A business analyst (BA) is working on a payment system (PS) implementation.

Interface analysis is a technique that identifies and specifies the information that is exchanged between systems or system components. It defines the communications protocol, data format, and timing of the information exchange. Interface analysis is useful for designing and documenting the interactions between the PS and other systems. It can also help to identify potential issues or risks related to the integration of the systems.

Vertical prototyping is a technique that creates a working model of one or more aspects of a solution, such as the user interface, functionality, or performance. It is used to validate the requirements and design of the solution, and to obtain feedback from stakeholders. It is not directly related to defining the communications protocol between the systems.

Concept modelling is a technique that defines the terms and concepts relevant to a domain or a problem. It establishes a common vocabulary and understanding of the domain among the stakeholders. It is not focused on the technical details of the system interactions.

System analysis is a broad term that refers to the process of studying and understanding the current or desired state of a system. It involves identifying the system components, functions, inputs, outputs, and relationships. It is not a specific technique for defining the communications protocol between the systems. References:

[A Guide to the Business Analysis Body of Knowledge (BABOK Guide), Version 3], Chapter 10: Techniques, pp. 402-403, 414-415, 420-421, 424-425.

[Business Analysis for Practitioners: A Practice Guide], Chapter 5: Requirements Analysis and Design Definition, pp. 123-124, 133-134, 140-141, 144-145.