Difference between revisions of "System Requirements"

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This process handles the ontology elements of Table 3.
 
This process handles the ontology elements of Table 3.
  
<center>'''Table 3. Main ontology elements as handled within system requirements definition (SEBoK Original)'''</center>
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[[File:SEBoKv05_KA-SystDef_ontology_elements_system_requirements.png|thumb|600px|center]]
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{|
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|+'''Table 3. Main Ontology Elements as Handled within System Requirements Definition.''' (SEBoK Original)
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!Element
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!Definition/Attributes (Examples)
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|'''System Requirement'''
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|Statement that expresses an expected characteristic or a constraint using a semantic code (such as natural language, mathematical, arithmetic, geometrical expression). A system requirement is feasible, objective, and verifiable.
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----
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Identifier; name; description; parent (result of a stakeholder requirement translation or of a design choice); type (classification); effectivity (immediately applicable, allocated to a later version, etc.); verification method (inspection, analysis, demonstration, test, etc.); decision (result of a trade-off study having led to choosing or rejecting the requirement); status (identified, analyzed, verified, allocated, satisfied). Criticality (importance related to the safety and the availability of the system); weighing/priority (relative importance compared to others); history records (date, author, identification, and contents of the modification, type of modification, reason for the modification); comments.
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|-
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|'''Rationale'''
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|Argument that provides the justification for the selection of an engineering element.
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----
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Identifier; name; description (rational, reasons for a requirement to be satisfied).
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|-
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|'''Scenario'''
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|A set of actions/functions representing the dynamic of exchanges between the functions allowing the system to achieve a mission or a service.
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----
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Identifier; name; description.
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|-
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|'''Risk'''
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|An event having a probability of occurrence and gravity degree on its consequence onto the system mission or on other characteristics (used for technical risk in engineering). A risk is the combination of vulnerability and of a danger or a threat.
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----
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Identifier; name; description; status.
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===Checking and Correctness of System Requirements===
 
===Checking and Correctness of System Requirements===

Revision as of 18:28, 2 August 2012

The System requirements are all of the requirements at the “system level” that have been properly translated from the list of stakeholders' requirements. The System Requirements form the basis of System Architecture and Design, verification, validation, and stakeholder acceptance.

System Requirements play major roles in the engineering activities. They serve:

  • as the essential inputs of the System Architecture and Design activities.
  • as reference for the System Validation activities.
  • as a communication means, between the various technical staff that interact within the project.

Principles Governing System Requirements

Origin in Stakeholder Requirements

The set of Stakeholder Requirements may contain vague, ambiguous, and qualitative “user-oriented” need statements that are difficult to use for design or to verify. Each of these requirements may need to be further clarified and translated into “engineering-oriented” language to enable proper architecture definition, design and verification activities. The System Requirements resulting from this translation are expressed in technical language useful for architecture and design; unambiguous, consistent, coherent, exhaustive, and verifiable. Of course, close coordination with the stakeholders is necessary to ensure the translation is accurate.

As an example, a need or an expectation such as "To easily maneuver a car to park it," will be transformed in a set of Stakeholder Requirements such as, "Increase the drivability of the car", "Decrease the effort for handling", "Assist the piloting", "Protect the coachwork against shocks or scratch", etc. Translating, for example, the Stakeholder Requirement "Increase the drivability of the car", results in a set of System Requirements specifying measurable characteristics such as the turning circle (steering lock), the wheelbase, etc.

Traceability and assignment of System Requirements during architecture and design

Requirements traceability provides the ability to trace information from the origin of the Stakeholder Requirements at the top level to the lowest level of the system hierarchy - see section "Top-down and recursive approach to system decomposition" in the System Definition article. Traceability is also used to provide an understanding of the extent of a change as an input to impact analyses conducted with respect to proposed engineering improvements or requests for change.

During architecture definition and design, the assignment of requirements from one level to lower levels in the system hierarchy can be accomplished using several methods, as appropriate - see Table 1.


Table 1. Assessment Types for a System Requirement. (SEBoK Original)
Assignment Type for a System Requirement Description
Direct Assignment The system requirement from the higher level is directly assigned to a system or a system element for a lower level (for example, the color used to paint visible parts of the product).
Indirect Assignment (Simply Decomposed) The system requirement is distributed across several systems or system elements and the sum or a more complex calculation for distribution is equal to the requirement of higher level (for example, a mass requirement, power distribution, reliability allocation, etc.) with sufficient margin or tolerance. A documented and configuration-managed "assignment budget" for each assignment must be maintained.
Indirect Assignment (Modeled and Decomposed) The system requirement is distributed to several systems or system elements using an analysis or mathematical modeling technique, and the resulting design parameters are assigned to the appropriate systems or system elements (with appropriate margin). For example, a radar detection requirement may be analyzed; lower-level parameters for output power, beam size, frequencies, etc., will then be assigned to the appropriate hardware and software elements. Again, the analysis (or model) must be documented and configuration-managed.
Derived Requirement (from Design) Such system requirements are developed during the design activities as a result of decision of the design team, not the stakeholder community. These requirements may include the use of commercial-off-the-shelf (COTS) items, existing systems or system elements in inventory, common components, and similar design decisions in order to produce a "best value" solution for the customer. As such these derived requirements may not directly trace to a stakeholder requirement, but they do not conflict with a stakeholder requirement or a constraint.

