Requirements Allocation is the act of decomposing higher-level requirements and assigning them to lower-level functions. All requirements of the top-level functions must be met by the aggregate of those for all lower-level functions. Step 3, Functional Analysis and Allocation, of the Systems Engineering Process is where requirements allocation occurs.
Requirements allocation is often difficult to prove when an upper-level performance requirement is achieved through a number of derived requirements. (For instance, system accuracy is composed of derived functional attributes that in sum determine its value.) Consequently, it is extremely important that higher-level requirements are allocated properly, but also that traceability to the originating requirement and rationale for the allocation be recorded and maintained. [1]
Requirements Traceability is an on-going record of the pedigree of requirements imposed on system and subsystem elements. Expressed in terms of “parents” and “children” and recorded on a suitable database. Traceability allows the System Engineer to ascertain rapidly what effects any proposed changes in requirements may have on related requirements at any system level. Because requirements are derived or apportioned among several functions, they must be traceable across functional boundaries to parent and child requirements. Design constraints defined in the Requirements Analysis must also be flowed down to the lower functions. The allocated requirements must be defined in measurable terms, contain applicable go/no go criteria, and be in sufficient detail to be used as design criteria in the subsequent Synthesis activity. [1]
Time-dependent operations are also allocated to the functions. If the total time required for the system to perform an operation is critical, the time allowed for each function to perform its portion of the process must be allocated and the sequence specified. For each sequence, the characteristics of the inputs and outputs between functions must be identified. In completion of the Requirements Loop, as the functional allocations are established they are continually evaluated against the original requirements. In addition, functional allocations are one of the criteria used in parallel activities of functional architecture and interfaces definition. If required, the allocations may be modified as a result of these activities. In some cases, this may reflect into reassessments of the Requirements Analysis results. The allocated requirements along with the associated architecture form the input to the Synthesis activity. Results of the Synthesis are validated against the allocated requirements and occasionally necessitate re-allocation. [1]
AcqLinks and References:
- [1] Space and Missile Systems Center (SMC) Systems Engineering Primer & Handbook
- Requirements Development Checklist
- DAU Systems Engineering Fundamentals Guide
- Space and Missile Systems Center (SMC) Systems Engineering Primer & Handbook
- NASA Systems Engineering Handbook (large 9M file)
- EIA-632 “Processes for Engineering a System” – 7 Jan 99
Updated: 7/17/2021
Rank: G1
