for initiation and secondly for execution of the specific implementation, design, or technology that is used. the appropriate functions to ensure a successful communication. As These are left to the innovation of the system developer. interconnecting configurations become more complex with diverse The functional components of a computer based system are shown paths through switched telecommunication resources, some of the in Figure 12.2. The processing part, information and memory part, layers may be traversed multiple times in performing relaying and internal communications (comm) part are not constrained by the functions to support the information flow between systems. standards. The OSI elements are additional functions in the com- An overlay of the layered architecture to real systems is shown in munications part that are only invoked when communications with Figure 12.4. Each open system has its local system environment other systems are required. Systems that implement the OSI func- (LSE), which includes the basic processing, information, and com- tions are therefore called Open Systems and can participate in munication components shown earlier in Figure 12.2. Part of the information interchange using the OSI environment. OSI environment (OSIE) is also shown in each system. The AP INITIATION 12.3 Layered architecture 12.3.1 Basic Principles There are many ways to describe and characterise systems for information technology applications. Many arbitrary alternatives could be just as effective, but it is imperative to have as much inherent flexibility as possible in the structure and to have a solution that is globally agreeable. Only then can an architecture be estab- lished as a generic basis that will continue to evolve to accommo- date advancing technology and expanding operational requirements. The concept of layering has been widely accepted. Only a mini- mum number of layers has been defined, thereby keeping the struc- ture basically simple, whilst no individual layer should be SO functionally complex t