Classification of System Requirements

Several classifications of System Requirements are possible, depending on the requirements definition methods and/or the architecture and design methods used. (ISO/IEC. 2011) provides a classification summarized below – see references for other interesting classifications. An example is given in Table 2.


Table 2. Example of System Requirements Classification. (SEBoK Original)
Types of System Requirement Description
Functional Requirements Describe qualitatively the system functions or tasks to be performed in operation.
Performance Requirements Define quantitatively the extent, or how well, and under what conditions a function or task is to be performed (e.g. rates, velocities). These are quantitative requirements of system performance and are verifiable individually. Note that there may be more than one performance requirement associated with a single function, functional requirement, or task.
Usability Requirements Define quality in use such as measurable effectiveness, efficiency, and satisfaction criteria.
Interface Requirements Define how the system is required to interact or to exchange material, energy, or information with external systems (external interface), or how system elements within the system, including human elements, interact with each other (internal interface). Interface requirements include physical connections (physical interfaces) with external systems or internal system elements supporting interactions or exchanges.
Operational Requirements Define operational conditions or properties under which the system is required to operate or exist. This type of requirement includes human factors and ergonomics, availability, maintainability, reliability, security.
Modes and/or States Requirements Define the various operational modes of the system in use, and events conducting to transitions of modes.
Adaptability Requirements Define potential extension, growth, or scalability during the lift of the system.
Physical Constraints Define constraints on weight, volume, and dimension applicable on system elements that compose the system.
Design Constraints Define the limits on the options open to a designer of a solution by imposing immovable boundaries and limits (e.g., the system shall incorporate a legacy or provided system element, or certain data shall be maintained in an online repository).
Environmental Conditions Define the environmental conditions to be encountered by the system in its different operational modes. Should be addressed the natural environment (e.g. wind, rain, temperature, fauna, salt, dust, radiation, etc.), induced and/or self-induced environment (e.g. motion, shock, noise, electromagnetism, thermal, etc.), threats societal environment (e.g. legal, political, economic, social, business, etc.).
Logistical Requirements Define the logistical conditions needed by the continuous utilization of the system. These requirements include sustainment (provision of facilities, level support, support personnel, spare parts, training, technical documentation, etc.), packaging, handling, shipping, transportation.
Policies and Regulations Define relevant and applicable organizational policies or regulatory requirements that could affect the operation or performance of the system (e.g. labor policies, reports to regulatory agony, health or safety criteria, etc.).
Cost and Schedule Constraints Define, for example, the cost of a single exemplar of the system, the expected delivery date of the first exemplar, etc.

Process Approach – System Requirements

Purpose and Principle of the Approach

The purpose of the system requirements analysis process is to transform the stakeholder, user-oriented view of desired services and properties into a technical view of the product that meets the operational needs of the user. This process builds a representation of the system that will meet Stakeholder Requirements and that, as far as constraints permit, does not imply any specific implementation. It results in measurable System Requirements that specify, from the supplier’s perspective, what performance and non-performance characteristics it must possess in order to satisfy stakeholders' requirements. (ISO/IEC. 2008)

Activities of the process

Major activities and tasks performed during this process include:

  1. Analyze the Stakeholder Requirements to check completeness of expected services and operational scenarios , conditions, Operational Modes, and constraints.
  2. Define the System Requirements and its rationale .
  3. Classify the System Requirements using suggested classifications – see examples above.
  4. Incorporate the derived requirements (coming from architecture and design) into the System Requirements baseline.
  5. Establish the upward traceability with the Stakeholder Requirements.
  6. Verify the quality and completeness of each System Requirement and the consistency of the set of System Requirements.
  7. Validate the content and relevance of each System Requirement against the set of Stakeholder Requirements.
  8. Identify potential risks (or threats and hazards) that could be generated by the System Requirements.
  9. Synthesize, record, and manage the System Requirements and potential associated Risks.

Artifacts and Ontology Elements

This process may create several artifacts such as:

  1. System Requirements Document
  2. System Requirements Justification Document (for traceability purpose)
  3. System Requirements Database, including traceability, analysis, rationale, decisions, and attributes, where appropriate.
  4. System External Interface Requirements Document (this document describes the interfaces of the system with external elements of its context of use; the interface requirements can be integrated or not to the System Requirements Document above).

