitiveness of business organisations. Such organisations have demanding requirements and these can be met in two ways. One approach is to provide a dedicated private network. Using private automatic branch exchanges (PABXs) at the various sites, a wide range of features can be employed between users at various locations. Over the years, a number of operators have produced signalling systems that specialised in connecting PABXs, e.g. the Digital Private Network Signalling System (DPNSS), produced by British Telecommunications in conjunction with several manufacturers. The Private Signalling System No.1 (PSS1), also known as 'QSIG', has now been specified for inter-PABX signalling by a collaboration of the European Association for Standardising Information and Communications Systems (ECMA), the International Standards Organisation (ISO) and the European Standards Institute (ETSI). A second approach to meet the requirements of business organisations is to use a virtual private network (VPN). In a VPN, the characteristics of a private network are emulated in a public network. Capacity is guaranteed to be provided on demand. Operations and maintenance are incorporated within the public network systems. The VPN solution lies in the specification of the necessary capabilities in appropriate modern CCS systems, e.g. DSS1 and CCSS7. The ITU has specified a Global Virtual Network Service (GVNS) on this basis. GVNS is specified for use within multiple networks and can therefore be applied globally as a basis for providing a worldwide VPN.

This chapter focuses on the higher layers for broadband inter-nodal signalling, namely the Broadband ISDN User Part (B-ISUP). Other standards for broadband signalling are also being proposed. The ITU-T standards are used here to describe the principles of evolving towards broadband operation.

Previous chapters have described the architecture, formats and procedures of the main CCS systems, namely CCSS7, DSS1 and DSS2. There are many principles that are common to the CCS systems. However, Chapter 2 explains that, because DSS1 and DSS2 are optimised for use in the access network and CCSS7 is optimised for inter-nodal use, the two types of system are different when con sidered at a detailed level. This chapter establishes the principles of interworking CCS systems by describing the interworking of CCSS7 and DSS1. Examples of interworking for basic and more complex calls are given.

This chapter discusses the signalling system, its requirements, standards and vision. Networks and services are evolving at a rapid pace, with the aim of meeting, and exceeding, the expectations of customers. Signalling, the vitalising influence of networks, is at the heart of this revolution. Without signalling, networks would be inert and passive aggregates of components. Signalling is the bond that provides dynamism and animation, transforming inert components into a living, cohesive and powerful medium. There is a great deal yet to come and signalling will remain at the heart of the evolutionary process.

Channel Associated Signalling (CAS) systems were once the mainstay of tele communications networks. Over time, they have been replaced by CCS Systems within networks, but some CAS systems are still common for gaining access to networks. Chapter 2 describes the basic tenet of CAS systems, i.e. dedicated sig nalling capacity is provided for each traffic circuit. This appendix describes the principles of six categories of CAS system. Examples are given to illustrate the techniques used. Details are well documented elsewhere .

ITU-T Signalling System No. 6 (CCSS6) 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 that was implemented widely, but it has now been superseded by ITU-T CCSS7. CCSS6 offers a wide range of features associated with CCS systems, including operation in the quasi-associated mode, error detection and correction mechanisms and re-routing capabilities in fault conditions. However, its main drawback is its limited evolutionary potential, caused by its lack of a tiered architecture. In a dynamic environment, CCSS7 offers far greater flexibility and evolutionary capability, particularly due to its structured architecture.

As mentioned in Chapter 13, private network signalling systems evolved to take advantage of the feature-rich nature of PABXs before the advent of virtual private networks (VPNs). A successful system was the Digital Private Network Signalling System (DPNSS). This appendix gives a brief outline of such a private network signalling system. The system is based on three layers, similar to those of DSS1. This appendix focuses on the Layer 3 aspects.