IT’S ALL ABOUT THE INTERFACES Jeffrey Eyster Indianapolis, IN

System Under Consideration Consider current banking transaction systems System includes internet transactions, telephone transactions, dedicated transaction machines, external transaction machines, and tellers System is very complex System requires considerable emphasis on security and data integrity

Banking System Context

Notional Banking System

System Design Issues Where will most problems be found with this system? Where will you spend most of your time fixing things with this system? Where should you spend more time making sure the definitions are complete for this system?INTERFACES

Defining Interfaces Interface requirements need to be defined with the same rigor as other system requirements Treat these the same as performance requirements The interface definition must satisfy the same criteria for requirements Complete Consistent Unambiguous Verifiable

Elements of the Interface Requirement Physical Construct Functional Behavior

Requirement Definition This is the “shall” of the definition The ATM System shall perform valid transactions of customers and non- customers. What does this requirement statement imply?

Physical Construct This is the definition of the physical elements of the interface. The ATM System shall extract information about the user and transmit this information to the main bank system to verify the user can perform transactions. What other physical elements are necessary for the ATM System?

Functional Behavior This is the functional definition of the desired behavior of the interface The ATM System shall verify the user has a valid card and identification number prior to initiating any transaction. What other functional behaviors are desireable about this interface? What functional behaviors should the interface NOT have?

Tools To Identify and Manage Interfaces Thorough understanding of the system and its context Be sure you understand what aspects are included and what are excluded Ensure everyone on the design team understands the system context CREATE THE SYSTEM CONTEXT DIAGRAM

Tools To Identify and Manage Interfaces (cont’d) Document the elements of the interface Text is fine to capture the requirement, but how many words does it take to describe the functional behavior and physical elements? Use graphics CREATE THE N-SQUARED DIAGRAM CREATE THE FUNCTIONAL BLOCK DIAGRAM CREATE THE SCHEMATIC BLOCK DIAGRAM

N-Squared Diagram

Functional Block Diagram

Schematic Block Diagram User Display Entry Panel Receipt Dispenser Cash Dispenser Deposit Collector Computer Home Bank

Verification and Validation Verification determines if the requirements have been satisfied Did I build it right? Validation determines if the design meets the intended purpose Did I build the right thing?

V&V for Interfaces Verify each interface meets the requirement by one of the verification methods (Inspection, Analysis, Demonstration, or Test) Requirement Functional Behavior Physical Construct

Summary It is essential to understand the interfaces of the system Most problems occur at the interface It isn’t the software’s fault that it didn’t work in the microprocessor It isn’t the microprocessor’s fault the software didn’t work The fault is the interface between the software and the microprocessor Time spent completely defining the interface is time well spent Pay me now or pay me later (but you will pay