This process handles the ontology elements of Table 3.


Table 3. Main Ontology Elements as Handled within System Requirements Definition. (SEBoK Original)
Element Definition/Attributes (Examples)
System Requirement Statement that expresses an expected characteristic or a constraint using a semantic code (such as natural language, mathematical, arithmetic, geometrical expression). A system requirement is feasible, objective, and verifiable.

Identifier; name; description; parent (result of a stakeholder requirement translation or of a design choice); type (classification); effectivity (immediately applicable, allocated to a later version, etc.); verification method (inspection, analysis, demonstration, test, etc.); decision (result of a trade-off study having led to choosing or rejecting the requirement); status (identified, analyzed, verified, allocated, satisfied). Criticality (importance related to the safety and the availability of the system); weighing/priority (relative importance compared to others); history records (date, author, identification, and contents of the modification, type of modification, reason for the modification); comments.

Rationale Argument that provides the justification for the selection of an engineering element.

Identifier; name; description (rational, reasons for a requirement to be satisfied).

Scenario A set of actions/functions representing the dynamic of exchanges between the functions allowing the system to achieve a mission or a service.

Identifier; name; description.

Risk An event having a probability of occurrence and gravity degree on its consequence onto the system mission or on other characteristics (used for technical risk in engineering). A risk is the combination of vulnerability and of a danger or a threat.

Identifier; name; description; status.

Checking and Correctness of System Requirements

System Requirements should be checked to gauge whether they are well expressed and appropriate. There are number of characteristics which can be used to check System Requirements. The set of System Requirements can be verified using standard peer review techniques and by comparing each requirement against the set of requirements characteristics listed in Table 2 and Table 3 of section "Presentation and Quality of Requirements".

The requirements can be further validated using the requirements elicitation and rationale capture described in section "Methods and Modeling Techniques."

Methods and Modeling Techniques

Requirements Elicitation and Prototyping

Requirements Elicitation requires user involvement, and can be effective in gaining stakeholder involvement and buy-in. QFD (Quality Function Deployment) and prototyping are two common techniques that can be applied and are defined in this section. In addition, interviews, focus groups, and Delphi techniques are often applied to elicit requirements.

QFD is a powerful technique to elicit requirements and compare design characteristics against user needs (Hauser and Clausing, 1988). The inputs to the QFD application are user needs and operational concepts, so it is essential that the users participate. Users from across the life cycle should be included so that all aspects of user needs are accounted for and prioritized.

Early prototyping can help the users and developers interactively identify functional and operational requirements and user interface constraints. The prototyping allows for realistic user interaction, discovery, and feedback, as well as some sensitivity analysis. This improves the user's understanding of the requirements and increases the probability of satisfying their actual needs.

Capturing Requirements Rationale

One powerful and cost-effective technique to translate Stakeholder Requirements to System Requirements is to capture the rationale for each requirement. Requirements rationale is merely a statement as to why the requirement exists, any assumptions made, the results of related design studies, or any other related supporting information. This supports further requirements analysis and decomposition. The rationale can be captured directly in the requirements database. (Hull, M. E. C., Jackson, K., Dick, A. J. J. 2010).

Some of the benefits include:

  • Reducing the total number of requirements. The process aids in identifying duplicates. Reducing requirements count will reduce project cost and risk.
  • Early exposure of bad assumptions.
  • Removes design implementation. Many poorly written stakeholder requirements are design requirements in disguise, in that the customer is intentionally or unintentionally specifying a candidate implementation.
  • Improves communication with the stakeholder community. By capturing the requirements rationale for all Stakeholders Requirements, the line of communication between the users and the designers is greatly improved. Adapted from (Hooks, I. F., and K. A. Farry. 2000) Chapter 8.

Modeling Techniques

Modeling techniques that can be used when requirements must be detailed or refined, or when they address topics not considered during the Stakeholder Requirements Definition and Mission Analysis include:

  • State-charts models (ISO/IEC. 2011) Section 8.4.2
  • Scenarios modeling (ISO/IEC. 2011) Section 6.2.3.1
  • Simulations, prototyping (ISO/IEC. 2011) Section 6.3.3.2
  • Quality Function Deployment (INCOSE. 2010) p. 83
  • Sequence diagram, Activity diagram, Use case, State machine diagram, Requirements diagram of SysML
  • Functional Flow Block Diagram for Operational Scenario

Presentation and Quality of Requirements

Generally, requirements are provided in a textual form. Guidelines exist for writing good requirements; they include recommendations about syntax of requirements statements, wording (exclusions, representation of concepts, etc.), characteristics(specific, measurable, achievable, feasible, testable, etc.). Refer to (INCOSE. 2010) section 4.2.2.2 and (ISO/IEC. 2011).

