2.1.1 Scope

Chapter 1 described the dramatic changes that are taking place in communications. This Chapter takes a step into the detail and outlines the principles of signalling. The role of signalling is explained and the importance of bringing networks to life is demonstrated.

2.1.2 Networks

At a physical level, information transfer takes place over transmission links consisting of metal cables (e.g. copper cables), optical fibres or radio spectrum (e.g. mobile radio paths, satellite links). Each user needs to be able to communicate with all other users. This is achieved by providing switches that permit sharing of the transmission links. The combination of switches and transmission links forms the backbone of networks. Readers unfamiliar with the structure and terminology of networks are advised to study Appendix 1 which explains the role of switches and transmission links and introduces the concept of a call.

2.1.3 Signalling

So, what is signalling? Signalling is the lifeblood, the vitalising influence, of networks. Signalling transforms the inert network backbone from a passive group of elements into a live entity. It provides the bond that holds together the multitude of transmission links and nodes in a network to provide a cohesive entity. Through the flow of signalling information the network becomes a tremendously powerful medium that provides customers with an expansive communications ability.

Signalling provides the ability to transfer control information. Control information is any form of data required by the network or user terminal to meet customers' needs. The type of control information...