 the following minimal interdependencies in the code. The resulting software tends headings: to be more robust, maintainable and extendable. Emphasis is placed on the fact that no unnecessary implementa- 1. Formal methods. tion constraints are imposed during design to allow a near perfect 2. Structured methods. specification to be produced first. This is a powerful technique 3. Object Oriented methods. which ensures that artificial constraints due to, for example, current 4. Knowledge Based Systems. hardware limitations are not allowed to influence the specification, 25/6 Software systems in telecommunications and therefore will be removed when a redesign is done for a new 3. Resilience and safety critical issues are not paramount. hardware platform. These techniques also provide a mechanism for communication It is likely that, as object oriented methods develop and tools are both with the software development teams, and with the user and/or available to fully support them, then structured methods may well customer, as no prior training is required to understand the diag- be superseded by object oriented methods. rams. The diagram elements have annotations which allow a more rigorous definition of the diagram element to be captured. Using this 25.3.2.1 SDL it is possible to combine the use of formal and structured methods to provide both an easy communication mechanism, and a for- SDL (Specification and Description Language) is a notation recom- malised definition of the software components. mended by the CCITT for the 'unambiguous specification and Structured methods are of most use where a system has one or description of the behaviour of telecommunications systems'. It is more of the following characteristics: to be used to describe both the required and the actual behaviour of telecommunications systems. 1. The system is large and requires splitting into subsystems with SDL has two different notations, a graphic representation clearly defined functionality before proceeding. (SD:/GR) and a textual phrase repr