Framework for Expressing Non-functional Requirements in System Engineering

System requirements are classified as FRs (Functional Requirements) and NFRs (Non-Functional Requirements) [1]. FRs focus on system goals. NFRs put constraints on the system. NFRs are often expressed in the form of properties that a system must fulfill in the process of realization of FRs. NFRs are usually categorized under headers such as performance, reliability, quality, security, maintainability without any mechanism to associate with the corresponding FRs in system architecture. This leads to challenges in further decomposition of requirements at subsystem levels. The paper proposes an approach for MBSE where NFRs can be identified and represented in a layered system architecture design. The proposed approach bifurcates NFRs to derive functions using SysML (extension to UML) to provide a functional or structural solution. The derived functions from NFRs are then either linked with an existing function or a new function is created in architecture design. The result is refined NFRs, which improve overall system architecture design quality.