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of mission and safety-critical applications development. For DO-178B, for example, traceability must be provided between system requirements and high-level software requirements, high and low level requirements, low-level requirements and tests, tests and code for structural coverage, and from top down and bottom up. The requirements diagram is used to integrate the system models with text based requirements that are typically captured in requirements management tools, to facilitate the required traceability. The UML containment relationship is used to decompose a requirement into its constituent requirements.
Structure in OMG SysML
A clear description of the system and its environment is essential for mission and safety- critical applications. SysML provides block diagrams to support this. The SysML «block» is a general purpose hierarchical structuring mechanism that abstracts away much of the software-specific detail implicit in UML structured classes. Blocks can represent any level of the system hierarchy including the top-level system, a subsystem, or logical or physical component of a system or environment. A SysML block describes a system as a collection of parts and connections between them that enable communication and other forms of interaction. Ports provide access to the internal structure of a block for use when the object is used within the context of a larger structure.
Two diagrams are used to describe block relationships. The Block Definition Diagram (bdd), similar to a traditional class diagram, is used to describe relationships that exist between blocks. The Internal Block Diagram (IBD) is used to describe block internals, which would see an avionics system represented as a block composed of other blocks, including several processing elements, 24 volt power, and two buses. The role names on the association ends correspond to the parts on the IBD.
Each of the components then has a number of flow ports that describe what can flow in and out, connected to other compatible ports to enable the required flows in this context.
The SysML allocation relationship allocate one model element to another. Allocation is the term used by systems engineers to denote the organised cross-association (mapping) of elements within the various structures or hierarchies of a user model. Often this is the allocation of function to form, such as the deployment of software on a hardware platform, or a use case to an organisation or system entity or a logical to a physical entity.
Allocations can be used early in the design as a precursor to more detailed rigorous specifications and implementations. The allocation relationship can provide an effective means for navigating the model by establishing cross relationships and ensuring the various parts of the model are properly integrated.
Fig. 2. Alstom Transport chose Artisan Studio as the core platform for its global systems engineering strategy based on SysML.
The integration of the software and hardware models means that safety integrity levels (SIL) for the various parts can be assigned and verified to ensure a consistent implementation. Hardware/software interfaces can also be verified, as well as architectural constraints, thus ensuring traceability and change analysis. Architectural frameworks are hugely important in defence procurement and a large proportion of mission and safety-critical applications are developed for military purposes using well-defined architectural frameworks, in particular the US Department of Defense Architecture Framework (DoDAF) and similarly the UK’s Ministry of Defence Architecture Framework (MODAF), as the standard means for describing them in model-based design tools. A version of the tool responds directly to the architectural framework needs of the US DoD, its suppliers and DoDAF users. It provides all the modelling features design engineers need when applying DoDAF/MODAF. UML, SysML, Data Modelling and the DoDAF profile are all packaged in a single, enterprise- scale design architecture modeling tool. Its underlying repository ensures architectural consistency across the full range of DoDAF views.
Fortunately, the use of UML/SysML, as an underlying mechanism for these frameworks, has made it feasible to work towards a unified UML/SysML profile for DoDAF/MODAF which is being fast-tracked through the OMG by the UPDM Group.. m
Matthew Hause is Chief Consultant at Artisan Software Tools in Cheltenham, UK, and Co-Chair of the UPDM Group.
www.ArtisanSoftwareTools.com
www.engineerlive.com
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