3.1 Introduction

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 architecture are outlined in Section 3.2.

Ideally, all modern signalling systems should have been specified using a common architecture, with variances only occurring to handle specific requirements. For example, CCSS7 and DSS1 could have used a common architecture, with the functions for CCSS7 optimised for inter-nodal signalling and the functions for DSS1 optimised for access signalling.

Unfortunately, various architectures have been adopted as technologies have evolved and as long-term objectives have changed. The types of architecture are outlined in Section 3.4. However, the various architectures use a common set of principles and these are explained in Section 3.3. The architectures for CCSS7, DSS1 and DSS2 are described in the following sections.

3.2 Architecture Requirements

A large number of factors influence the architecture adopted for CCS systems. The important requirements are given...