#5 Defining the System Boundary and Interfaces

In the systems engineering lifecycle, boundary definition is the final conceptual framing before formal system requirements are specified. It marks the transition from "understanding the world" to "defining the system." Within the V-Model, this step sits between Concept Definition and System Requirements, ensuring we’ve scoped the problem space correctly before moving into design commitments.

This directly follows from our earlier activities:

• In #3 From Bid to Baseline, we reconciled bid commitments with contractual realities. That work helps us validate that everything we include (or exclude) from the system boundary aligns with what we are obligated to deliver, and flags risky assumptions we might inherit at the interfaces.

• In #4 Establishing the System Context, we defined the world the system must live in its environment, stakeholders, and constraints. That understanding helps us ensure we don’t overlook external actors or boundary influences during interface identification.

Now, we turn inward: what exactly is our system, and where does it begin and end?

We use this step to:

• Clearly define the System of Interest (SoI)

• Identify interfaces to the outside world

• Ensure clarity of scope ownership

• Stabilise the input conditions for requirement derivation and interface specification

Without a validated boundary, requirements become ambiguous, interface responsibilities drift, and test planning becomes incomplete.

Defining the System of Interest (SoI)

The SoI is the anchor point for system requirements and architecture. It defines the engineering scope and allows us to apply clear responsibilities, traceability, and V&V boundaries.

We explicitly state:

• What the system includes (components, subsystems, software/hardware)

• What is external but required (e.g., third-party services)

• What is excluded entirely (e.g., processes, devices, stakeholders outside delivery scope)

Locker System Example:

• In Scope: Locker cabinet hardware, embedded controller, user interface, SMS delivery trigger, admin UI

• External Dependencies: 230V mains power, LTE network, third-party SMS gateway

• Out of Scope: Package delivery operations, end-user mobile phones, payment processing

This boundary statement directly supports the System Requirements phase by anchoring requirement ownership and avoiding future scope creep.

Identifying External Interfaces

Every interface across the system boundary introduces integration risk. Identifying these early allows us to:

• Assign responsibility

• Estimate interface stability and testability

• Plan for interface specification and validation

We group interfaces by type:

• Physical: Mechanical mounts, enclosures

• Energy: Power, battery, backup

• Information: Networks, APIs, protocols

• Human: Maintenance access, user controls

Categorised Interface Table – OTP Locker System:

These entries later become interface requirements and form the interface control baseline.

Clarifying Interface Ownership

Interface confusion is one of the most common causes of integration failure and delivery disputes. Clearly documenting who owns each side of an interface and who is responsible for verification reduces downstream ambiguity.

We ask:

• Who supplies the external system?

• Who implements or integrates the interface?

• Who verifies the interaction in testing?

Example:

• SMS Gateway: Provided by client; we integrate to API

• Power Supply: Provided by site; we define electrical spec

• Admin Interface: Developed and hosted by us; secured by ISO measures

This information supports integration planning, acceptance testing, and contract alignment.

Validating the Boundary

The boundary definition isn’t complete until it has been validated against:

• Contract scope: Are we including/excluding the right elements?

• System context: Have we accounted for all external influences?

• Risk register: Are any interfaces associated with high-risk assumptions?

This step links directly to our earlier risk capture in #3 From Bid to Baseline, where we identified unverified bid claims and risky assumptions that may influence boundary decisions (e.g., who provides power, or what is required for "remote access"). It also leverages our findings in #4 Establishing the System Context, where we understood the operational, environmental, and stakeholder constraints that might impact interface design or necessitate exclusions.

System Boundary Diagram

Visual models provide shared understanding across disciplines. A System Context or Boundary Diagram reduces misinterpretation and helps align engineering, test, and stakeholder expectations.

It typically includes:

• The System of Interest (boxed)

• External systems and stakeholders

• Directional labelled interfaces

Locker Example Sketch:

• Box: Locker Unit

• Arrows: SMS Gateway (API), Power (AC), Admin Portal (HTTPS), Maintenance Access (human)

This becomes a control artefact in interface design and verification.

Closing Reflection

Defining the system boundary is where systems engineering turns context and commitment into a structured delivery perimeter. It prevents scope creep, defines responsibility, and supports test planning and requirement writing.

A clear boundary means:

• Stakeholders know what’s being delivered

• Test teams know what must be verified

• Designers know what must be integrated

This step closes the Concept Definition phase and opens the door to formal system specification.

What’s Next?

In the next post, we begin defining what the system must do within the boundary we’ve just drawn. In #6 Defining the System Functions and Requirements We’ll walk through functional decomposition, context-derived requirements, and the start of formal, traceable system behaviour.