5 Part 5: V-Modell Reference Work Products

5.3 Products

5.3.9 System Specifications

5.3.9.2 System Specification

Process module: System Development

Responsible: System Architect (when using process module System Development)

Activity: Preparing System Specification

Participating: Logistics Developer, System Integrator, Safety Manager, Ergonomics Manager, Logistics Manager, Inspector

Purpose

The »System Specification describes the functional and non-functional requirements posed on a system element (system, »Enabling System or segment). In order to prepare the System Specification, the requirements will be derived from the specifications of higher system elements or from the Overall System Specification. The specification provides standards and tools for designing and decomposing the architecture. If changes are required in the course of the development of the system element, the System Specification shall be adapted at first. The »Evaluation Specification System Element defines the evaluation cases required for demonstrating the requirements of interfaces and specifications.

The System Specification mainly describes the requirements posed on the system element and specifies the connected interfaces. In addition, requirements and interfaces will be refined and allocated to lower system elements.

The requirements tracing ensures that all requirements posed on the respective elements will be taken into account when the next hierarchy level is refined. The System Specification will be prepared together with the architecture design of the system or a sub-system. The »System Architect is responsible for the preparation of these products, thus ensuring the consistency between specification and architecture.

Requirements of the System Specification may influence the Logistic Support Specification, just as logistic requirements may influence the System Specification.

Is generated by

System Implementation, Integration and Evaluation Concept, System Architecture (see product dependency 4.23)

Enabling System Implementation, Integration, and Evaluation Concept, Enabling System Architecture (see product dependency 4.24)

Overall System Specification (see product dependency 4.25)

Overall System Specification (see product dependency 4.26)

Depends on

Man-Machine Interface (Style Guide), Hardware Specification, Software Specification (see product dependency 5.7)

External Hardware Module Specification, Hardware Specification, Logistic Support Specification, External Software Module Specification, Software Specification, External Unit Specification (see product dependency 5.23)

5.3.9.2.1 System Element Overview

The summary of system elements provides a brief survey of the system element to be realized. It describes tasks and objectives of the system element and its role within the system or »Enabling System.

5.3.9.2.2 Interface Specification

An interface represents the boundary between a system element and its environment. It describes the data exchanged at the system boundary and their logic dependencies. Thus, the interface defines the services to be provided by the system element. One system element can support several interfaces.

The interface description collects all functional requirements posed on the system element, specifies all interfaces and presents them in their environment. Together with the non-functional requirements, the interface description defines the information required for developing the system element. The interface description describes the interfaces to other system elements and the interfaces to the environment, e.g. the man-machine interface or interfaces to »Enabling Systems.

The description of the functional interface is subdivided into the description of the static and dynamic behavior. The static behavior specifies the structure of the interface, through which the functionalities of the system element can be used. The dynamic behavior determines the sequence of use and the logic dependencies of the transmitted data and signals.

Contents and description of the interfaces may vary, depending on the fact as to whether the interface belongs to hardware or software components of the system element. Hardware components will be specified by electrical and mechanical data, while software components will be specified by the description of methods, parameters and information on the behavior.

Static elements of a hardware interface include, e.g., information on electrical performance parameters (power, voltage, current, frequency, polarity), information on the mechanical design (type of connector, connector assignment, type of cable), or information on the technical design (function call and parameter list, transmission device, layout of a user interface). The description of the dynamic behavior includes, e.g., the determination of communication protocols and their specification, the description of synchronization mechanisms and references to the use and operation of the interface.

The static behavior of a software interface determines the structure of the calls through which the services of the software elements can be used. The description is mainly based on method signatures and definitions of data types. The dynamic behavior determines the possible sequence of the calls. The description of the dynamic behavior is frequently based on flowcharts (sequence charts, message sequence charts) or state transition diagrams.

The interface description is based on the summary of interfaces in the architecture and on the interface realizations of the »System Specifications of higher system elements.

The interface description should consider if a re-use of already existing system elements is possible. In addtion, it should be ensured that the interface is stable, thus allowing a long use of the system element.

5.3.9.2.3 Non-Functional Requirements

In addition to the functional requirements, a system element must fulfill several non-functional requirements. Frequently demanded non-functional requirements include quality characteristics like performance, safety and security, availability and maintainability.

The non-functional requirements will be described in detail and specified by the actually required values. The non-functional requirements relevant for the system element will be derived from the specifications of higher system elements or the Overall System Specification.

5.3.9.2.4 Interface Realization

The interface realization refines the functional requirements of the interface description. Requirements and interfaces will be concretized, refined and allocated to the system elements of the lower hierarchy levels.

The interface realization is based on the system architecture or a »Enabling System Architecture of the higher system. The hierarchical structure will be identified in the architectures with in the scope of the decomposition process.

5.3.9.2.5 Refining Non-Functional Requirements

Non-functional requirements are refined in parallel to functional requirements in the interface realization. The non-functional requirements will be concretized, refined and allocated to the system elements of the lower hierarchy level.

The refined requirements remain in existence as autonomous requirements or will be integrated into the interface realization.

5.3.9.2.6 Requirements Tracing

The requirements tracing summarizes the allocation of functional and non-functional requirements posed on the system element to the refined requirements and lower system elements. It is based on the results of the interface realization and the refinement of non-functional requirements. The bi-directional trackability (i.e. from higher to lower system elements and vice versa) must be ensured. The data may be presented, e.g., in form of a matrix.