บทที่ 2. วิศวกรรมระบบ (Systems Engineering) การออกแบบ การพัฒนา และ การนำไปใช้งาน ทั้งฮาร์ดแวร์ (Hardware) ซอฟต์แวร์ (Software) และ พีเพิลแวร์ (People Ware)

วัตถุประสงค์ 1. เพื่ออธิบายหลักการของ system software ซึ่งมีผลต่อขอบข่ายของ system engineering 2. เพื่อแนะนำหลักการและคุณสมบัติของระบบ เช่น ความน่าเชื่อถือ การรักษาความปลอดภัย 3. เพื่ออธิบายสิ่งแวดล้อมของระบบซึ่งต้องคำนึงถึงในการ ออกแบบระบบ 4. เพื่ออธิบายขั้นตอนในการจัดหาระบบ

ระบบ (system) คืออะไร? 1. เป็นการรวบรวมองค์ประกอบ (component) ที่เกี่ยวข้องกัน และทำงานร่วมกันเพื่อทำงานตามจุดประสงค์ 2. ระบบจะรวมถึง software กลไก อุปกรณ์อิเลคทรอนิกส์ ซึ่งควบคุมการทำงานโดยคน 3. ส่วนประกอบของระบบอาจจะขึ้นอยู่กับส่วนของระบบอื่นๆที่เกี่ยวข้องได้ 4. คุณสมบัติและพฤติกรรมขององค์ประกอบ จะประสานแนบ แน่นจนแยกไม่ออก

ระบบและสิ่งแวดล้อมของระบบ 1. ระบบจะไม่เป็นอิสระต่อกันแต่จะคงอยู่ในสิ่งแวดล้อมเดียวกัน 2. หน้าที่ของระบบอาจมีการเปลี่ยนแปลงในสิ่งแวดล้อมของมัน 3. สิ่งแวดล้อมมีผลต่อการทำงานของระบบ

ตัวอย่างของ System hierarchies

Component types in alarm system 1. Sensor : Movement sensor, door sensor. 2. Actuator : Siren. 3.Communication : Telephone caller. 4. Co-ordination : Alarm controller. 5. Interface : Voice synthesizer.

Intruder alarm system

Functional system components 1. Sensor components 2. Actuator components 3. Computation components 4. Communication components 5. Co-ordination components 6. Interface components

องค์ประกอบของระบบ (System components) 1. Sensor components : Collect information from the system’s environment e.g. radars in an air traffic control system. 2. Actuator components : Cause some change in the system’s environment e.g. valves in a process control system which increase or decrease material flow in a pipe. 3. Computation components : Carry out some computations on an input to produce an output e.g. a floating point processor in a computer system.

4. Communication components : Allow system components to communicate with each other e.g. network linking distributed computers. 5. Co-ordination components : Co-ordinate the interactions of other system components e.g. scheduler in a real-time system 6. Interface components : Facilitate the interactions of other system components e.g. operator interface All components are now usually software controlled.

The system engineering process

Inter-disciplinary involvement

นิยามความต้องการของระบบ สามารถนิยามความต้องการดังนี้: 1. Abstract functional requirements. System functions are defined in an abstract way. 2. System properties. Non-functional requirements for the system in general are defined. 3. Undesirable characteristics. Unacceptable system behaviour is specified.

System objectives 1. Functional objectives : To provide a fire and intruder alarm system for the building which will provide internal and external warning of fire or unauthorized intrusion. 2. Organisational objectives : To ensure that the normal functioning of work carried out in the building is not seriously disrupted by events such as fire and unauthorized intrusion.

System requirements problems 1. Changing as the system is being specified. 2. Must anticipate hardware/communications developments over the lifetime of the system. 3. Hard to define non-functional requirements (particularly) without an impression of component structure of the system.

The system design process 1. Partition requirements : Organise requirements into related groups. 2. Identify sub-systems : Identify a set of sub-systems which collectively can meet the system requirements. 3.Assign requirements to sub-systems : Causes particular problems when COTS are integrated. 4. Specify sub-system functionality. 5. Define sub-system interfaces : Critical activity for parallel sub-system development.0

The system design process

System design problems 1. Requirements partitioning to hardware, software and human components may involve a lot of negotiation. 2. Difficult design problems are often assumed to be readily solved using software. 3. Hardware platforms may be inappropriate for software requirements so software must compensate for this.

Sub-system development O Typically parallel projects developing the hardware, software and communications. o May involve some COTS (Commercial Off-the-Shelf) systems procurement. O Lack of communication across implementation teams. o Bureaucratic and slow mechanism for proposing system changes means that the development schedule may be extended because of the need for rework.

System integration O The process of putting hardware, software and people together to make a system. O Should be tackled incrementally so that sub-systems are integrated one at a time. O Interface problems between sub-systems are usually found at this stage. O May be problems with uncoordinated deliveries of system components.

System installation O Environmental assumptions may be incorrect. O May be human resistance to the introduction of a new system. O System may have to coexist with alternative systems for some time. O May be physical installation problems (e.g. cabling problems). O Operator training has to be identified.

System operation bring unforeseen requirements to light, Users may use the system in a way which is not anticipated by system designers, May reveal problems in the interaction with other systems; a. Physical problems of incompatibility, b. Data conversion problems, Increased operator error rate because of inconsistent interfaces.

System evolution Large systems have a long lifetime. They must evolve to meet changing requirements. Evolution is inherently costly; a. Changes must be analysed from a technical and business perspective. b. Sub-systems interact so unanticipated problems can arise. c. There is rarely a rationale for original design decisions. d. System structure is corrupted as changes are made to it. Existing systems which must be maintained are sometimes called legacy systems.

System decommissioning O Taking the system out of service after its useful lifetime. O May require removal of materials (e.g. dangerous chemicals) which pollute the environment : Should be planned for in the system design by encapsulation. O May require data to be restructured and converted to be used in some other system.

System procurement O Acquiring a system for an organization to meet some need. O 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 commercial off-the-shelf (COTS) system. Almost always cheaper than developing a system from scratch

The system procurement process

Procurement issues 1. Requirements may have to be modified to match the capabilities of off-the-shelf components. 2. The requirements specification may be part of the contract for the development of the system. 3. There is usually a contract negotiation period to agree changes after the contractor to build a system has been selected.

Contractors and sub-contractors 1. The procurement of large hardware/software systems is usually based around some principal contractor. 2. Sub-contracts are issued to other suppliers to supply parts of the system. 3. Customer liases with the principal contractor and does not deal directly with sub-contractors.

Contractor/Sub-contractor model