em before the final design is produced. The number of descriptions will 25.4.2 Structured methods lifecycle depend on the size and complexity of the system, and should be defined at the start of the design stage. This allows the design stage The structured methods most widely used in the telecommunica- to be broken up into several sub-stages, each sub-stage producing to tions industry are SSADM and Yourdon based techniques. In each new description of the system. case the lifecycle is essentially the same, although the terminology Modelling, simulation and rapid prototyping can all be used with will differ. Figure 25.3 shows the stages used in RT-SASD (Real formal methods, and will be based on the mathematical descriptions Time Structured Analysis Structured Design) a Yourdon extended Lifecycle for each software design technique 25/9 Requirements Identity DFDs, ERDS, STDs Requirements System Testing SCs, DSCs, STDs Produce Test Plans Design Requests for Clarification Code & Unit Test Integrated S/W Requests for Integrate minor changes System System Tested Requests for Bug Fixes Requests for Acceptance Bug Fixes Feedback/Change Requests Release Figure 25.3 Real Time Structured Analysis Structured Design (RT - SASD) method providing a stronger support for the user than classic Your- ified, they need to be classified, SO that commonality amongst objects is separated out. For example, if two objects in the system Modelling, simulation and rapid prototyping may all be used in are cars and trucks, it may be possible to separate out properties conjunction with the main method. The stages where they are best common to both which relate to a vehicle. Cars and trucks are then utilised depends on the nature of the software being developed and simply special types of vehicles, inheriting the properties of a should be decided at project initiation. vehicle, but with some additional properties. This classification is not simple, and can require considerable effort with much rework. 25.4.3 Object oriented methods lifecycle In bo