Design · 3.1
From use case to verifiable requirement
Time: about 11–14 minutes including the task; estimate
Guiding question: How does an operational intention become traceable evidence?
Learning objective: You translate a use case into a verifiable requirement with traceability.
Prerequisites: Previous lessons through 2.22, particularly the case file developed there.
The familiar role handover becomes binding
In 2.7 Amir took over the role for train 1234 from Lea. Talbahn wants to prevent the wrong person from being reached after a confirmed change. “Role change must work” is too vague. It lacks an observable result, the state being considered and conflict handling.
A requirement describes a property to be fulfilled or required behaviour. It must be bounded so the parties can assess the same claim. Traceability means the documented, followable links between need, requirement, implementation and evidence. It reveals why something is built and which evidence supports its fulfilment.
One row is enough to start
The following row is a teaching draft, not a normative FRMCS requirement:
| Field | TB-REQ-017 |
|---|---|
| Operational need | A call to the role of train 1234 reaches the responsible person after handover |
| Requirement | After confirmed takeover by Amir, newly initiated calls to this role address Amir's authorised session; Lea's previous assignment no longer applies to them |
| Scope | One FRMCS service domain, two authorised users, newly initiated calls |
| Outside this row | Already ongoing calls, cross-border assignment, interworking |
| Implementation | Assignment management and participating clients; specific product versions still to be entered |
| Test | TB-T-017: record states before, during and after handover; then initiate a role-based call |
| Open decision | When is takeover considered confirmed? What happens with conflicting or simultaneous registrations? |
| Responsible | Operational owner for expected behaviour; system owner for implementation and evidence |
Identifiers link documents without copying the same content repeatedly. A requirement may need several tests; a test may concern several requirements. These relationships are not necessarily one-to-one.
Take the unresolved word seriously
“Confirmed” is the decisive reference point. An indication on Amir's device and an effective assignment in the service may occur at different times. While it remains unclear which state counts, the supplier and Talbahn may regard different results as successful.
Talbahn therefore adds a decision on confirmation and establishes a permissible transition time before accepting the test. It must not take the value from a generic 5G marketing promise. It derives it from operational needs and applicable requirements.
The conflict case also needs a decision. Options might include rejecting a second takeover or controlled replacement after a permission check. The applicable variant must be defined by the specification and project; this example does not prescribe a general FRMCS rule.
Changes have consequences for evidence
If confirmation or handling of ongoing calls changes, Talbahn must be able to locate affected requirements, implementations and tests. This is exactly where traceability helps. An unconnected list of passed tests does not reliably reveal those effects.
In the case file, TB-REQ-017 remains a draft until its open points are resolved. Its status prevents a seemingly precise identifier from being confused with an approved requirement.
Check your understanding
A test report states “TB-T-017 passed”, but contains only one successful call to Amir. Which gaps remain against the row?
Model answer
Evidence is missing for confirmed handover, treatment of Lea's old assignment and the agreed transition or conflict conditions. Product versions and scope must also match the test. The individual call proves reachability, not the complete role change. Before acceptance, open decisions on expected behaviour must be resolved and suitable observations added.
What to remember
Traceability connects the reason for a requirement to bounded evidence that can be found again.
Your Talbahn case file
Record TB-REQ-017 and TB-T-017. Mark confirmation, transition time and conflict behaviour as open decisions, each with a responsible party.
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