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Lecture Software requirements engineering - Lecture­ 22: Requirements specification

After this chapter the student should have acquired the following knowledge and skills: Specifying NFRs, specifying NFRs for safety critical systems, requirements validation, NFRs for critical systems, usability metrics, safety requirement definition, non-functional requirements checklist,. | Requirements Specification Lecture-22 Recap 2 Specifying NFRs Deriving NFRs Concern decomposition Goal decomposition Specifying NFRs for safety critical systems Today’s lecture 3 Specifying NFRs Specifying NFRs for safety critical systems Requirements validation NFRs for critical systems A failure of a critical system may result from a failure to meet a functional requirement but, more often, it results from a failure of the system to satisfy some non-functional constraint on its operation. The principal non-functional constraints which are relevant to critical systems: Reliability Performance Security Usability Safety Reliability Constraints on the run-time behaviour of the system Can be considered under two separate headings: Availability - is the system available for service when requested by end-users. Failure rate - how often does the system fail to deliver the service expected by end-users. Reliability requirements can often be expressed quantitatively. Availability may be . | Requirements Specification Lecture-22 Recap 2 Specifying NFRs Deriving NFRs Concern decomposition Goal decomposition Specifying NFRs for safety critical systems Today’s lecture 3 Specifying NFRs Specifying NFRs for safety critical systems Requirements validation NFRs for critical systems A failure of a critical system may result from a failure to meet a functional requirement but, more often, it results from a failure of the system to satisfy some non-functional constraint on its operation. The principal non-functional constraints which are relevant to critical systems: Reliability Performance Security Usability Safety Reliability Constraints on the run-time behaviour of the system Can be considered under two separate headings: Availability - is the system available for service when requested by end-users. Failure rate - how often does the system fail to deliver the service expected by end-users. Reliability requirements can often be expressed quantitatively. Availability may be specified in terms of the time when the system is unavailable for service (e.g. 3 minutes in 24 hours) The acceptable failure rate may be specified in terms of the rate of occurrence of failures in a given time period (ROCOF) or the mean time between system failures (MTTF). Performance Constrain the speed of operation of a system Types of performance requirements: Response requirements which specify the acceptable response of the system to end-user input: for example, it might be specified that the system should respond to a user request for service within 2 seconds. Throughput requirements which specify the amount of data which must be processed in a given time: for example, it might be specified that the system must process at least 10 transactions per second. Timing requirements which specify how quickly the system must collect input from sensors before it is overwritten by the next input value, or which specify how quickly outputs must be produced for processing by other systems: for .