There are several characteristics of requirements and sets of requirements that ensure the quality of requirements. These are used both to aid the development of the requirements and to verify the implementation of requirements into the solution. Table 4 provides a list and descriptions of the characteristics for individual requirements and Table 5 provides a list and descriptions of characteristics for a set of requirements, as adapted from (ISO/IEC. 2011) sections 5.2.5 and 5.2.6.

Table 4. Characteristics of Individual Requirements (SEBoK Original)
Table. Charac of Ind Req AF 052312.png


Table 5. Characteristics of a Set of Requirements (SEBoK Original)
Table. Charac of Set of Req AF 052312.png

Requirements in Tables

Requirements may be provided in a table, especially when specifying a set of parameters for the system or a system element. It is good practice to make standard table templates available. For tables, the following conventions apply:

  • Invoke each requirements table in the requirements set that clearly points to the table.
  • Identify each table with a unique title and table number.
  • Include the word “requirements” in the table title.
  • Identify the purpose of the table in the text immediately preceding it and include an explanation of how to read and use the table, including context and units.
  • For independent-dependant variable situations, organize the table in a way that best accommodates the use of the information.
  • Each cell should contain, at most, a single requirement.

Requirements in Flow Charts

Flow charts often contain requirements in a graphical form. These requirements may include logic that must be incorporated into the system, operational requirements, process or procedural requirements, or other situations that are best defined graphically by a sequence of interrelated steps. For flow charts, the following conventions apply:

  • Invoke flow charts in the requirements set that clearly points to the flow chart.
  • Identify each flow chart with a unique title and figure number.
  • Include the word “requirements” in the title of the flow chart
  • Clearly indicate and explain unique symbols that represent requirements in the flow chart.

Requirements in Drawings

Drawings also provide a graphical means to define requirements. The type of requirement defined in a drawing depends on the type of drawing. Following conventions apply:

  • Drawings are used when they can aid in the description of the following:
    • Spatial requirements
    • Interface requirements
    • Layout requirements
  • Invoke drawings in the requirements set that clearly points to the drawing.


Practical Considerations about System Requirements

There are several pitfalls that will inhibit the generation and management of an optimal set of System Requirements. See Table 6.

Table 6. Major pitfalls with definition of System Requirements (SEBoK Original)
SEBoKv05 KA-SystDef pitfalls System Requirements.png

The following proven practices in system requirements engineering have repeatedly been shown to reduce project risk and cost, foster customer satisfaction, and produce successful system development. See Table 7.

Table 7. Proven practices with definition of System Requirements (SEBoK Original)
SEBoKv05 KA-SystDef practices System Requirements.png

References

Works Cited

Hauser, J. and D. Clausing. 1988. "The House of Quality." Harvard Business Review. (May - June 1988).

Hooks, I. F., and K. A. Farry. 2000. Customer-centered products: Creating successful products through smart requirements management. New York, NY, USA: American Management Association.

Hull, M. E. C., Jackson, K., Dick, A. J. J. 2010. Systems Engineering. 3rd ed. London, UK: Springer.

INCOSE. 2011. Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities Version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1.

ISO/IEC/IEEE. 2011. Systems and Software Engineering - Requirements Engineering. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission/ Institute of Electrical and Electronics Engineers (IEEE), (IEC), ISO/IEC/IEEE 29148.

ISO/IEC/IEEE. 2008. Systems and Software Engineering - System Life Cycle Processes. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electronical Commission (IEC), Institute of Electrical and Electronics Engineers. ISO/IEC/IEEE 15288:2008 (E).

Primary References

ISO/IEC/IEEE. 2011. Systems and Software Engineering - Requirements Engineering. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission/ Institute of Electrical and Electronics Engineers (IEEE), (IEC), ISO/IEC/IEEE 29148.

ISO/IEC/IEEE. 2008. Systems and Software Engineering - System Life Cycle Processes. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electronical Commission (IEC), Institute of Electrical and Electronics Engineers. ISO/IEC/IEEE 15288:2008 (E).

INCOSE. 2011. INCOSE Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities. Version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1.

Lamsweerde, A. van. 2009. Requirements Engineering. New York, NY, USA: Wiley.

Additional References

Faisandier, A. 2012. Systems Opportunities and Requirements. Belberaud, France: Sinergy'Com.

Hooks, I.F., and K.A. Farry. 2000. Customer-Centered Products: Creating Successful Products through Smart Requirements Management. New York, NY, USA: American Management Association.

Hull, M.E.C., K. Jackson, A.J.J. Dick. 2010. Systems Engineering. 3rd ed. London, UK: Springer.

Roedler, G., D. Rhodes, C. Jones, and H. Schimmoller. 2010. Systems Engineering Leading Indicators Guide. Version 2.0. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2005-001-03.


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