【正文】 siness perspective ? Subsystems interact so unanticipated problems can arise ? There is rarely a rationale for original design decisions ? System structure is corrupted as changes are made to it ? Existing systems which must be maintained are sometimes called legacy systems 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 40 System demissioning ? Taking the system out of service after its useful lifetime ? May require removal of materials (. dangerous chemicals) which pollute the environment ? Should be planned for in the system design by encapsulation ? May require data to be restructured and converted to be used in some other system 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 41 System procurement ? Acquiring a system for an anization to meet some need ? Some system specification and architectural design is usually necessary before procurement ? You need a specification to let a contract for system development ? The specification may allow you to buy a mercial offtheshelf (COTS) system. Almost always cheaper than developing a system from scratch 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 42 The system procurement process C h o o s es u p p l i e rI s s u e r e q u e s tf o r b i d sC h o o s es y s t e mA d a p tr e q u i r e m e n t sS u r v e y m a r k e t f o re x i s t i n g s y s t e m sL e t c o n t r a c t f o rd e v e l o p m e n tN e g o t i a t ec o n t r a c tS e l e c tt e n d e rI s s u e r e q u e s tt o t e n d e rO f f t h e s h e l fs y s t e m a v a i l a b l eB e s p o k e s y s t e mr e q u i r e d169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 43 Procurement issues ? Requirements may have to be modified to match the capabilities of offtheshelf ponents ? The requirements specification may be part of the contract for the development of the system ? There is usually a contract negotiation period to agree changes after the contractor to build a system has been selected 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 44 Contractors and subcontractors ? The procurement of large hardware/software systems is usually based around some principal contractor ? Subcontracts are issued to other suppliers to supply parts of the system ? Customer liases with the principal contractor and does not deal directly with subcontractors 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 45 Contractor/Subcontractor model S u b c o n t r a c t o r 2S u b c o n t r a c t o r 1 S u b c o n t r a c t o r 3P r i n c i p a lc o n t r a c t o rS y s t e mc u s t o m e r169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 46 Key points ? System engineering involves input from a range of disciplines ? Emergent properties are properties that are characteristic of the system as a whole and not its ponent parts ? System architectural models show major subsystems and interconnections. They are usually described using block diagrams 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 47 Key points ? System ponent types are sensor, actuator, putation, coordination, munication and interface ? The systems engineering process is usually a waterfall model and includes specification, design, development and integration. ? System procurement is concerned with deciding which system to buy and who to buy it from 169。Ian Sommerville 2021 Software Engineering, 6th edition. Chapter 2 Slide 48 ? Systems engineering is hard! There will never be an easy answer to the problems of plex system development ? Software engineers do not have all the answers but may be better at taking a systems viewpoint ? Disciplines need to recognise each others strengths and actively rather than reluctantly cooperate in the systems engineering process Conclusio