Telecommunications Signalling offers an introduction to the principles of signalling systems along with an in-depth examination of their architecture and appeals to a wide range of readership, from those who want to expand their range of knowledge to communications experts.
Inspec keywords: intelligent networks; protocols; mobile communication; private telephone exchanges; ISDN; telecommunication signalling; Internet
Other keywords: signalling connection control part; intelligent network; inter-PABX signalling; mobile communication; private network; broadband access signalling; DSS1 network layer; CCSS7 message transfer part; channel associated signalling; Internet protocol; ISDN
Subjects: Other computer networks; Protocols; Protocols; Telecommunication
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.
The backbone of networks is provided by switches and transmission links. In circuit switching, a circuit is dedicated to a call for the whole period of a call. In ATM switches, information is split into cells and the cells are transmitted over the transmission links. Each transmission link is shared on a real time basis with many users and a virtual circuit is provided when there is information to transfer. IP networks use routers, search engines and databases. Signalling is the vitalising influence, the lifeblood, of networks. Signalling transforms the inert network backbone into a live entity and provides customers with an expansive communications ability. There are two types of signalling system for telecommunications networks. In Channel Associated Signalling (CAS) systems, signalling capacity is provided on a dedicated basis for each traffic circuit. In Common Channel Signalling (CCS) systems, signalling capacity is provided in a common pool and is allocated on a dynamic basis, as and when required.
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 Message Transfer Part (MTP) consists of Levels 1 to 3 of the 4-level structure adopted for CCSS7. The MTP is responsible for transferring messages between Signalling Points. The objective of the MTP is to transfer messages without loss or duplication and deliver them to the intended destination in an error-free condition and in the sequence in which they were transmitted. The MTP enables a signalling network to meet demanding message transfer requirements, even during fault conditions.
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.
This chapter discusses telecommunications signalling. The ISDN User Part (ISUP) defines the messages and procedures for the control of switched services in integrated services digital networks (ISDNs). The ISUP covers both voice (e.g. telephony) and non-voice (e.g. circuit-switched data) 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.
This chapter discusses mobile communications. The aim of mobile communications systems is to offer services to customers wherever the customers are located. This requires the provision of radio spectrum over the geographic area to be covered. Terrestrial mobile systems aim to achieve this target by providing a network of radio stations.
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).
Network management includes performance management, fault management and configuration management. CCSS7 is a key part of telecommunications networks. More than this, it forms a signalling network in its own right. It is therefore essential that CCSS7 should have a comprehensive management capability. This chapter briefly reviews the management aspects of CCSS7.
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.
The DSS1 Network Layer (Layer 3) is responsible for establishing, maintaining and clearing connections between users and the narrowband ISDN (e.g. between users and local nodes). The procedures in DSS1 control (a) circuit switched connections, (b) user-to-user signalling connections and (c) packet switched connections. The specifications of DSS1 Layer 3 adopt the term 'user' to describe a calling/called party . This chapter follows this terminology.
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.
Customer requirements are becoming more exacting as quality expectations rise and the demand for new services escalates. With the increasing range of services, bandwidth requirements are also increasing. Although 64 kbit/s circuits are adequate for telephony, the emergence of multi-media services is spawning massive increases in bandwidth requirements. The convergence of the computing, IP, broadcast/media and telecommunications industries is consis tent with this demand. This chapter describes the ITU-T signalling platform for broadband networks. Section 14.2 summarises the signalling relations that are available. Section 14.3 describes the architecture for access and inter-nodal signalling. The rest of the chapter focuses on the lower layers of the architecture that provide a platform for broadband signalling, namely the ATM layer and ATM Adaptation layer (AAL). The higher layers (B-ISUP and DSS2) are described in Chapters 15 and 16, respectively. 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.
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.
This chapter describes the Digital Subscriber Signalling System No.2 (DSS2), which defines the ITU-T procedures and formats for establishing and clearing broadband connections in the access network. DSS2 is the broadband equiva lent of DSS1, which is described in Chapter 12. DSS2, in conjunction with the CCSS7 Broadband ISDN User Part (B-ISUP), provides the opportunity for users to invoke broadband services. DSS2 applies to the control of broadband ISDN (B-ISDN) point-to-point calls on virtual channels. The point-to-multipoint mode of operation that is an option for DSS1 is not used in DSS2 at this stage. The procedures are specified at the interface between B-ISDN terminal equipment and the network. The signalling system is allocated its own virtual channel. The procedures and formats for DSS2 are similar to those for DSS1.
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.
As described in Chapter 1, the telecommunications, broadcast/media, computing and Internet Protocol (IP) industries have a mutual goal to generate, store and manipulate information. As the Information Age evolves, the amount of information that is transferred across networks continues to increase substantially and there will be much more focus on data communica tions. The IP technologies are a key part of this evolution. Hence, whereas the focus of this book is on telecommunications signalling, this chapter gives a summary of IP signalling systems. IP technologies were introduced to support open interfaces and facilitate the inter-operability of computers. IP was originally intended to provide a trans parent transport capability over packet-switched data networks, particularly in wide area networks (WANs). A WAN is a data network designed for use over a large geographical area, thus imposing constraints and requirements over the way communications are provided. IP has now expanded to become a provider of communications to a wide range of users. IP is defined to operate primarily in a multiple-network environment. The structure, organisation and routing approaches used in telecommunications and IP networks are different. However, the signalling systems in the two forms of network have many principles in common.
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.
This appendix gives a brief explanation of networks to assist in the under standing of the role of signalling.
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 .
One of the major factors influencing the development of signalling systems in the past was the relationship between signalling and the control functions of nodes. This appendix traces the evolution of signalling systems in relation to call control technologies.
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.
The TUP is an effective system, but the fact that it is designed primarily for telephony-based networks constrains its applicability and it is being replaced by the more flexible ISUP. The TUP Signalling Information Field consists of a Label, a Heading Code and sub-fields. The Label consists of the Routing Label and a Circuit Identification Code (CIC). The Heading Code defines the class of message and, for simple messages, defines the message type. For complex messages, the Heading Code defines the format of the rest of the message. The sub-fields can be mandatory or optional. In both cases, sub-fields can be of fixed or variable length.
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.